Northern California

Geolo
Geology of Northern
California
Frank DeCourten
Department of Earth Science Sierra College

Standing more than
10,000 feet (3,000 m) above the surrounding terrain, Mt. Shasta is the largest volcano in northern California and symbolizes the dynamic geologic processes that have shaped a spectacular landscape. 63829_02_insidecover.qxd

11/25/08

12:53 AM

Page ii

ESSENTIAL QUESTIONS TO ASK
Northern California.1 Introduction
Ⅲ What are northern California’s physiographic provinces?
Ⅲ What is the Farallon subduction zone? al Ⅲ What two types of plate boundaries exist in northern California today? th Ⅲ What are terranes, how do they originate, and why are they important in northern
California?

Northern California.2 The Sierra Nevada: California’s Geologic alifornia’s Ge
Backbone
Ⅲ What is the Sierra Nevada batholith? rra batholi
Ⅲ What kinds of rocks surround the Sierra Nevada batholith? ra Ⅲ When and how was the modern Sierra Nevada uplifted? e Ⅲ What types of gold deposits occur in the Sierra Nevada? e? Ⅲ What is the Mother Lode?
Northern California.3 The Klamath M
Mountains
t ath an
Ne
evada
Ⅲ In what ways are the Klamath Mountains and the Sierra Nevada similar? ds ro o ath M
Ⅲ What kinds of rocks comprise the ophiolites in the Klamath Mountains and what tectonic events do they signify?

ineral occu th ntai Ⅲ What mineral resources occur in the Klamath Mountains?

Northern California.4 The Gr
Ca rnia e Great Valley fa s th i he
Valle
Ⅲ What factors have led to the formation of fertile soils in the Great Valley? at t ime y
Grea
Seq
Ⅲ What was the origin of the sedimentary rocks in the Great Valley Sequence? ori l p duced
Gr
Ⅲ What is the origin of the natural gas produced in the Great Valley? y Gre chron ooding
Ⅲ Why is the Great Valley so prone to chronic flooding?
Northern California.5 The Northe Coast Ranges rn Ca
Northern Coa
Ⅲ What is the Franciscan complex and how did it form?
Andr s or Ⅲ How did the San Andreas fault system originate? inian Ⅲ What is the Salinian block? norther Ra
Ⅲ How old are the northern Coast Ranges and what tectonic forces elevated them?
Northern California.6 Volcanoes of the Cascade Range and Modoc ornia.6 olcan
Pl
Plateau fornia volca
Ⅲ What California volcanoes are part of the Cascade Range?
Cascad
Ⅲ What is the Cascadia subduction zone? o Ⅲ What kind of volcanic activity typifies the Cascade Range?
Mou
Ⅲ Why is Mount Lassen an especially interesting volcano in the Cascade Range?
Northern California.7 The Basin and Range of Northeast California
Ca
Wh
Ⅲ What tectonic forces are responsible for the pattern of alternating mountains and valleys in the Basin and Range province?

Ⅲ What mountains and basins in northern California belong to the Basin and Range province?

Northern California.8 The Northern California Ice Ages
Ⅲ During what time periods did northern California experience Ice Age conditions?
Ⅲ What landscape features resulted from the Pleistocene Epoch glaciations in northern
California?

Ⅲ How did the Pleistocene landscape of northern California differ from the modern setting?

Northern California.9 Northern California Earthquakes
Ⅲ What plate tectonic settings are associated with northern California earthquakes?
Ⅲ What is the likelihood of another major earthquake in northern California?
Ⅲ What might be the effects of a large northern California earthquake?
Ⅲ Given the severity of the potential hazards, how can the effects of earthquakes be minimized in northern California?

Northern California.10 Living on the Edge: Coastal Hazards in
Northern California
Ⅲ In comparison to the coast of southern California, why is the northern California shoreline so rugged and scenic?

Ⅲ What coastal hazards exist in northern California?
Ⅲ How do human activities affect coastal hazards?

Geology of Northern California

Northern California.1
Introduction
California’s Varied Landscape: California is arguably the best place in the world to study geology. Few areas of comparable size are as geologically varied, physiographically diverse, or so spectacularly scenic as the Golden State. California’s abundant natural resources reflect in large measure its rich geological history, and earthquakes, floods, and mass wasting events underscore the importance of ongoing geologic processes for residents and visitors. With the boundary between two of the largest lithospheric plates on the planet running for more than 1,300 kilometers along the western side of the state,
California is an outstanding natural laboratory for studying
Earth processes and plate interactions.
During the 1960s and 1970s, our understanding of earth dynamics shifted dramatically as the modern theory of plate tectonics was developed and refined. Many of the new concepts of that era either were developed in California or were specifically formulated to explain and/or reinterpret its incredibly varied geological features. Today, with a new theoretical framework and vastly improved tools for exploration ration and analysis, scientists continue to consider California a nia geologic paradise.
The rocks in California tell an amazing story of the evolury tion of land and life at the edge of North America. Although merica. Altho it would require many years to explore all the geologic wonders in California, a first course in physical geology is an physi logy excellent start to a lifetime of fascinating adventure in the inat e
Golden State. The purpose of this chapter is threefold: (1) to cha 1 introduce the broader aspects of California’s geologic setting, lifornia’s (2) to outline the major tectonic events that have shaped its nts s landscape over geologic time, and (3) to examine the impacts on humans of the state’s ongoing geologic evolution. I also hope you will consider this chapter an invitation to begin consid chap n your own lifelong exploration of one of the most magnifiexplo n on t magn cent landscapes in the world. pes wo
California’s Physiographic Provinces: The California
Physio
hic Pro rnia landscape can be divided into a dozen regions of distinctive o doz and characteristic geology, landforms, climate, geomorphic aracterist cl trends, soils and vegetation, and drainage. These natural an d draina areas are known as physiographic provinces. After the geophic provinc logic alignment of the state’s major mountain systems, most ent o ajor mou of the geomorphic provinces are oriented in a northwest-toorphi ince southeast trend (Figure NC.1). Hence some of the provinces, end (F e H such as the Sierra Nevada and the Coast Ranges, extend from ada the northern part of the state to the southern portion. For the te purposes of the present discussion, we will arbitrarily define northern California as the region between Monterey Bay
(latitude approximately 36.5° N) and the California–Oregon border. In this portion of California, seven natural physiographic provinces comprise the landscape: the northern
Sierra Nevada, the northern Coast Ranges, the northern
Central Valley, the Klamath Mountains, the Cascade Range, the Modoc Plateau, and portions of the Basin and Range.
Though there are consistent geologic patterns within each of these regions that make them distinctive and identifiable,

none can be regarded as simple or monotonous. Each of the physiographic provinces in northern California is a varied and fascinating geological realm with endless opportunities d for applying the knowledge you have gained in your physical geology course. Collectively, they represent a region of such sent complex origin that scientists have yet to develop completely o co satisfactory explanations of all of the geologic features in this ogic featu varied terrain. The common attribute of all the physiographic ribute he physiograp nia on ay an provinces of northern California is that, one way or another, each reflects the consequences of plate tectonic interactions es tecton nt along the western margin of North America over the past f 500 million years. f Calif m Geologic Map of California: Geologic maps are indispensable assets in exploring the natural history and geologic setoring natu ting of any region. Such maps show the distribution of rock uch tion types and ages on the surface, along with information about d age surfac ma mati the orientation of the various rock bodies, the nature of their entation t ario e na contact with adjacent rock masses, and the trends and extent ct ck m e of geologic structures such as faults and folds. This informaologic su olds. tion is essential in unraveling the geologic history of a region tio unra eolog regi because it reveals spatial patterns in the distribution of rocks becau sp dis bution ro of various types, ages, and degree of deformation that reflect variou nd ee deformat n re the tectonic and geologic events of the past. In addition, geotectoni ven ii logic maps are of critical importance in locating surface and ogic mporta e l subsurface deposits of earth resources and in identifying the sourc areas most susceptible to various geologic hazards. are ible variou geol

ᮣ

Figure NC.1 The Physiographic Provinces of California
1

Map courtesy of the U.S. Geological Survey

2

Northern California.1 Introduction

Figure NC.2 Geological Map of California

Reproduced with permission, California Department of Conservation, California Geological Survey
Californ
Conservatio eologica ᮣ

3

The geologic map of California (Figure NC.2) has been compiled by the California Geological Survey over many decades of geologic mapping. This map is scaled to show the entire state and, as such, it portrays only the broader distribution of various California rocks and structures.

Nonetheless, you will notice at a glance the strong similarity between the physiographic provinces in California and the distribution of various rock types. For example, notice the similarity in location, trend, and extent of the Sierra
Nevada province and the large area of red, blue, and green

4

Geology of Northern California

ᮣ

Figure NC.3 Interactions between The Farallon, Pacific, and North American plates over the past 40 million years.

40 M.Y.A

20 M.Y.A

0 M.Y.A
Queen
Charlotte transform fault

Farallon plate Pacific- remnant
Farallon
Ridge

Pacific plate Farallon plate North
American
plate

Farallon plate remnant

colors on the geologic map. These colors represent the nt e
Mesozoic-age plutonic igneous rocks that comprise the r ro hat core of the Sierra Nevada (red) and the older rocks that th were metamorphosed by the emplacement of the magma
(blue and green). A very similar pattern is observed in the
Klamath Mountains physiographic province northwest of ovi tw ions the Sierra Nevada, suggesting that the two regions share e si een th some common geologic traits. The similarity between the physiographic provinces of California and the distribution rni und of various types and ages of rock in the state underscores the importance of the geologic foundation in shaping the ndation sha character of the landscape. ard The Westward Migration: For the past 500 million years convergent plate tectonic interactions have prevailed along tect terac ailed ern o A y the western margin of North America. Early in the
Mesozoic Era, about 200 million years ago, the rate of plate ra, 20 llion ye plat convergence increased significantly as the ancient superconreased ficantly continent of Pangaea began to break up. The North American ang u n plate separated from the northern part of the supercontinent eparated f via the opening of the Atlantic Oceanic basin. Seafloor e openin c Ocean spreading in this basin propelled the North American plate led Nor to the west while the Eurasia and African plates moved in
Afric
the opposite direction. Several different oceanic plates were e direc
Seve
ff subducted under the leading edge of the North American ed plate as it slowly moved west. The last and largest of the ved T oceanic plates to descend beneath North America was the bene Farallon plate, remnants of which still exist along the western margin of North and Central America. This plate tectonic interaction along the western edge of North
America produced the Farallon subduction zone, which was established in mid-Mesozoic time and persisted from some 160 million years ago until about 30 million years ago.
Many of the major geologic trends in California are the result of this long history of plate convergence.
Origin of the Modern Transform Plate Boundary: The
Farallon plate originated at an oceanic ridge to the west and

North
American
plate

Mexico

Incipient ent San Andreas
Sa
transform transfo fault

te p pla i fic a if
Pac

Gulf of
Mexico

e plat ific
Pac

PacificFarallon
Ridge

Incipient
San Andreas transform fault

Juan de Fuca
Fuc
plate e North
American
plate
Present day volcanoes Gulf of
Mexico
Cocos plate thwes
Ame
othe ide o southwest of North America. On the other side of that spr he Pa fic plat ancient spreading center, the Pacific plate was sliding to the lon p e mov west, while the Farallon plate moved in the opposite directio ancing ma in tion toward the advancing margin of North America. A little ion less than 30 million years ago, the western edge of North ded ith, eve America collided with, and eventually overran, the Pacificdge n e latit
Farallon ridge near the latitude of modern Los Angeles. As a th li consequence of this collision, the North American plate came into contact with the Pacific plate (Figure NC.3). e con ct wit
The collision between North America and the Pacificn en Farallon ridge ended plate convergence in the California region and established the modern transform plate regio e bounda boundary. After the initial collision of the ridge and the continent, the transform plate boundary expanded north continen so and south as North America continued moving west, overru running more of the ridge in the process (Figure NC.4).
The relatively small Cocos, Rivera, and Juan de Fuca plates represent modern remnants of the ancient Farallon plate.
The famous San Andreas fault system developed as a consequence of the transform plate boundary between the North
America and the Pacific plates.
The geologic setting of modern northern California is thus influenced by two different kinds of boundaries between the North American and oceanic plates to the west: a transform boundary from Monterey Bay to Cape
Mendocino and a remnant convergent boundary to the north. We will explore the consequences of these interactions in more detail in the sections that follow.
California and Accreted Terranes: One of the consequences of the long history of plate convergence along the western margin of North America was the accretion of numerous blocks of rock to stable crust of the continent.
Such blocks added to the edge of a continent by plate convergence are known as accretionary terranes, or simply terranes. Five hundred million years ago, there was no land where the Pacific Coast now stands. The crust of California

Northern California.2 The Sierra Nevada: California’s Geologic Backbone

Figure NC.4 Major terranes of western North America. In addition to the Franciscan Complex and the Salinian block illustrated here, dozens of smaller terranes have been identified in northern California.

ᮣ

Seward
Peninsula

North
Slope
En di co

A
Yu las k o ka n tt

Ruby
Nixon Fork

Wrangellia

Yukon–
Tanana

Alexander

Wrangellia

5

200 million years ago. Several others were added in earlier accretionary events in the Paleozoic Era, before North
America separated from Pangaea. It was not until Cenozoic wer time that all of the terranes were in place along the Pacific fornia Coast and the modern California landscape began to emerge.
,
California, as a whole, therefore represents a geologic ces assem collage, an amalgam of pieces assembled through the cons vergence of plates along the west edge of North America
0
year orthern over the past 500 million years. Northern California is espebo r cially intriguing because here both a remnant of an ancient ndary m convergent boundary and the modern transform boundary tinue t to the south continue to shape the landscape. It is not surg ists no prising that geologists find northern California such a fascinating region. It is a place where the geologic past meets the c dynamic present, and there is no place in the world better s r geolo et’s lo s suited for geologic exploration. Let’s look a little closer.

Section Northern California.1 Summary alifornia.1 S

Stikine

● The geology and landscape of northern California is d landsca

extremely varied, with seven different physiographic y v d, provinces, each with distinctive rock assemblages and distincti k geologic histories. ories. Wrangellia

● Convergent plate boundaries have existed for 500 milnvergent ate b

Cana
U. S da
. A.
Wrangellia
gellia
Franciscan
n
Complex

Klamath
Moutains
s

n n lion years in the northern California region. After North
America separated from Pangaea early in the Mesozoic
America
Era, e
E the rate of convergence increased as several oceanic plat were subducted under the west-moving continent. es plates
● The subduction of the Farallon plate in the Mesozoic

Era resulted in many of the geologic trends that can be ob observed in modern California. About 30 million years ago, plate convergence ended as the transform boundary between the Pacific and North American plates began to develop. The unique geologic setting of modern northern California is shaped by both a remnant of the
Mesozoic subduction zone and the continuing evolution of the more recent transform plate boundary.

Salina

U. S. A.
Mexico
300 km

has since been assembled in piecemeal fashion as seamounts, n asse island arcs, coral reefs, and small continental blocks that were carried on subducting oceanic plates collided with the western edge of the continent and were embedded into the existing margin. The rocks in the various terranes were metamorphosed and deformed as each was sutured into
North America like pieces of a mosaic. In this manner, North
America grew incrementally westward with the addition of each fragment. Geologists are in general agreement that about 100 such terranes were accreted to the western margin of North America since the breakup of Pangaea about

Northern California.2
The Sierra Nevada: California’s
Geologic Backbone
The Sierra Nevada is California’s best known mountain system. Stretching for more than 700 kilometers from
Lake Almanor in the north to Tehachapi in the south, this northwest-trending mountain system is home to three national parks and the highest peak in the coterminous
United States at Mount Whitney (14,495 feet/4,418 meters above sea level). Winter storms passing east from the Pacific
Ocean over the high Sierra Nevada produce heavy snowfall that is an essential supply of water to the entire state.
Materials released from the weathering of Sierra Nevada

6

Geology of Northern California

bedrock help sustain the fertility of California’s rich agricultural soils. It was in the Sierra Nevada foothills that gold was discovered in the 1800s, and the course of California history was forever changed. Without the Sierra Nevada,
California would simply not be California.
The bedrock of the Sierra Nevada is dominated by the
Mesozoic Sierra Nevada batholith, one of the largest and most complex masses of granitic rock in the world. Adjacent to the batholith, and sometimes as xenoliths and roof pendants within it, are older Mesozoic and Paleozoic metamorphic rocks that were invaded by the subterranean magma. More recent episodes in the evolution of the Sierra
Nevada are documented by Cenozoic volcanic and sedimentary rocks that rest on the granite-metamorphic basement.
The Sierra Nevada Batholith: The geologic map of
California clearly indicates that the Mesozoic granite of the
Sierra Nevada batholith comprises the core of the Sierra
Nevada. Vast exposures of such light-colored plutonic rock in the high country of Yosemite, Kings Canyon, and
Sequoia National Parks, is in part what led John Muir to refer to the Sierra as “the range of light” (Figure NC.5).
The Sierra Nevada batholith is a complex assemblage of perhaps as many as 200 individual plutons representing enting magma bodies emplaced mostly between 140 million and
80 million years ago at depths of 10 to 30 kilometers
0 ki rs Figure NC.5 Vast exposures of granitic rock characterize the Sierra Nevada, John Muir’s “Range of Light”. erize Rang f Light

Frank DeCourten

ᮣ

beneath the surface. Most of the plutons comprising the
Sierra Nevada batholith consist of felsic rock such as granite, rich in light-colored quartz, potassium feldspar, and sodium-rich plagioclase (Figure NC.6). Some of the pluSom tons are richer in the darker ferromagnesian minerals and nesian consist of rock more similar to diorite or gabbro.
The Sierra Nevada batholith represents the deep roots nts of a Mesozoic volcanic arc that developed along the westat deve he w ern margin of North America above the Farallon subducca allon su tion zone. In this ancient subduction zone, magma was zon mag produced by partial melting and migrated upward through up older crustal material to build a chain of andesitic volcanoes ande mod ndes Moun that rivaled the modern Andes Mountains of South
America. Recent geologic studies have revealed evidence of eologic studi large calderas formed by explosive eruptions in this ancient ed volcanic chain. Late in the Cenozoic Era, the uplift of the chain up upli Sierra Nevada resulted in the nearly complete erosion of res d plete the volcanic edifice that originally concealed the batholitic olcanic t or ealed rocks. However, isolated remnants of the ancient volcanoes
.
isolate e can still be seen in places along the crest of the range ca ill g rest ran (Figure NC.7).
(Figu
Paleozoic-Mesozoic Accretionary Terranes of the
Paleo
A tionary Terr nes
Northern Sierra Nevada: The magma that formed the d : T e th Sierra Nevada batholith was emplaced into older rocks that
Nev
as emp ced in either accumulated on the seafloor west of the margin of afloor Northern California.2 The Sierra Nevada: California’s Geologic Backbone

7

Figure NC.8 Folds in these Paleozoic sedimentary rocks along the
Yuba River were developed during the collision between a block of oceanic rocks and western edge of North America in the Mesozoic Era.

Figure NC.6 Close up of granite from the Sierra Nevada
Batholith. Dark crystals are biotite and hornblende, white crytals are plagioclase and potassium feldspar, and the gray translucent crystals are quartz.

ᮣ

Frank DeCourten

ᮣ

Figure NC.7 Dark-colored rocks of the Ritter Range in the eastd rock t ern Sierra Nevada are remnants of the Mesozoic Sierra volcanic arc. ants o zoic volc

Dick Hilton

Paleozoic North America or were accreted to the edge of the continent in convergent plate interactions that preceded the Farallon subduction zone. These old rocks were deformed and metamorphosed during several different accretion events and altered by the heat and fluids associated with magma rising from the Farallon subduction zone in later Mesozoic time. Metamorphic rocks such as marble, slate, schist, serpentinite, and greenstone are common in the pre-batholithic terranes of the Sierra Nevada. Some of

De
Frank DeCourten

ᮣ

these rocks have been affected by multiple periods of metamorphism. In addition, these rocks are commonly folded and faulted by the compressional stress generated along ancient convergent plate boundaries (Figure NC.8).
It has taken geologists many decades to unscramble the complicated history recorded in the metamorphic rocks surrounding the Sierra Nevada batholith. Within the granitic core of the Sierra Nevada, exposures of these rocks are found as roof pendants and xenoliths (Figure NC.9) preserved in the plutonic rocks. In the foothills north and west of the batholithic core of the Sierra Nevada, extensive exposures of metamorphic rocks have revealed a complex history of metamorphism and accretion. Because there has been much greater Cenozoic uplift in the southern Sierra Nevada than in the north, erosion there has progressed to a deeper level in the crust and much of the older rock that formerly surrounded the granitic plutons has been removed.
Within the western foothills metamorphic belt, geologists have identified three primary units of metamorphosed rocks that are separated from each other by major faults.
These faults probably were produced when seamounts, undersea volcanic arcs, oceanic crust, or other fragments of lithosphere collided with the edge of North America. The metamorphosed and deformed rocks comprising these

8

Geology of Northern California

Figure NC.9 Dark-colored xenoliths in Sierra granite represent fragments of older rock assimilated into the magma prior to crystallization. Frank DeCourten

ᮣ

terranes are almost entirely of oceanic origin, and generally nerally older than the tectonic events that sutured them to the ma North American continent. Some represent mafic magma erupted by undersea volcanoes, and the characteristic pillow acteris structures can still be observed (Figure NC.10). Other
O
roc uch terranes include oceanic sedimentary rocks such as chert, mudstone, and limestone that have been metamorphosed e morphosed to varying degrees. There is strong evidence that some of ge ome o the accreted fragments traveled thousands of kilometers k before the now-vanished plates carrying them sank into arrying sa ancient subduction zones. Ophiolites, sequences of rocks es, representing oceanic lithosphere, are present in some terresent i ranes, and probably originated as fragments splintered from obab the descending oceanic plates sutured into the various terding ocean es sutu vario ranes. Adding complexity to some of the terranes is the ding complexi o es
Figure NC.10 The rounded pillow structures in this outcrop of ro illow stru mildly metmorposed basalt in the western metmorphic belt indicate morp eruption of lava on the seafloor. on o

Frank DeCourten

ᮣ

presence of mélanges, chaotic mixtures of deformed and metamorphosed rocks that are formed in subduction zone settings. We will learn more about ophiolites and mélanges ang when we explore the northern Coast Range, where they on su formed within the much younger Farallon subduction zone. da: e
Cenozoic Rocks of the Sierra Nevada: At the end of the
,
Sier
Mesozoic Era about 65 million years ago, the Sierra Nevada n perch b was an elevated volcanic terrain perched on a complex basecks conc ment comprised of older rocks intruded by the concealed s e
Sierra Nevada batholith. As Cenozoic time (the Tertiary ity hav
Period) began, igneous activity appears to have temporarily wa subsided in the Sierra region, and erosion was beginning to s th ant volcano attack the summits of the dormant volcanoes. The volcanic highland extended into what is now western Nevada and the n e
Pac
ancient shore of the ancestral Pacific Ocean was located he foo f d c along the foothills of the dormant volcanic arc in central
California. Rivers draining the volcanic highland ran west rnia. ning ghlan s ode ada. The across the site of the modern Sierra Nevada. These ancient rivers steadily wore away the volcanic bedrock, and transaw a ported great quantities of sediment to the ocean basin to the po t th t his peri Ea
E
west. This long period of erosion in the Early Cenozoic Era produced a pronounced plain, which has since been elevated oduce pl een elev o he flo rs hundreds of feet above the floors of modern stream valleys in the Sierra Nevada region (Figure NC.11). n Sierr n (Figu NC.
During the Eocene Epoch, about 50 million years ago, ne Epo , abou 5 river-deposited sediments began to accumulate in the riv ediments be an ancient river systems of the Sierra Nevada. In the stream anc sys ems
Sie
channels, coarse gravel deposits formed. On the floodoarse ravel plains, fine sand, silt, and mud accumulated. These riversand deposited sediments are still preserved in many places in ited s iment the foothills of the Sierra Nevada as colorful exposures of o he S conglomerate, sandstone, and mudstone. Geologic mapglomerate, sa ping in the early 1900s demonstrated that exposures of the ancient river gravels were aligned as elongated ribbons, indiancien g cating the location and drainage pattern of the 50-millionth year-old river channels. Soon after the California Gold
Rush began, prospectors discovered that some of the
R h
Eocene-age river gravels contained rich concentrations of
E
gold in the form of nuggets, flakes, and fine particles. The richest gravel deposits became known as the “auriferous gravels.” Such gold-bearing sediments were intensely mined and processed in the mid-1800s, primarily by washing away the weakly-cemented material with powerful steams of water and separating the small gold particles from the loosened sand, pebbles, and cobbles (Figure NC.12).
Known as hydraulic mining, this process was phased out after 1884 because so much sediment had been washed from the Sierra slopes that agriculture and river navigation downstream in the Central Valley was being adversely affected. The best exposures of the auriferous gravels today are in the scars and excavations left from the hydraulic mining activities more than century ago.
During the Oligocene and Miocene Epochs, 35 million to 5 million years ago, volcanic activity resumed along what is today the crest of the Sierra Nevada and areas to the east.
The initial eruptions were violent caldera-forming explosions that sent great flows of ash and fragments of volcanic

Northern California.2 The Sierra Nevada: California’s Geologic Backbone

9

Figure NC.11 Beyond the deep canyon of the Yuba River on the Sierra Nevada west slope, the flat surface reflects a lengthy interval of erosion in the early Tertiary Period.

Frank DeCourten

ᮣ

Figure NC.13 Oligocene tuff near Soda Springs, California consists of ash partcles and white pumice fragments welded into a coherent rock.

ᮣ

Figure NC.12 Hydraulic mining in the Sierra Nevada involved washing Eocene sediments with powerful jets of water. Photo taken wash hoto take in 1905 at Junction City, California. on City

Frank DeCourten

Bettmann/Corbis

ᮣ

rock surging west through the canyons carved by the
Eocene-age rivers. These pyroclastic deposits hardened into tuffs and welded tuffs that partially filled the ancient canyons (Figure NC.13). The later eruptions produced basalt flows, volcanic breccias, and volcanic mudflow deposits known as lahars. The Tertiary-age volcanic rocks completely filled some of the valleys of the western slope of

the ancestral Sierra Nevada, forcing the ancient streams from their channels. In the past few million years, as the modern Sierra Nevada rose, vigorous down-cutting has left the erosion-resistant valley-filling Tertiary volcanic rocks perched high above modern river beds. Sinuous ridges

Geology of Northern California

capped by Tertiary volcanic rocks demonstrate the phenomena of inverted topography in many places in the
Sierra foothills (Figure NC.14).
Recent Uplift of the Sierra Nevada: Though the rocks of the Sierra Nevada document a long history of geologic unrest, good evidence suggests that the modern range reflects a relatively recent pulse of uplift. Although the
Sierra Nevada was probably an elevated tract of land since the mid-Mesozoic Era, recent geologic studies suggest that
5 million to 10 million years ago, the mountain system rose to its current elevation, primarily by westward tilting along normal faults located along the eastern escarpment.
These faults, many of which remain active today, have uplifted and tilted the range to the west, producing a spectacularly rugged eastern escarpment and a gently inclined western slope. The steep eastern escarpment of the Sierra
Nevada was a formidable barrier to the migration of people during the Gold Rush. Even today, only a few highways cross the crest of the northern Sierra Nevada through passes that range in elevation from more than 3,000 meters
(9,945 feet) to just over 2,200 meters (7,259 feet) above sea level. Geologists have not yet determined the precise cause for use the recent uplift of the Sierra Nevada. The forces affecting ting the Sierra region are complicated by its proximity to prox o regions of differing plate tectonic interactions. In the Basin ns. B and Range province to the east, tensional stresses are al stretching the crust and producing the numerous normal he n rous faults, such as those along the eastern escarpment of the tern nt
Sierra Nevada. To the west, the modern transform boundmod bound ary between the Pacific and North American plate produces p shear stresses that tend to move the Sierra Nevada block to e the northwest. Recent studies also suggest that the dense uggest lower crust of the southern Sierra Nevada may have been d removed in late Cenozoic time by heat associated with material upwelling from the mantle. The removal of the fro e mantl oval o dense root beneath the Sierra Nevada would have increased t S a Nevad ncrea the buoyancy of the Sierra Nevada, causing it to rise. All of
Sie
these factors may have been involved in the recent ascent of involve t the Sierra Nevada. Whatever the precise cause of uplift may a Nev r pr be, there is no doubt that it is still continuing. The northern re d ntinu portion of the Sierra Nevada is currently rising at a rate of 2 n th r to 3 millimeters per year, estimated on the basis of stream mated th incision rates and the increased tilt of ancient streambeds. s ilt a
The Mother Lode: Gold and Geology of the Sierra ther L
Foothills: Califor
California is known as the “Golden State” for a s a reason: since the initial discovery in the mid-1800s, more l discover than 115 million ounces of gold have been produced in the state, an amount equivalent to a volume of more than 190 cubic meters! At current prices, the cumulative value of
California gold is more than $110 billion. Though gold has been found in many places in California, more than 75% of the amount recovered historically has come from the western foothills of the Sierra Nevada. It was also in this region that James Marshall first noticed the glittering nuggets in the American River in 1849 and ignited one of the most dramatic human migrations in history.

Figure NC.14 Inverted topography in the Sierra Nevada foothills is a common consequence of Miocene volcanic activity.

ᮣ

(a)

(b)

James S. Monroe

10

(c)

Northern California.2 The Sierra Nevada: California’s Geologic Backbone

Gold was initially discovered in California as flakes and nuggets resting in modern river gravels. Such deposits are known as placers, and early miners invented several techniques to wash the gold particles from the loose river sediments. Soon, miners began to exhaust the placer deposits, and looked upstream from the gravel bars to find the source of the gold particles. They eventually found two sources for the placer gold. First, it was discovered that the Eocene-age river deposits were also rich in gold, particularly the lower channel-filling conglomerates. The “auriferous gravels” were more difficult to work than modern placers because they were more consolidated and generally exposed high above the modern streams. The invention of the hydraulic mining methods mentioned earlier helped to solve these difficulties, but the disastrous environmental consequences ended this practice in the 1880s. At the same time, prospectors discovered a second source of the placer gold: quartz rtz veins in the bedrock in the western foothills metamorphic hic belt were found to contain large masses of native gold. Such uch gold-bearing quartz veins are called lodes, and the largest est concentration of such ores in the Sierra foothills became s bec known as the Mother Lode (Figure NC.15).
Geologists are still not certain about the source of all the he gold in California, but the variety of ore types suggests mulpes tiple origins. Much of the Mother Lode gold may have been
Lod
h originally disseminated in the oceanic metamorphic rocks or ceanic meta hic roc in the younger igneous bodies. Some of the gold may have th originated in the volatile gases associated with the granitic gase ociated t magma that intruded the Sierra Nevada basement during the he Sie da basemen

11

Mesozoic era. During metamorphism related to either burial, tectonic accretion, or the emplacement of magma, hot fluids were introduced into the rocks of the Mother Lode belt and migrated through them along fractures, faults, or hem shear zones. Interactions between the circulating hot bet (hydrothermal) fluids and the metamorphic rock concentratmetamo ed gold in the quartz-rich fluids. The hydrothermal fluids
T
eventually cooled as they circulated through the fractured the ulated throug metamorphic rock, leaving veins of quartz laced with pure ck, v quar gold. When the gold-rich metamorphic rocks were eroded meta ph by Cenozoic-age streams, the heavy gold particles accumue he lated in nearby stream sediments and the less dense compotream th k wash nents of the bedrock were washed farther downstream.
In the Mother Lode belt, all three types of gold
Mothe
deposits—placers, auriferous gravels, and lodes—were press—placers, auri s, an ent. It is not surprising that this region was the primary tare su egio ion get of the Gold Rush prospectors and miners. Though o e G ector production of gold in California fell dramatically after the roducti ornia
1860s, people occasionally still find gold in the streams of peo the Sierra Nevada. erra Section Northern California.2 Summary ion No thern C
● The Sierra Nevada is California’s geological backbone
Nev

and he lar an the largest mountain system in the state. It is a region of majestic scenery with three national parks, and reg f ma highest peaks in the coterminous United States. the h
● The core of the Sierra Nevada is comprised of the he Figure NC.15 Specimen of gold from Grass Valley, California.
G
ifornia.
Inset map shows location of the Mother Lode region in the western
Mothe
e foothills of the Sierra Nevada.

National Museum of Natural History, Specimen #R12197, Photo by D. Penland. o ᮣ

Sie Nevada batholith, a large mass of granite emplaced
Sierra
i into older rocks during the Mesozoic Era. Magma rising from the Farallon subduction zone constructed the batholith beneath a volcanic arc similar in origin to the modern Andes Mountains of South America. Surrounding the Sierra Nevada batholith are older metamorphic rocks in which numerous accreted terranes can be recognized.
The metamorphic rocks form roof pendants and xenoliths in the batholithic rocks and extensive bedrock exposures in the western foothills. Cenozoic sedimentary and volcanic rocks record more recent periods of erosion and pyroclastic volcanic eruptions in the Sierra Nevada region.
● Though the Sierra Nevada was an elevated region

during the Mesozoic Era, the uplift of the modern mountains appears to have occurred primarily during the past 5 million to 10 million years, when normal faulting along the eastern side was initiated. The Sierra Nevada was lifted and tilted to west during this time, creating the gentle western slope and the abrupt eastern escarpment.
● Gold in the Sierra Nevada was concentrated in plac-

ers, Eocene stream gravels, and in lode deposits of the western foothills. In the famous Mother Lode belt of the
Sierra Nevada foothills, all three types of gold deposits are present.

12

Geology of Northern California

Northern California.3
The Klamath Mountains
The Klamath Mountain region is a rugged highland with a general elevation of 1,500 to 2,150 meters (5,000 to 7,000 feet) above sea level. This elevated terrain is deeply incised by the Klamath River and its major tributaries, the Trinity and Salmon Rivers. These rivers carry water to the Pacific
Coast through winding and spectacularly rugged canyons
(Figure NC.16). The deep river gorges separate the
Klamath highland into several distinct mountain ranges, including the Trinity Alps and the Siskiyou, Marble, and
Salmon Mountains, some of which extend northward into southern Oregon. These rugged mountains are generally lower than the Sierra Nevada to the south, with the maximum elevations reaching only about 2,750 meters (9,000 feet). The Klamath Mountains separate the northern Coast
Range on the west from the volcanoes of the Cascade Range to the east. Unlike most other geologic provinces in northern California, the Klamath Mountains lack a prominent northwest orientation. Instead, the individual mountains in
Figure NC.16 The rugged canyon of the Salmon River in the
Klamath Mountains privince.

Dick Hilton

ᮣ

this province have a crude north-to-south alignment or a weakly curved trend.
Even a quick glance at the geologic map of California rs a reveals a strong similarity in outcrop colors and patterns of the Klamath Mountains region and the Sierra Nevada (see
Sier
Figure NC.2). These similarities arise from comparable, se com though not identical, geologic histories in the two regions. tw In a general sense, the Klamath Mountains can be considth Mo s con geolo o ered a northwest extension of the geologic trends of the
Sierra Nevada. However, the continuity between the two he be n regions is broken by the young volcanic rocks of the southng ern Cascade Range and by the sediments of the northern he o al Valle
M
part of the Central Valley. The Klamath Mountains shares with the Sierra Nevada a long history of subduction-related vada h accretion, beginning in the Paleozoic Era during which g Paleo uring numerous oceanic terranes collided with the western edge us oce rranes wes est of North America. The dozen or so terranes recognized by rth America e do es re geologists in the Klamath region are separated from each gists Kla ath parated f other by major east-dipping fault zones. Granitic plutons of east-dip p
Mesozoic age were emplaced into the accreted terranes in
M zoic em o creted several areas of the Klamath region, though such rocks are sever ion, thou not as widespread in the Klamath region as they are in the ot w e K math t ey
Sierra Nevada. Both the Sierra Nevada and the Klamath
Ne
h Sie
K
Mountains experienced glaciation during the Pleistocene aciati duri ice ages, though the lower elevations in the Klamath region e vation resulted in smaller and less extensive glaciers. The Klamath res r ext nsive
Mountains region also experienced gold mineralization
Mo
gion experi during the late Mesozoic Era, though none of the Klamath ate M ozoic gold districts were as rich as the Mother Lode of the Sierra ts wer ch a
Nevada. Despite these similarities, the geologic story of the da. De ite th
Klamath Mountains is not exactly the same as the Sierra math Mou ains
Nevada. In the following sections, we focus on the unique vada. f attributes of the Klamath Mountains with respect to the attri tes similar Sierra Nevada. simila Klamath Mountain Terranes and Ophiolites: In generKlam al, the Klamath Mountains region consists of numerous oceanic terranes representing fragments of crustal material that were embedded into the western margin of North th America since Early Paleozoic time. The fragments include metamorphosed volcanic and sedimentary rocks that represent volcanic island arcs, submarine plateaus, reeflike bodies of limestone, and deep ocean sediments that were intensely deformed during accretion. It has taken geologists many years to unravel the complicated tectonic pattern in the Klamath Mountains, but it now appears that as many as a dozen different accreted terranes exist in the region. Some of these terranes no doubt represent the same fragments recognized in the western metamorphic belt of the Sierra Nevada. In general, the Klamath terranes become younger from east to west, a pattern that probably reflects the westward growth of North America through a series of plate collisions along the western edge of the continent. Some of the Klamath terranes may have originated thousands of kilometers from North America, whereas others were probably of local origin. Fossils discovered recently in the eastern Klamath terranes suggest that rocks as old as Late Precambrian, nearly 600 million years old, are included in the oldest accreted fragments.

Northern California.3 The Klamath Mountains

younger, about 162 million years old, and is exposed in the northwestern part of the Klamath region. The Josephine ophiolite may be in part equivalent to similar rocks in the western metamorphic belt of the Sierra Nevada. Neither the ophio Josephine nor the Trinity ophiolite sequences are preserved lamath Mou in their entirety in the Klamath Mountains; they were disrupted and metamorphosed during accretion. Nonetheless, t o tes Kla there is little doubt that ophiolites of the Klamath Mountains ents accrete eanic lit represent fragments of accreted oceanic lithosphere. tonic th am Mesozoic Plutonic Rocks of the Klamath Mountains: The on zon
Mesozoic Farallon subduction zone that generated great volagma
Sie
umes of felsic magma in the Sierra Nevada region extends th math region north to the Klamath region. However, the magma that ed Klam intruded the Klamath basement was emplaced mostly as isoutons relativ ize lated plutons of relatively limited size and did not coalesce in er ma
Si
Sier into larger masses similar to the Sierra Nevada batholith. er vely lig The relatively small exposures of light-colored granitic rocks n K mat in the Klamath Mountains contrast with the darker metaro s morphic rock of the terranes to create spectacular landforms
C
(Fig re
Th
such as Castle Crags (Figure NC.18). The Mesozoic-age ath ange plutons of the Klamath region range in age from about 136 lion end million to 174 million years, and tend to be somewhat more nin ferrom i mafic (containing more ferromagnesian silicate minerals) tic roc comp than the granitic rocks comprising the Sierra Nevada. hrom Gold and Chromite Mineralization: The geologic similarity between the Klamath Mountains and the arity betwe th Mo her regio Mother Lode region of the Sierra Nevada did not escape the notice of prospectors and miners in the mid-1800s. tice o
Gol
disc
Gold was discovered in placer gravels along the Klamath nd T and Trinity rivers soon after the Gold Rush began.
Additional gold deposits were eventually located in
A ition anci ancient river sediments and in the metamorphic basement rocks. Ultimately, the Klamath Mountains became the rock second largest gold producing region in California. s During and just after World War II, deposits of chromite were extensively mined in the Klamath region. Chromite is an iron chromium oxide mineral (FeCr2O4) that is the only source of chromium, a valuable metal used in the manufacture

Figure NC.17 Ophiolites are sequences of rock that represent oceanic lithosphere emplaced on land at convergent plate boundaries.

ᮣ

Deep-sea sediments Pillow lavas Oceanic crust Sheeted dikes Massive gabbro Layered gabbro Peridotite

Of particular interest in the Klamath region are extenKlam n know sive exposures of mafic and ultramafic rocks known as igne ophiolites, sequences of igneous rocks thought by geologists to represent disrupted oceanic lithosphere. Ophiolites pted o ithosphere. or mant te consist, in ascending order, of upper mantle peridotite overlain by layered and massive gabbro, sheeted basalt ga asalt dikes, and basaltic pillow lavas (Figure NC.17). Ophiolite low (Figur iolite aced te con rsequences can be emplaced on land during plate convergence simultaneously with the accretion process. As relaac ess. re ively tively thin and dense oceanic plates sink into subduction zones, fragments of the descending lithosphere are somefr ts th g lith e times incorporated into the accretionary mass. The heat, incor ated int nary m he pressure, and chemically active fluids that accompany plate a hemical at accompan convergence usually result in significant deformation, disvergen sually re eformat ruption, and metamorphism of the ophiolite sequences. In ru etamor many exposures of the Klamath Mountains ophiolites, the ma Klam mafic igneous rocks have altered into the metamorphic rock s alte serpentinite, but careful studies can still reveal their origin stud as components of oceanic lithosphere. We will learn more co about sepentinite in our examination of the northern Coast abou ntinit
Ranges, where they are also abundant.
Rang
In the Klamath Mountains region, the Trinity and
Josephine ophiolites are two of the largest and best known ioli rock sequences of their type in the world. These two rock assemblages represent slivers of oceanic lithosphere that were incorporated into the accreted terranes at different times and different places in the Klamath region. The Trinity ophiolite is part of the Paleozoic–Early Mesozoic Eastern
Klamath-Yreka superterrane, which is very similar to the
Northern Sierra terrane of roughly the same age to the south. Most geologists feel that the Trinity and the Northern
Sierra terranes were originally part of the same accreted oceanic plate. The mid-Mesozoic Josephine ophiolite is

Figure NC.18 Castle Crags is an exposure of 167-million-year old granite in the Klamath Mountains.

ᮣ

Frank DeCourten

Upper mantle 13

14

Geology of Northern California

of specialized steel alloys and in corrosion-resistant steel plating. Chromite is very rare in crustal rocks, but comprises a significant fraction of the upper mantle material. In the
Klamath Mountains region, the mantle slices preserved in the Trinity and Josephine ophiolites contained numerous pods of chromite-bearing rock that were rich enough to be mined. Much of the ore was depleted after the 1960s, and production of chromite from the Klamath Mountains has since declined dramatically. For a time, however, the
Klamath Mountains was the primary source for this valuable strategic metal.

Section Northern California.3 Summary
● The Klamath Mountains province is a mountainous

upland deeply incised by the Klamath River and its tributaries. Several distinct mountain ranges comprise the region, with elevations up to 2,750 meters (9,000 feet).
In contrast to the strong northwest orientation of
California’s major landforms, the ranges in the Klamath
Mountains province have a weak north-to-south alignment or a curved trend.
● The bedrock of the Klamath Mountains province has

strong similarities with the Sierra Nevada to the south, o nuati and can be considered as a northwest continuation of the geologic patterns of the later. In both areas, metamoreas, metamo rrane ntrude phic bedrock comprising multiple terranes is intruded by
Mesozoic granite plutons and overlain by Cenozoic-age rlain zoic-age sedimentary and volcanic rocks. The rocks comprising
Th
rising some of the Klamath Mountains terranes are almost ns identical in type, age, and origin to those of the northern o no
Sierra Nevada. At least a dozen terranes have been idenranes be tified in the Klamath Mountains, some of which can be f assembled into larger superterranes. nto la

(50 miles) wide. The northern portion of the Great Valley is known as the Sacramento Valley, and the southern two thirds is designated as the San Joaquin Valley. These names originate from the two large rivers systems that drain the ms interior basin from the north and south, respectively. These
, resp rivers meet southwest of Sacramento in the Delta region
Del
and eventually drain into the San Francisco Bay. co The Sacramento Valley is a remarkably flat interior basin remark lat b surrounded by elevated terrain to the west (Coastal errain th
(C
Ranges), north (Klamath Mountains and Cascade Range), ountains C de and east (the Sierra Nevada). The Sacramento River system receives runoff from these adjacent highlands via several highlan includ e major tributaries including the Feather, Yuba, American,
Pit, and Bear Rivers (Figure NC.19). Because northern ers California receives significantly more rain than southern m an s
California, the Sacramento River is California’s largest ia, th amento rni nia’s river, carrying about 18 million acre-feet of water, six times abo 8m f wat more than the San Joaquin River, toward the sea annually.
Jo uin d This water is a critically important resource for agriculture, criticall rce ag industry, commerce, and domestic use statewide. in ry, a s tewide.
Soils and Agriculture in the Great Valley: The surface
So
Agricu
Va y: surfa of the Sacramento Valley is covered by recent and f Vall
Pleistocene-age alluvium washed into the bottomlands by
Pleistocen
wa ed b l streams draining the adjacent highlands. These stream seddra ent hig ands iments consist of a heterogeneous assemblage of channel hetero neous gravels, river bank sands, silt, and clay deposited on the gra nk silt broad floodplain, and in some places, peat deposits reprebro ain p senting plant litter that accumulated in lakes and wetlands. nt litt hat ac
The rivers draining the Sacramento Valley typically follow drain e Sa meandering courses that shift continuously across the dering course
ᮣ

Figure NC.19 The drainage basin of the Sacramento River. ure T

● Ophiolites, representing fragments of oceanic lithoolites, represen fragm nic lit

re ter amath sphere, are prominent in the terranes of the Klamath gion. Th sephine ites
Mountains region. The Josephine and Trinity ophiolites m neous ro are dominated by mafic igneous rocks commonly altered eenstone to greenstone and serpentinite.
● Gold and chromite have been mined from the ld m

Northern California.4
The Great Valley
A Great Depression: Nestled between the Sierra Nevada on the east and coastal mountains to the west, the Great Valley of California is a vast elongated basin extending nearly 700 kilometers (430 miles) and averaging about 80 kilometers

Map courtesy of the U.S. Geological Survey.

pr
Klamath Mountains region. The gold probably originatanner
Mothe
ed in a manner similar to the Mother Lode deposits of
Neva
ch the Sierra Nevada. The chromite deposits are concenm pre trated in the mantle rocks present within the Klamath
Mountains ophiolite sequences. ophi equences.

Northern California.4 The Great Valley

nearly flat floodplain. Over time, the migrating rivers have deposited thick sequences of interlayered river sediments.
These alluvial deposits are generally several hundred meters thick, but can be much thicker in the lower areas on the west side of the valley.
The soils developed on the alluvium of the Great Valley are among the most fertile in the world. These soils, coupled with abundant water, mild climate, and a lengthy growing season have made the Great Valley one of the most agriculturally productive regions in North America. The annual value of California agricultural products exceeds $32 billion and most of the products are grown in the Great Valley. In the Sacramento Valley, the principal crops are rice, almonds, walnuts, orchard fruits, grapes, and feed grains. Even before cultivation, the fertility of the Great Valley was noted by early explorers who reported that the Sacramento River flowed through an incredibly lush grassland, with large ge stands of oaks trees and numerous wetlands.
Soil fertility is the consequence of several inter-related ted factors, including the texture, chemistry, micro flora and flo nd fauna, moisture, and organic content of the soil. However,
Howe
some vital nutrients can be obtained by plants only from y chemicals released during the weathering of mineral grains g in the soil. These mineral nutrients include potassium, clude magnesium, and calcium. The alluvium of the Sacramento uviu e Sac
Valley is naturally rich in minerals such as feldspar and mica als ar that release these nutrients as they undergo chemical un che weathering. These mineral grains, in turn, originated priral g orig marily in the plutonic rocks and associated metamorphic c rock sociated m terranes of the Sierra Nevada and the Klamath Mountains.
Ne
Klama
s.
The agricultural productivity of the Sacramento Valley soils uctivity Sa reflects, in part, the vast exposures of these rocks in the o n upper reaches of the Sacramento River watershed. ramento Riv
d.
Deeper Structure of the Great Valley: Beneath the alluG ath al vium and soils at the surface, the bedrock of the Great f Valley is comprised of a thick sequence of mostly Mesozoic c ised nce o y and Cenozoic sedimentary rocks that are downfolded in a
Cenozo edimen downfold d i d great asymmetrical syn asym ical syncline (Figure NC.20). These strata
C.20).
are called the Great Valley Sequence, an enormous mass of alled Va ormou oceanic sediments exceeding 6,000 meters in maximum oc ents exc
Figure NC.20 Generalized diagram of the subsurface structure of dia Great Valley, Coast Ranges, and Sierra Nevada. the G t a

Diagram courtesy of the U.S. Geological Survey.

ᮣ

15

thickness. Because they are folded downward to depths of
15,000 feet or more, the strata of the Great Valley
Sequence are only exposed at the surface in the foothills of the highlands on either side of the valley. To the east, along f th ills, t the Sierra Nevada foothills, the rock layers dip gently toward the center of the valley, although they are steeply alth upturned to the west, where they are exposed in the here foothills of the Coast Ranges (Figure NC.21). oast Ra
Figure NC.2
The Great Valley Sequence is comprised mostly of sandalley Sequenc compris sed t stone and shale representing sediment that accumulated in a deep forearc basin that developed in the Mesozoic Era. This sin basin was located between the offshore trench and the ancesof tral Sierra volcanic arc above the Farallon subduction zone ierra (Figure NC. 22). Much of the sediment eroded from the vole
M
canic and metamorphic rocks in the Klamath and Sierra nd metamorph
Nevada regions was transported to the deep seafloor by
N
region o turbidity currents. These great undersea landslides probabid curr eat un bly were triggered by volcanic eruptions, earthquakes, or rigg anic erup storm events along the ancient continental margin. Repeated even nt deposition of sediment by turbidity currents leads to the osition nt urbidity curren development of submarine fans on the seafloor, and geolodevelop marine fa seaf gists recognize sediment of this type in many portions of the niz diment t e man
Great Valley Sequence. Some of the sediments in the Great
Sequ nce.
Valley Sequence are rich in organic matter, primarily the ce ich i remains of microorganisms that continually rained down to s mic organ the seafloor from the shallow waters above. Over time, this eafloor e sh organic matter has become transformed in the subsurface to rganic the combustible hydrocarbons that comprise natural gas. mbusti Where these gases are trapped in the porous subsurface sandWh se g stones, productive gas wells can be developed. From 1977 to ones, 1998, the annual production of natural gas in the Sacramento
19 8, th
Valley region averaged about 100 trillion cubic feet. The
Valle
majority of this gas originates in the Great Valley Sequence. maj Above and Below the Great Valley Sequence: The sediments of the Great Valley Sequence were deposited on top of oceanic lithosphere that can still be recognized by geophysical techniques. The deeply buried oceanic crust in the Great
Valley is 7 to 8 kilometers thick and the moho beneath is 11 to 16 kilometers below the surface. Surprisingly, this relatively dense oceanic basement appears to be perched above the western extension of the Sierra Nevada batholith. The presence of a thick slab of dense oceanic rock deep beneath the
Great Valley may act like a gravitational anchor, helping to explain why this region has remained so low with respect to the surrounding terrain.
The forearc basin in which the Great Valley Sequence accumulated persisted into the early part of the Cenozoic
Era. By about 25 million years ago, the rate of subsidence decreased and the basin began to fill in the north. Starting in
Late Cenozoic time, the oceans gradually drained away from the Great Valley, and all of the sediments washed into the lowland accumulated in terrestrial environments such as river beds, lakes, and swamps. As we will soon see, volcanoes in the Sierra Nevada and Cascade Range became active at roughly this same time, and some of the erupted material flowed or fell into the Great Valley. Consequently, along the modern periphery of the Great Valley, the Late Cenozoic

16

Figure NC.21 Submarine fan deposits of the Great Valley Sequence exposed in the foothills of the Coast Ranges dipping steeply to the east (left).

Frank DeCourten

ᮣ

Geology of Northern California

Fi re NC.2
Figure NC.23 The Sutter Buttes are the only prominent mountains tain in the Great Valley of northern California.
ᮣ

Figure NC.22 The Mesozoic forearc basin in which sediments of the Great Valley Sequence accumulated was located between the ancestral Sierra Nevada volcanic arc and oceanic trench related to a Neva ate the Farallon subduction xone. x ᮣ

Volcanic chain n De

sc

en

Continental crust of
North American plate

di

ng

Fa

ra

llo

n

pl

at

e

rocks are characterized by interlayered volcanic and terrestrial sedimentary rocks.
The Sutter Buttes: About 12 kilometers northwest of
Marysville stand the Sutter Buttes, a circular cluster of ragged volcanic peaks, about 17 kilometers (10 miles) in diameter, that rise to an elevation of more than 650 meters
(2,100 feet). These peaks are such a striking feature in the

Frank DeCourten

Oceanic crust c Great Valley sequence alley seque in forearc basin orearc Diagram courtesy of the U.S. Geological Survey,

Franciscan subduction c n complex

monotonously flat interior of the Great Valley that they have attracted attention ever since humans first arrived in northern California (Figure NC.23). The core of the Sutter
Buttes consists of several rhyolite domes emplaced as shallow intrusions from 1.6 million to 1.3 million years ago.
Surrounding this core is a circular zone of slightly older pyroclastic material of andesitic composition. Geologists

Northern California.4 The Great Valley

17

Figure NC.24 Flowing on a low gradient with an extensive flood plain, the Sacramento River is subject to seasonal flooding that places millions of people at risk.

James S. Monroe

ᮣ

Figure NC.25 Flooding in the Sacramento Valley, 2005–2006.
Photo on the left was taken from space on Dec. 10, 2005 immediately prior to heavy late winter rains. Photo on the left is from Jan. 4,
2006. Note that the Sutter Buttes were entirely circled by flood waters from the Sacramento River.

ᮣ

Photos from NASA Earth Observatory.

generally consider the Sutter Buttes to be the southernmost st expression of volcanic activity related to the Cascade Range ivity relate scade Range to the north. However, development of the Sutter Buttes utter Bu may also have been influenced by the migration of the al y th ation Mendocino triple junction. We will review both of these ple junc l revi h thes aspects of northern California geology in later sections. n hern Cal section Flooding n
Floodin in the Sacramento Valley: Because it is a
S
nearly flat surface drained by California’s largest river fl face dr lar (Figure NC.24), the Sacramento Valley is prone to recur(F ring rin floods (Figure NC.25). The annual flood cycle generally peaks in late winter and early spring as heavy winter e rains give way to the melting of the snow pack in the bordering derin mountains. Before people settled there, the floodunt
Bef
plain adjacent to the Sacramento River and its tributaries cent was repeatedly inundated in a cyclic pattern. The annual atedly inun floods renewed the floodplain soils and scoured the channel ed of accumulated sediment and debris.
Today, the intense agricultural and residential development in the Sacramento Valley has resulted in a serious flood hazard for millions of people. Billions of dollars have been invested in flood control measures that include large dams in the foothills, miles of levees, and river bypass channels. Even with these measures, the threat of seasonal floods has not been eliminated from the Sacramento Valley. Attempts to reduce flooding along the Sacramento River have also diminished the beneficial effects of this natural process.

18

Geology of Northern California

Section Northern California.4 Summary
● The Great Valley is an elongated lowland situated

between the Sierra Nevada on the east and the Coast
Ranges on the west. The northern portion of the Great
Valley is referred to as the Sacramento Valley, named for the large south-flowing river system that drains it. The watershed for the Sacramento River includes the highlands of the northern Sierra Nevada, Cascade Range,
Klamath Mountains, and the northern Coast Ranges.
Because these mountainous regions receive more precipitation than other regions of the state, the Sacramento is
California’s largest river.
● Great quantities of alluvium have been deposited across the floor of the Great Valley by rivers draining the adjacent highlands. The fertility of the soils of the
Great Valley reflects, in part, the concentration of vital mineral nutrients derived from the weathering of rocks in the adjacent watersheds. The natural fertility of the
Great Valley soils, coupled with the prevailing climate and abundance of water, has made the Great Valley one of the most productive agricultural regions in the n world.
● In the subsurface of the Great Valley, Mesozoic and
Mesoz

Vall
Cenozoic sedimentary rocks of the Great Valley rge syncli
Sequence are downfolded in the form of a large syncline. nts a ulated The Great Valley Sequence sediments accumulated in a h ubducdeep forearc basin associated with the Farallon subduction zone. Most of the sandstone and shale in these rocks ock n turbidi was transported to the deep ocean floor by turbidity curne
Organ
rents and accumulated as submarine fans. Organic matter in parts of the Great Valley Sequence has produced uence significant quantities of natural gas.

Northern California.5
The Northern Coast Ranges
The rugged north coast of California is one of the most sceo aces nic seashores in the world. In most places along this rocky coast, beaches are narrow and sea cliffs rise from the water’s t edge to elevations of several thousand meters in just a few ousan ters mples tal landf kilometers. Spectacular examples of coastal landforms such as sea stacks, sea arches, and marine terraces rch nd m e t
(Figure NC.26) provide evidence of recent emergence of the ence em he erosio northern California Coast. The vigorous erosion that accompanies such an actively rising coast is responsible for the tivel resp spectacular ruggedness of the Northern California seaside. ness The relatively young coastal mountains of California oung m
C
define the Coast Ranges physiographic province that extends e Co ges physiograp e th more than 600 kilometers (960 miles) from the Transverse han 6 ters th T
Ranges in the south to the Oregon border, and beyond. es o th rder, an
Within this region, coastal mountains such as the Santa Cruz in coas l h S
Mountains, the Mendocino Range, the Gabilan Range, and
M ntains,
Mendo
a the Diablo Ranges of northern California are all aligned in a blo alifornia i consistent northwest trend. The Coast Ranges are generally onsist th end he
Rang
gener oriented parallel to the Great Valley and Sierra Nevada e Gr rra N provinces to the east, and have a similar extent. This suggests t milar e that the geologic evolution of these regions may be linked to lution f gio Figure NC.26 Steep cliffs, sea stacks, and sea arches charac6 teep sta terize northern California’s emergent coast.
Cali
a’s em

ᮣ

● The Great Valley Sequence accumulated on dense
Valle
uence d th de e Gre oceanic lithosphere that lies deep under the Great s ock th ed Valley. This dense slab of rock is thought to be related to the low elevation of the region in relation to the suron i urg hig rounding highlands.
● The Sutter Buttes are a distinctive cluster of volcanic e ive c

domes that erupted in the Sacramento Valley 1.6 million s tha acramento V tha to 1.3 million years ago. The magma that surfaced in this location may be related to the Cascade Range volcanoes dt Casc th t migrat to the north or to the migration of the Mendocino triple junction along the coast. long t oast. largest river, the Sacramento Valley is naturally prone to cyclic flooding. Over geologic time, the recurring floods that inundated the floodplain have added alluvium to the valley surface and renewed soil components. Flood control measures, such as levees and upstream dams, are necessary to protect floodplain communities and industries, but they also limit the beneficial effects of the annual flood cycles.

Sue Monroe

● Because it is a nearly flat valley drained by California’s at va

Northern California.5 The Northern Coast Ranges

Figure NC.27 Chert of the Franciscan complex exposed in
Marin County. Most of the layers are 5 cm thick.

ᮣ

Sue Monroe e Mon

some of the same tectonic events. However, the recent emergence of the Coast Ranges was strongly affected by the evolution of the San Andreas fault system and the development of the modern transform boundary between the Pacific and
North American plates. Thus, though the older bedrock of the Coast Ranges can be related to broader tectonic patterns, the story of their more recent emergence involves plate tectonic events that affected only the coastal region.
The California Coast Ranges have traditionally been divided into northern and southern portions, with San
Francisco Bay arbitrarily dividing the two. However, for sound geologic reasons, it is more meaningful to divide the
Coast Ranges into northern and southern portions separated by the San Andreas fault system. This great fault zone slices diagonally across the Coast Range, and the areas on opposite sides have unique and distinctive geologic histories. By this definition, the northern Coast Ranges extend northward ard from approximately Monterey Bay and the Gabilan Range to e the Oregon border. It is this part of the larger Coast Ranges ges province that we will refer to as the North Coast Ranges, and
Rang
nd on which we will focus in the following sections.
The Chaotic Franciscan Complex: In most of the th Northern Coast Ranges, the basement rock consist of an incredibly complex assemblage of intensely deformed and ly metamorphosed sandstone, shale, chert, basalt, and plutonic cher t, rocks. In many exposures of these rocks, layering and other ese l g internal structures have been obliterated by internal movebliterated b al m ments such that some geologists have described it as a geolo describ
“churned” rock assemblage. In the early 1900s, the name blage 1900s
Franciscan was applied to these mangled rocks for their ed led r eir prevalence in the San Francisco Bay region. Since then, the re ,
Franciscan rocks have become one of the most famous and s well studied rock units in the world. For decades the comn s mplexities of the Franciscan assemblage defied geologic ed geolo explanation. With the advent of the plate tectonic paradigm expla nic in the 1970s, geologists began to unscramble the origin of
19
eologist nscra he o this chaotic rock sequence. Today, there is general agreeck seque ere general ment that the Franciscan rocks represent a subduction
Franci
sent subd complex, a mixture of rocks that form in association with plex, xture associat subducting oceanic lithosphere. su anic lith
The age of rocks comprising the Franciscan complex omp ranges from about 200 million to 80 million years. Recall t milli that about 140 million years ago, the Farallon plate began to illion a subduct beneath western North America, generating the subd magma that ultimately formed the granite batholith of the magm at ulti
Sierra Nevada (see Figure NC.22).Offshore of that ancient
Sierr
evada
F
continental margin, a deep trench developed on the oceanic cont l floor. In this trench, sand and rock fragments were washed from the continent and accumulated in great thickness. Such sediments comprise graywacke, an impure sandstone that is the most common rock type in the Franciscan assemblage.
The Franciscan graywacke deposits commonly exhibit graded bedding, suggesting that turbidity currents transported them into the deep basin. Other deep ocean sedimentary rocks in the Franciscan complex also include fine-grained shale and chert (Figure NC.27). In addition to the sedimentary components of the Franciscan rocks, mafic igneous

19

rocks such as pillow basalt, gabbro, and peridotite are press peri ent. The igneous materials probably comprised portions of
Th
aterials pr ably compr the Farallon plate that originated at ancient spreading ridges, np as submarine lava flows, or in the upper part of the mantle. ows, th
As the Farallon plate collided with North America, and was n plat ollide forced downward, the igneous rocks were subjected to high downwa , temperatures, extreme pressures, and chemically active fluids peratures, extr me p including hot seawater. Alteration of the iron and magnencl ding seaw sium-bearing silicates in the igneous components of the sium earing
Farallon plate produced such minerals as chlorite, pumpelFara late lyite, antigorite, chrysotile (asbestos), and lizardite. These yite, a greenish-colored minerals are abundant in greenstone and g nish serpentinite (Figure NC.28), both of which are common in serp the Franciscan complex. Serpentinite plays an important role in creating the “churned” appearance of the Franciscan complex. It is significantly less dense than the igneous rocks from which it develops. Under the temperature and pressure conditions that prevail in subduction zones, serpentinite can become mobile and plastic, squeezing upward into the overlying rocks to disrupt the original layering.
The internal structure of the Franciscan rocks is so complex that it bewildered geologists for decades. The original layers of the sedimentary rocks are highly deformed (Figure NC.29) or even completely obliterated by serpentinite intrusion and tectonic deformation. The igneous rocks are usually metamorphosed, fragmented, and broken into isolated blocks. The entire rock assemblage is further complicated my numerous faults and shear zones that separate the sequences into discontinuous blocks. Such a mixture of deformed, jumbled, and altered rocks is known as a mélange, a French term for a mixture of different components. The origin of these rocks was a mystery to geologists until the plate tectonic theory provided a new model for interpreting the complexity of the Franciscan assemblage.
The Franciscan rocks, as a whole, are now thought to be a subduction complex, a heterogeneous mass of deformed and altered rocks that forms in and near subduction zones.
The sedimentary components of the Franciscan complex

20

Figure NC.28 Serpentinite, like this sample from the northern Coast Ranges, is California’s official State Rock.

Frank DeCourten

ᮣ

Geology of Northern California

represent sand, silt, and ooze that accumulated in the trench mulated or on the Farallon plate as it approached North America
Mesozo
men during the Mesozoic Era. Some of these sediments were scraped off the plate as it descended beneath the leading ff descend he lea rth a deforme se pre edge of North America and deformed by the immense pressure generated in the zone of plate convergence. Some sedizo f c mentary rocks were also pulled downward by the descending al ed down ng Farallon plate to depths of 15 kilometers or so beneath the n t surface. In this high-pressure and low-temperature environ. h low-tem ment, blueschist was produced, a metamorphic rock rich in blue d, metamo the bluish mineral glaucophane. The igneous components ne. ign of the Farallon plate descended to even greater depths and lon pl sce e encountered higher temperature conditions where the serd high mperatu pentintite and greesnstone formed. Slices of rock were g nstone form sometimes broken from the descending slab and thrust over he des or crushed against the edge of the continent. In addition, the
Farallon plate was evidently carrying isolated seamounts, lava plateaus, and coral reefs that became detached and embedded into the Franciscan mélange. Geologists currently recognize no fewer than nine such micro-terranes in the
Franciscan Complex in the San Francisco Bay region. The chaotic nature of the Franciscan complex thus reflects the diversity of rocks types, the varying degrees of metamorphism, the accretion of exotic terranes, and the intensity of deformation that result from plate subduction.

The subduction of the Farallon plate persisted under the
Th subduc northern Coast Ranges for more than 130 million years, north C extendin extending well into the Cenozoic Era. During this time, a thick wedge of Franciscan rocks developed along the leadw in ing edge of North America, but the coastal mountain ranges had not been elevated from the seafloor. About r 30 million years ago, the plate boundary along the west coast began to shift from a convergent type to a transform boundary. The emergence of the modern Coast Ranges is linked to development of the modern transform boundary between the Pacific and North American plates.
Evolution of the San Andreas System: Recall that the
Farallon plate originated at an offshore spreading ridge that separated it from the Pacific plate moving to the west (see
Figure NC.3). Segments of this spreading ridge were offset by transform faults to create an irregular divergent boundary. Through the Mesozoic and Early Cenozoic Eras, the
North American plate was moving faster to the west than was the Farallon plate to the east. Consequently, the west edge of North America eventually collided with the spreading ridge as the intervening Farallon plate was consumed by subduction. This collision first occurred at the approximate latitude of modern Los Angeles about 30 million years ago
(Figure NC.30). One consequence of this collision was the creation of a point where three plates—the Farallon, Pacific,

Northern California.5 The Northern Coast Ranges

Figure NC.29 Deformed chert in the Marin Headlands terrane of the Franciscan complex.

21

Figure NC.30 Evolution of the San Andreas fault system as a transform boundary between the Pacific and North America Plates.
The Juan de Fuca Plate north of the Mendocine Triple Junction is a remnant of the Farallon plate.

ᮣ

Diagram courtesy of the U.S. Geological Survey. tesy o

ᮣ

Figur
Figure NC.31 Major Fault of California, including the San r rnia,
Andreas
Andre and related stike-slip faults that represent the boundary t between the Pacific and North American plates. e Pa
America

and North American—met. Such points are called triple erican— oints lled tripl junctions, and they signify places where different kinds of sig re k ate boun s plate boundaries exist in proximity to each other.
After the initial triple junction developed, the North r nitial tri ed, t
American plate continued to overrun more of the spreading
Am
rid ridge. This caused the original triple junction between the e origi
Farallon, Pacific, and North American plates to separate into two (see Figure NC.30). The continued westward gure NC.3 motion of the North American plate caused the two triple moti hA junctions to migrate in opposite directions along the contijunct migra nental margin. The northern triple junction migrated nent Th through the San Francisco Bay area about 10 million years
F
ago. Today, the Mendocino triple junction is located near
Cape Mendocino, more than 200 kilometers (130 miles) north of San Francisco. The corresponding point to the south is the Rivera triple junction, located near the southern tip of Baja California.
The collision between the spreading ridge and the North
American continent brought the Pacific plate into contact with the North America plate. Because both plates had westerly motions, this new boundary ended the long era of plate convergence and subduction. Instead, differences in the rates

Map courtesy of the U.S. Geological Survey.

James S. Monroe

ᮣ

and directions of plate motion gave rise to a transform plate boundary, in which the two plates slide laterally past each other. The development of the transform boundary between the North America and Pacific plate initiated the famous
San Andreas fault system (Figure NC.31). The San Andreas fault is perhaps the best known fault in the world, but it is

22

Geology of Northern California

only one of hundreds of faults in the zone that bears the same name. Collectively, these faults accommodate the relative motion between the North American and Pacific plates through right-lateral strike-slip displacement (Figure NC.32). It is important to understand that the San

Figure NC.33 Major faults in the San Francisco Bay area. The blue arrows represent their average displacement rates.

ᮣ

123°

122°

A

0

30 Kilometers

Ro ers dg

Figure NC.32 The right-lateral displacement on the San
Andreas Fault is illustrated by the offet of this stream channel in the southern Coast Ranges. View is to the west from the North
American plate (foreground) to the Pacific plate (distance).

ᮣ

ee
Cr
au

kf

Gre Valley fault
Green

lt

38°

Gre

Francisco

d ar John S. Shelton

lt

au

sf

era

lt

au

sf

ea dr An

Vector scale—In millimeters imete per year

ult u fa

n

20

v a a a ala
Ca

yw
Ha

Sa

lt ult io fa g gor

0

Displacement-rate vector— ment-rate vec
Showing direction and ng amount of relative movement nt moveme

Active fault—Dotted where w concealed

37°

Map courtesy of the U.S. Geological Survey

San

PACIFIC
OCEAN

Figure NC.34 Granite of the Salinian Block with dark-colored xenoliths at Point Reyes National Seashore. This granite is very similar to the pluark-co s Po ashore. nite tonic core of the Sierra Nevada,

Dick Hilton

ᮣ

Northern California.5 The Northern Coast Ranges

Andreas fault system is actually a zone of dominantly strikeslip faults that ranges from 100 kilometers to nearly 300 kilometers wide. The collective motion of these faults allows the
Pacific plate to slip to the northwest relative to the North
American plate at an average rate of about 5 centimeters
(2 inches) per year. This may sound very small, but it is enough to displace older geologic features by more than 300 kilometers (185 miles) since the transform boundary originated.
In the San Francisco Bay area, the San Andreas fault is one of several large right-lateral faults that comprise the plate boundary (Figure NC.33). Other major faults in this region include the Calaveras, Hayward, and San Gregorio faults that are nearly, but not exactly, parallel to the plate boundary. The San Andreas fault extends offshore from
Point Arena to the Mendocino triple junction. The San
Andreas fault zone produces continuous earthquakes, some of which pose serious threats to California residents. We will examine the pattern of seismic activity along the San
Andreas in a later section.
The Salinian Block—An Immigrant from the South: In e Sou the northern Coast Ranges, rocks of the Franciscan complex n comp are absent west of the San Andreas fault. In places such as uch a
Point Reyes National Seashore, the basement rocks consist ment of Mesozoic-age plutonic and metamorphic rocks overlain ic by Cenozoic-age sediments (Figure NC.34). These rocks ure N
The
comprise a block that can be traced south from Point Arena aced A to the southern Coast Ranges. Because such materials are su ria especially prominent around the Salinas Valley, geologists und refer to this mass as the Salinian block (Figure NC.35). e Salin k N
Figure NC.35 Right lateral displacement along the San Andreas ateral a as Fault has moved the Salinian Block more than 500 kilometers to the an h northwest. ᮣ

Fault–Dotted where concealed: arrows indicate direction of ent relative movement C. Ca
Calaveras;
H
H. Hayward; S. Sargent
Val
ence
Great Valley sequence and ra lite
Crust range ophiolite at base
Parkfield
window

Cholamc

Andre

Franciscan assemblage

as fa ult New IDRIA window LL

EY

San

HEC
HEC
EC
PACHECO
ndo ndo d do pass window

Upper Cretaceous and
Tertiary strata

S

OC

C
H

RE

AT

M
Mount Diablo window EA

VA

N

Salin
Salinian block

IC
CIF
PA

Diagram courtesy of the U.S. Geological Survey.

G

San Francisco

0

100

Kilometers
(approximate)

?

N

23

The granites of the Salinian block are 78 million to 110 million years old, and are remarkably similar in age and composition to the granites of the southern Sierra Nevada.
In addition, metamorphic rocks associated with the granite cks basement of the Salinian block are almost identical to the roof pendants of the Sierra Nevada. These similarities sugra
T
gest that the Salinian block was part of the Sierra Nevada th he dev ent S batholith before the development of the San Andreas fault system. The Salinian block has since been transported a linian b minimum of 550 kilometers (340 miles) to the northwest by
0
right-lateral displacement along this fault. placement t
The Rise of the Northern Coast Ranges: Overlying the he C basement rocks of both the Salinian block and the ment t
Franciscan complex are as much as 6,000 meters (nearly scan comple
20,000 feet) of Cenozoic-age sedimentary rock that
Cen
sedi records the emergence of the modern Coast Ranges. This r he em ode ern sedimentary record is complicated by motion along the im ry r icate San Andreas system, but it is clear that there were several
And as s t different basins where oceanic and terrestrial sediments anic accumulated. The older deposits in these basins are mostly umula der de osits ba mud, ooze, and sand that settled out on the continental o nd se ed shelf. Some sequences, such as the Monterey Formation me ences, he Mon
(Miocene Epoch; 11 million to 22 million years old), accuoch 1 mulated in deep water, where organic material became eep w er, w concentrated in the mud and ooze (Figure NC.36). The trated i m Monterey Formation is believed to be the source rock of terey Forma n i much of California’s petroleum. much California
By Late Pliocene time, 3 million to 4 million years ago,
B ate P the coastal basins were receiving mostly terrestrial sedil ba ments, indicating that the northern Coast Ranges were rising from the seafloor during this time. However, the in complex association of Late Cenozoic river, estuary, and com beach deposits in the northern Coast Ranges indicates that bea the precise timing and rate of uplift was somewhat variable. t Accompanying the uplift of the modern northern Coast
Ranges was the eruption of volcanic ash and lava in the areas around Sonoma (3 million to 8 million years ago) and
Clear Lake (2 million to 10,000 years ago). Ash from these eruptions is commonly found within the layers of Late
Cenozoic sedimentary rocks. This relatively recent volcanic activity in the northern Coast Range is probably related to the northward migration of the Mendocino triple junction.
In the Clear Lake volcanic region, shallow residual magma is probably responsible for the steam that has produced as much as 2,000 megawatts of electricity annually at The
Geysers geothermal field.
The forces that raised the north Coast Ranges in Late
Cenozoic time are continuing to elevate the region. The general northwest alignment of mountain ranges and drainages is attributable to the orientation of the strike-slip faults that influenced the Late Cenozoic uplift. Several of the mountain ranges are still being squeezed upward by compression generated by small differences in the alignment of active faults. The coastal mountains of northern
California have only recently emerged as land, and they will grow even higher as the Pacific plate continues to slide past the North American plate.

24

Figure NC.36 Folded layers of the Monterey Formation in the Monterey Peninsula area of the northern Coast Ranges.

Dick Hilton

ᮣ

Geology of Northern California

Section Northern California.5 Summary nN liforn
● The northern Coast Ranges include mountains such e northe

M
C
as the Mendocino Range, the Santa Cruz Mountains, iablo aligne and the Diablo Range that are aligned parallel to the ley an rra d
Great Valley and Sierra Nevada. These are relatively ountain nges, young mountain ranges, and their continuing uplift is related to the emergence of the northern coast. em ence
● The bedrock of the northern Coast Ranges is domith

nated by the Mesozoic-age Franciscan complex, an incredibly varied mixture of sedimentary and metamorphic rocks known as a mélange. The Franciscan complex accumulated in or adjacent to the Farallon subduction, and includes several exotic terranes that originated far from northern California.
● The San Andreas fault zone cuts diagonally across the

northern Coast Ranges and is part of the modern transform boundary between the Pacific and North American

plates. This boundary evolved when North America collided with and overran the Farallon-Pacific plate spreading center, beginning about 30 million years ago.
Right-lateral strike slip motion on the San Andreas and other faults helps to accommodate the northwest motion of the Pacific plate relative to the North American plate.The Salinian block is a large fragment of crust in the northern Coast Ranges that has been transported at least 550 kilometers to the northwest by right-lateral displacement along the San Andreas fault zone.
● The mountains of the northern Coast Ranges are all rel-

atively young, with uplift beginning only 3 million to 4 million years ago. The rise of the Coast Ranges is documented by the shift from marine to terrestrial sediments in Late
Pliocene time. The forces that lifted the Coast Ranges appear to be related to the interaction of blocks separated by various faults within the San Andreas fault system and to the northward migration of the Mendocino triple junction.

Northern California.6 Volcanoes of the Cascade Range and the Modoc Plateau

Northern California.6
Volcanoes of the Cascade Range and the Modoc Plateau
The landscape of northeastern California is dominated by the Cascade Range and the adjacent Modoc Plateau (see
Figure NC.1), which together create a scenic volcanic wonderland in this part of the state. The Cascade Range includes majestic Mount Shasta (4,319 meters/14,161 feet high; Figure NC.37), Mount Lassen (3,188 meters/10,457 feet high), and scores of other smaller volcanoes of all types.
The Modoc Plateau is a high lava plain east of the bordering Cascades averaging about 1,350 meters (4,500 feet) in elevation (Figure NC.38). Many volcanoes are perched on this arid plateau, but the largest of them is the Medicine e Lake volcano (Figure NC.39) near the eastern margin of
Cascade Range. This immense shield volcano rises to an elevation of 2,375 meters (7,795 feet) and covers more than han 2,000 square kilometers (770 square miles).

Diagram courtesy of the U.S. Geological Survey.

ᮣ

25

Both the Modoc Plateau and the Cascade Range in northern California represent only a portion of larger geologic provinces that extend into adjacent states. The hw Cascade Range extends northward from the southern terminus at Mount Lassen for more than 800 kilometers (500 r mor miles) into southern British Columbia. Other prominent tish Columb
Cascade volcanoes include Oregon’s Crater Lake, Mount
Oregon
Baker, and Mount Hood, along with Mount Rainier and t Hoo ng Mo ens Washi
n.
Mount St. Helens of Washington. The Modoc Plateau is likewise continuous with the Columbia Plateau of Oregon, uous Co bi Washington, and Idaho. The Modoc Plateau encompasses d M nearly 26,000 square kilometers; the Columbia Plateau is quare kilometer hly t arger. roughly 20 times larger.
The principle volcanic features of the Cascade Range e v and Modoc Plateau are all relatively young, generally odoc tive resulting from eruptions that occurred during the past r occu cur 3 million years (Figure NC.40). Earlier Cenozoic-age volill year 0). E canic activity did occur in the region, but the structures anic act ty regi built during this time have either been eroded or obscured durin bee beneath the younger lava flows. The most recent activity ath t ows. Figure NC.37 Mt. Shasta is the second highest composite in the Cascade Range and measures about 20 km in diameter at the base.
Casc
ures a k 26

Figure NC.38 Crater Mountain in Lassen County is one of several young shield volcanoes that erupted the basaltic lava in the Modoc Plateau region.

James S. Monroe

ᮣ

Geology of Northern California

Figure NC.39 Map showing location of Mt. Shasta and the
M
Medicine Lake shield volcano, 60 kilometers to the east. ers Figure NC.40 Recent eruptions of the volcanoes of the
40 Re
Cascades Range. Note that Mt. Shasta and Mt. Lassen in California ades Ra have both been active in the past several centuries. ve ve

occurred in the years 1914 to 1917, when Mount Lassen experienced a series of small summit eruptions (Figure NC.41). The recent volcanic activity in the Cascade
Range and Modoc Plateau is related to the plate convergence

Map courtesy of the U.S. Geological Survey Cascade Volcano Observatory.

ᮣ

Map courtesy of the U.S. Geological Survey.

ᮣ

that exists along the margin of North America, north of the
Mendocino triple junction. Here, oceanic plates are descending beneath the continent in the Cascadia subduction zone, in the process generating magma that continues to rise into the Cascade region.
The Cascadia Subduction Zone: The Cascadia subduction zone extends for 1,200 kilometers (750 miles) along the west edge of North America from southern British
Columbia to northern California (Figure NC.42). Along this convergent plate boundary, the Juan de Fuca plate (and

27

Northern California.6 Volcanoes of the Cascade Range and the Modoc Plateau

ᮣ

Figure NC.41 A summit eruption from Mt. Lassen in 1915.

ᮣ Figure NC.42 Plate tectonic setting of the Pacific Northwest.
Subduction of Juan De Fuca plate beneath North America accounts for the volcanism of the Cascade Range.

Cascade Range

A
Juan de Fuca
Ridge

Juan n de Fuca uca Plate

Mount St. Helens
Mount Adams

B

North
American
Plate

Pac
Pacific
Pla
Plate

La
Lassen Peak g Spreading ge Ridge

A

Juan de Fuca Ridge e Fu dge Diagram cour iagram courtesy of the U.S. Geological Survey.
Geolog

P
Pacific
Plate
Pla

its southernmost segment, the Gorda plate) is moving under so rnmost s the advancing continent at a rate of 3 to 4 centimeters per a ncing contin release year. Water released from the sinking slab causes subsurface rock to melt, forming magma bodies beneath the Cascade
Range. Magma produced in this way is dominantly andesitic in composition and commonly contains large amounts of dissolved gases such as water vapor, carbon dioxide, and sulfurous vapors. The larger volcanoes in the Cascade chain are composite cones built through a series of explosive eruptions, such as the 1980 eruption of Mount St. Helens, alternating with less violent effusions of viscous magma.
The Cascadia subduction zone is terminated to the south by the Mendocino fracture zone, a transform fault

Subduction uction Zone
Mount St. Helens un Juan de Fuca
Plate

Mount Adams

B

North
American
Plate

th that separates the Juan de Fuca (Gorda) plate from the
Pacific plate (see Figure NC.42). Because there is no plate convergence south of the Mendocino fracture zone, no subduction-related volcanic activity occurs in that part of
California. Mount Lassen, at the southern end of the
Cascade Range, is located just slightly north of the landward projection of the Mendocino fracture zone.
California’s Cascade Volcanoes: Among the hundreds of volcanoes in the California portion of the Cascade Range, three merit special attention because of their size and potential for future activity. Though future eruptions could occur almost anywhere in the region, Mount Shasta,
Medicine Lake, and Mount Lassen volcanoes appear to have been the most active over the past several centuries.
Each of these volcanic centers has a unique history, and poses distinctive threats to residents and visitors.
Mount Shasta (see Figure NC.37) is one of the largest composite volcanoes in the Cascade chain. This volcano was constructed over the past 100,000 years by hundreds of eruptions, including at least 13 in the past 10,000 years.
These eruptions produced andesitic lava flows, pyroclastic flows, and debris flows, some of which extended well beyond the base of the volcano. Shastina, a small volcanic cone on the western flank of Mount Shasta (Figure NC.43), resulted from eruptions that occurred between about 9,300 and

28

ᮣ

Geology of Northern California

Figure NC.45 Mt. Lassen, the southern most peak in the
Cascade Range.

Figure NC.43 Mt. Shasta and Shastina, seen from the north
Shastina

9,700 years ago. The most recent activity at Mount Shasta was about 200 years ago, when several small debris flows, possibly triggered by steam eruptions, originated from the summit region.
At Medicine Lake, eruptions began about 700,000 years ago, when great volumes of basalt lava flowed onto the sure ano ure face, building the base of this large shield volcano (Figure
NC.44). The eruption of voluminous basalt flows is generally associated with divergent plate boundaries or hot ndaries a spots, and is unusual in subduction-related volcanic areas.
The basalt eruption at Medicine Lake is probably related to e pr y the proximity of the region to the Basin and Range
. I ea province immediately to the east. In this area, the early stages of continental rifting may be in progress as a new a evada. divergent boundary develops in Nevada. Perched atop or along the gently sloping flanks of the Medicine Lake vole cano are numerous cinder cones that provide sources for ock, con g dm decorative rock, concrete aggregate, and roadbed material.
The more recent eruptions at Medicine Lake, some probae eruptio Medi me pr bly less than 100 years old, have produced silica-rich han yea hav ica-ric magma that resulted in obsidian flows and rhyolite domes sulted idian flo omes on the higher slopes.
Figure NC.44 Basalt Flow and Cinder Cone in the Medicine re NC nder Lake Highland of Northern California.
a.

Rick Hazlett

ᮣ

Sue Monroe

Lyn Topinka/USGS

Mount Shasta

ᮣ

ount
(Figu e N sual Mount Lassen (Figure NC.45) is unusual with respect to her ge no the other volcanoes of the Cascade Range in that it is not a com site
R
do b composite volcano. Rather, it is a lava dome volcano, a buls p-sided st med viscou ab bous steep-sided structure formed by viscous magma above olcani er ava dom a volcanic conduit. The rock comprising the lava dome at
Las
dacit repres
Mount Lassen is mostly dacite, representing sticky silicac duit rich magma that oozed up the conduit a little more than
1
th deve
11,000 years ago. Before the development of the Mount
Las
e, large Lassen lava dome, a much larger composite volcano existed to the west. Geologists have named this ancient volcano
Ge ogists n hama ure N
Mount Tehama (Figure NC.46), and it was the site of iolent ani numerous violent volcanic blasts over the past 600,000 years. Mount Lassen is the only California volcano to be
s. Moun erved erupti observed erupting in historic time. Between 1914 and 1921, a series of small eruptions, mostly producing ash and steam, e occurred near the summit of the lava dome. In May of occur ed
1915
1915, however, somewhat larger eruptions sent glowing avalanches of debris down the flanks of Mount Lassen, and avalanch produc produced a small dacite lava flow on the. Today, visitors to
Lassen Volcanic National Park can explore hot springs,
L
boiling mud pots, and gas-discharging fumeroles that prob vide hints that the volcanic system is still active.
Volcanic Hazards in Northern California: All of the volcanoes of the Cascade Range and Modoc Plateau region are geologically young, and it is almost certain that there will be future eruptions in the region. In addition, the presence of magma at shallow depths may be responsible for the geothermal activity around Clear Lake in the northern
Coast Ranges. Though these areas are distant from large population centers, future eruptions could produce widespread impacts, adversely affecting distant communities as well as local residents and visitors. A major volcanic eruption in the Cascade Range or elsewhere in northern
California could also affect agriculture, transportation, communications, water quality, timber resources, and recreation over a broad area.
Volcanic eruptions can threaten human welfare by generating destructive flows of lava or debris, by filling the skies with clouds of ash and other fine rock particles, and by emitting noxious gases such as carbon dioxide, chlorine, or acidic

Northern California.6 Volcanoes of the Cascade Range and the Modoc Plateau

ᮣ

29

Figure NC.46 The original profile of Mt. Tehama near the lava dome of Lassen Peak.

Eagle Peak 2810 m
3397 m
3154
2790

Brokeoff Mountain 2815 m
Lassen Peak 3187 m original form of ed Moun Ski Heil 2713 m um t Teh
Pres
ama
Sulphur Works 2164 m

Diagram courtesy of the U.S. Geological Survey.

2426

0

SW

1

NE

km
1915 dacite from Lassen Peak

Breccia (vent filling) of Mount Tehama
Decomposed andesites
Decompo

Talus
Dacite domes
Pre-Lassen Peak dacites from the Lassen Peak vent en compounds. Of these multiple potential hazards, large flows ge flo of pyroclastic material from Cascade volcanoes would be the anoes e greatest threat in northern California. The geologic evidence indicates that between 300,000 and 360,000 years ago, an d 3
0 year explosive eruption at Mount Shasta produced a volcanic hasta d v debris flow that traveled 45 kilometers from the volcano and meters lcan buried an area exceeding 450 square kilometers in rubble quare kilo arger nic mudflows
(Figure NC.47). Even larger volcanic mudflows, known as lahars, resulted from prehistoric eruptions at both Mount preh ions
Lassen and Mount Shasta. And during the relatively small has he mall eruptions of 1915, ash particles from Mount Lassen fell as far
M
f away as Elko, Nevada, 500 kilometers (300 miles) to the east.
00
(
t.

Andesites of Mount Tehama ehama Mindful of the serious implications a large eruption in
Mi
e imp ations la northern California would have, geologists have prepared
Cal
ia have geologi rd m s th volcanic hazard maps of the most sensitive areas and have helped develop evacuation plans for disaster response agenevacu on p cies. In addition, both Mount Shasta and Mount Lassen n additio
(along with other areas in central and southern California) ng are continuously monitored by scientists from the U.S. re ontinuously m
Ge
cal Su
Geological Survey for signs of renewed activity. In the absence of any means to accurately predict future volcanic absen an eruptions, the combination of hazard assessments beforeuption hand, strategic preparedness, and monitoring will help limit han st the harmful consequences of future eruptions. h Figu
Figure NC.47 The hummocky terrain in the foreground was produced by an avalanche of volcanic debris from Mt. Shasta more than
Th
n d produce years ago.
300,000 ye go. Dick Hilton

ᮣ

30

Geology of Northern California

Section Northern California.6 Summary
● The Cascade Range of northern California is part of a

chain of mostly composite volcanoes that extends north into British Columbia. This chain includes the famous volcanoes of the Pacific Northwest including Mount
Rainier, Mount St. Helens, Mount Hood, and Crater
Lake. In northern California, Mount Lassen and Mount
Shasta are the most prominent volcanoes of the Cascade
Range. The Modoc Plateau to the east is a high volcanic table land with numerous shield volcanoes, such as that in the Medicine Lake highland, and cinder cones.
● The magma that sustains the volcanic activity in the

Cascade Range arises from the Cascadia subduction zone, in which the Juan de Fuca plate descends beneath
North America. Water released from the subducting plate induces the formation of magma bodies more than
100 kilometers beneath the surface. This magma is generally intermediate to felsic in composition, relatively viscous, and laden with magmatic gases. These factors commonly lead to explosive eruptions and the construction of composite volcanoes in the Cascade Range.
● Mount Shasta is a large composite volcano built in a bu series of eruptions over the past 100,000 years. Mount ears. t
Lassen is a lava dome volcano that was last active in the early 1900s. The Medicine Lake highland is a large d la r con d obsi shield volcano, with smaller cinder cones and obsidian flows on its flanks and summit. The most recent erupt tions in the Medicine Lake region may have occurred on rred po only 100 years ago. All of these volcanoes are potential sites of future eruptions in northern California. rn ● Because they all have the potential for future activity, futu the Cascade Range volcanic centers pose serious volcanic
Ran
hazards for people in northern California. The geologic or i hern C geo record of previous eruptions demonstrates that the e ions d hat t
Cascade volcanoes can produce pyroclatic flows, volcanic lcanoes duce py canic mudflows known as lahars, and ash falls that could affect wn l as ect communities hundreds of kilometers away. These threats ities kilomete can be mitigated by continuous monitoring of the volcanic e mitigate tor areas, assessment of the potential hazards beforehand, and assessm al preparations for responses to the future eruptions. e Northern California.7 ern lifornia
The Basin and Range of
Northeast California
East of the Modoc Plateau in extreme northeastern
California, the volcanic table land is broken by numerous large normal faults into a series of elevated blocks and lower basins. Principal among these mountains and valleys are the Warner Range, rising to 3,017 meters (9,892 feet), and Surprise Valley, lying 1,600 meters (5,200 feet) lower

immediately to the east. The displacement between the
Warner Range and Surprise Valley is a result of the motion along normal faults along the base of the steep eastern ef escarpment of the mountain block. These faults comprise as to the Surprise Valley fault zone, which has a total cumulative of more than 14,000 feet. Normal faulting in this area ut began as early as 14 million years ago, but much of the disre rec mes. Ho placement has occurred in more recent times. The Honey urprise de
Lake basin to the south of Surprise Valley is also a deprests. sion bounded by normal faults. g The pattern of alternating high mountains and intervenas
C
ing basins in these small areas of northern California conr hun tinues eastward for hundreds of kilometers into Nevada and acent portions of the adjacent states. This corrugated landscape, n squa rs ( covering more than 770,000 square kilometers (300,000 tern N kno ow square miles) of western North America is known as the p nce. regi Basin and Range province. In this immense region, tensionces stretchi t fra al forces are stretching the Earth’s crust and fracturing it he hangin into blocks bounded by normal faults. The hanging walls of th ormal mo d orm the normal faults move downward to form the basins or grab , ocks rem fo grabens, while the mountain blocks remain high to form orsts an e asin horsts. Surprise Valley and the Honey Lake basin are only hun ssio th two of hundreds of depressions formed as the crust in the
R
tensi
Basin and Range province yields to tensional stresses. eotherm l Res
Geology and Geothermal Resources of the Warner
Ra
he o Range region: The bedrock of the Warner Range consists mo canic erupt mostly of volcanic rocks erupted since about 36 million
Inclu
t years ago. Included in this thick accumulation are lahar uffs us t deposits, tuffs of various types, and felsic to intermediate ows. hese o lava flows. These older volcanic rocks appear to be related ubduction to subduction of the Farallon plate. Younger basaltic (more fic) mafic) lava flows in the region are about 3 million to 8 milars an lion years old and probably signify a shift from subductionrelate volcan related volcanic activity to the rifting-related eruptions. pro The prominent normal faults are relatively young, displacing the entire succession of volcanic rocks.
N
Numerous hot springs and “mud volcanoes” occur along a north-to-south zone near the base of the Warner Range, within the Surprise Valley fault zone. Exploratory drilling along the fault zone has encountered water as hot as 170° F at depths from 500 to 1,500 meters (1,600 to 4,900 feet).
This geothermal activity probably reflects the interaction between water moving down from the surface and hot young volcanic rock in the subsurface. Since 1984, the hot water from the Surprise Valley springs has been used to heat schools and hospitals in the small communities of the valley.
In 1993 alone, this saved roughly $50,000 in fossil fuel costs.
The Lake Tahoe Graben: Though it rests along the eastern edge of the Sierra Nevada physiographic province, the geological structure of the Lake Tahoe basin is similar to the grabens of the Basin and Range province. This magnificent lake fills a deep depression created by normal faulting to a depth of nearly 500 meters (more than1,600 feet).
Volcanic activity 3 million to 4 million years ago created a natural dam of basalt and andesite at the north end of the graben that blocked the drainage of the upper Truckee
River. The basin-forming normal faults are still active in

Northern California.8 The Northern California Ice Ages

Figure NC.48 Several scarps mark the trace of active faults on the floor of the Lake Tahoe Basin. Note the large blocks of rock on the lake bottom that document an ancient catastophic landslide.

ᮣ

31

volcanoes along the fault zone at the base of the eastern escarpment indicate the presence of young and hot volcanic rock in the shallow subsurface. These hot springs bs have been used locally as a source of energy for space s so heating. ● The Lake Tahoe basin lies in a graben that developed e ba es Image courtesy of the U.S. Geological Survey

wnwar cement ro through the downward displacement of rocks along normal faults to the east and west. The elongated graben w slopes gently to the north, and was blocked by lava flows o mill that erupted 3 million to 4 million years ago. The natural volcanic dam impounded the waters of the ancestral vol m
Truckee River, creating the beautiful alpine lake. ckee Riv oing ac
Ongoing seismic activity in the Lake Tahoe region suggests that the norma faults are still active and the basin normal hat st a till is still evolving. volvin Nort e Californ a.8
Northern California.8
The Northern California Ice
Northern Ca f
Ages
s

the Lake Tahoe region, producing many small to moderate n, p sm rate earthquakes. Ancient landslide rubble on the floor of Lake
Lak
Tahoe is thought to have resulted from a major landslide ave fro dslide triggered by a powerful Ice Age earthquake. Recent imagear ecent im ging of the lake (Figure NC.48) reveals numerous fault merous fa scarps resulting from recent displacement, a further indicaem rther tion of active faulting. acti ulting.

Section Northern California.7 Summary ction rthern umma ● The Basin and Range province of extreme northeast ge p

California is a landscape of alternating mountains and andscape o ends eastwa valleys that extends eastward into Nevada and adjacent states. The corrugated terrain is the result of intense stat ugated normal faulting caused by the tensional stresses that nor faultin affe the entire Basin and Range region of western affect he
B
North America. The mountain blocks such as the
No
Warner Range represent the footwall (or horsts) of the normal faults, whereas basins such as Surprise Valley represent the downthrown grabens. The Honey Lake and the Lake Tahoe basins also developed as grabens displaced downward by normal faults along the eastern edge of the Sierra Nevada province.
● The Warner Range is the most prominent mountain

range of northeast California, and is composed primarily of Cenozoic-age volcanic rocks. Hot springs and mud

During the Pleistocene Epoch, 1.8 million to about 10,000 ing Pleisto ear th years ago, all of the regions of northern California were affe mu affected by multiple climatic oscillations. In the higher elevations of the Sierra Nevada, Klamath Mountains, and the vatio he ascade Cascade Range, numerous valley glaciers formed, and in the basins of the Great Valley and Basin and Range, lakes asi grew as water accumulated in the lower terrain (Figure
N
NC.49). Even the coastal regions were affected, because sea level fell when the glaciers were expanding and rose when they were in retreat. Based on evidence from the Sierra
Nevada, where the effects of the Pleistocene glaciers are most striking, geologists have identified four major glaciations corresponding to periods of cool climatic conditions.
Some of these glaciations appear to be comprised of multiple cycles of glacial growth and decline. The glacial periods were separated by warm interglacial intervals when glaciers melted back and the lowland lakes diminished. The multiple ice ages of the Pleistocene Epoch in northern California signify a period of erratic oscillation in climate, not a long interval of enduring cold.
Because of their oscillatory nature, establishing a detailed and accurate history of glacial events from the geological evidence is difficult in most places. Each glacial advance tends to obliterate or obscure the evidence for the previous one. Geologists are in general agreement, however, that the two most recent glacial epochs in northern California peaked at about 160,000 years ago (the Tahoe glaciation) and 20,000 years ago (the Tioga glacation). Of these two glaciations, the earlier Tahoe appears to be longer and more severe than the later Tioga. There is also geologic evidence for earlier glaciations, but it is more fragmentary and localized than for either the Tahoe or the Tioga event.

32

Figure NC.49 Donner Lake in the northern Sierra Nevada, rests in a U-shaped glacial trough carved by Pleistocene glaciers.

Frank DeCourten

ᮣ

Geology of Northern California

Fi re NC.5
Figure NC.50 Glacial striations on bedrock the northern Sierra
Nevada.
Neva
ᮣ

Frank DeCourten

Effects of Valley Glaciers: The overall effect of glacial verall erosion in mountainous regions was to increase the relief ss th op and sharpness of the landscape through the development of
U-shaped glacial troughs, horns, arêtes, and cirques. trou d cirq landform re displa lar fas
Although these landforms are displayed in spectacular fashion around Yosemite National Park in the central Sierra osemite N nal Par
Nevada, they are also present in many other mountain als sent ain regions of northern California. Wherever the Pleistocene nor i W glaciers existed, glacial polish and striations on bedrock s existed stria surfaces are common (Figure NC.50). Such erosional feas
Su
tures are widespread in the Trinity Alps, in the higher peaks nity of the Cascade Range, and in the alpine terrain west and ade R he a south of Lake Tahoe. The Pleistocene-age valley glaciers ake Ta
P
also left extensive deposits of moriane of several types in tensive posits m these same mountain areas, along with glacial erratics reas, al
(Figure NC.51).
Lowland Lakes and Wetlands: During the cool climatic intervals of the Pleistocene Ice Ages, increased precipitation and reduced evaporation led to the development of large lakes and wetlands in the lower areas of northern
California. Ice Age lakes that form in response to cool climate cycles are known as pluvial lakes and there were several of them at various times in the Honey Lake basin, in
Surprise Valley, and in the Great Valley. In the Great Valley,

large lakes formed repeatedly during the Pleistocene
Epoch. One of the largest Ice Age lakes in the Great Valley existed between 800,000 and 600,000 years ago and covered the entire valley from its north end to the southern extremity. At other times, there were several smaller lakes with

Northern California.8 The Northern California Ice Ages

Figure NC.51 Erratics of light-colored granite perched on dark-colored volcanic bedrock in the northern Sierra Nevada.

Dick Hilton

ᮣ

33

glaciers melted, and sea level rose. By about 8,000 years lac ers an ago, sea water had once again invaded San Francisco Bay to ago re-establish the estuary of modern times (Figure NC.52). re-es h th

F
Figure NC.52 Sea level rise over the past 20,000 years has converted the San Francisco Bay area from a river valley to an estuary. v ᮣ

N

20 km

8.3ka
–18m

10.5ka

–1 m 1m

10

–9

Map courtesy of the U.S. Geological Survey.

–5

5m

–7

3m

7m

–3

ka

.5

11 ka 14

ka

a

18

.2k

14

extensive wetland habitats between them. The lush native een h vegetation provided forage for a remarkable menagerie of age fo emarkable me
Ice Age creatures, including giant bison, elk, ground sloths, udin n, gr mammoths, large cats, and bears. The remains of these s, rem ese creatures have been found in excavation sites throughout excavat ghou the Great Valley.
The Ice Age Landscape of San Francisco Bay: Earlier ape Ea er than about 600,000 years ago, the Sacramento and San nto S
Joaquin Rivers did not flow to the Pacific Ocean via San
Joaqui
P cean Francisco Bay as they now do. Instead the drainage from tead t nage the surrounding highlands accumulated in one or more surroun g highl ed lakes in the Central Valley. Tectonic activity in the northern kes th ntral Val ty no
Coast Ranges had already produced a valley where the bay
Ra
alre wher now is, but only local streams flowed into and through it. no l l
Sometime after 600,000 years ago, water from central
Som
000 y
California began to flow into the San Francisco Bay, where it mingled with ocean water to create the large estuary that cean exists today. However, cycles of climate change during the ver, c
Late Pleistocene caused sea level to fluctuate several times. tocene During cool intervals leading to glacial advances, sea level
Duri
ool interval fell by as much as 120 meters (390 feet), causing water to l withdraw from San Francisco Bay to the west. About
20,000 years ago, for example, the shoreline of the Pacific
Ocean was located beyond the Farallon Islands, more than
30 kilometers (18 miles) west of the modern Golden Gate
Bridge. At that time, San Francisco Bay was a broad river valley where streams from the Great Valley met with local tributaries before turning west through a valley that then existed in the Golden Gate area. Since about 20,000 years ago, the climate has become warmer, the Late Pleistocene

34

Geology of Northern California

Section Northern California.8 Summary
● During the Pleistocene Epoch, California was affected

by multiple cycles of climatic oscillations. Cool intervals resulted in the growth of valley glaciers in the higher elevations of the Sierra Nevada, Klamath Mountains, and the Cascade Range, and pluvial lakes developed in the lowlands.
● The two best documented glacial intervals are the

Tioga glaciation about 20,000 years ago and the preceding Tahoe glaciation that peaked about 160,000 years ago. Both of these glacial intervals involved multiple advances and retreats that reflect the climatic oscillations of the Pleistocene. There is evidence of earlier glaciations, but the precise chronology and extent for them has yet to be determined with certainty.
● Valley glaciers in the mountainous regions carved U-

shaped valleys and sculpted arêtes, horns, and cirques in bedrock exposures. Moraines and glacial erratics can also be identified in most mountains above 2,000 meters in elevation. ● In the lower elevations of the Basin and Range and

decli
Great Valley, pluvial lakes expanded and declined in es. Th rhythm with the Pleistocene climatic cycles. The largest r ent of these Ice Age lakes submerged the floor of the entire
00,00
ars ago
Great Valley between 600,000 and 800,000 years ago.
● Sea level rose and fell multiple times during the ple ng

Pleistocene Ice Ages. During times of glacial advances, mes nces sea level fell as much as 120 meters. Under such condiers. tions, San Francisco Bay was a river valley through t which rivers carried water to the Pacific coast located coas about 30 kilometers west of the modern shore. When the California glaciers melted during warm interglacial rnia gla ter intervals, sea level rose. The modern estuary in San
,
ro mo y
Francisco Bay formed about 8,000 years ago as glaciers o ut 8,00 lacier in the interior mountains were melting under the influr mount m fluence of a warming climate. gc Northern California.9 rn C ia.9 Northern California Earthquakes ern aliforn
Earthquakes in California are an inevitable consequence of
C
nia the interactions between tectonic plates along the forward t edge of a west-moving continent. The forces generated along the transform boundary between the Pacific and
North American plates, and at the convergent boundary between the Juan de Fuca and North American plates, result in thousands of earthquakes every year. The ground literally shakes continuously in California, but most of these tremors go unfelt by people and rarely cause damage.
Nonetheless, the seismic history of northern California

includes some devastating events that provide a chronic reminder of the natural hazards facing those who live near a plate boundary. The infamous 1906 San Francisco earthe quake (estimated magnitude 7.8 on the Richter scale)
Andr
occurred along a segment of the San Andreas fault more ce than 400 kilometers long, ignited fierce building fires, and mag claimed more than 2,000 lives. In 1989, the magnitude 6.9 e Sant z Loma Prieta earthquake in the Santa Cruz Mountains left a c
62 people dead in the San Francisco Bay area and caused ges. dyn i nearly $10 billion in damages. In the dynamic tectonic lifornia, m environment of northern California, where millions of residents live in close proximity to active faults, future seismic tain h disasters are as certain as those of the past have been tragic.
Ca
Earthquakes in northern California are associated with e sett an A three different plate tectonic settings. The San Andreas tem, i g score ults, g s fault system, including scores of subsidiary faults, generates th an F a many tremors in the San Francisco Bay area and the northT se e lease stre ern Coast Ranges. These earthquakes release stress associnorthwar p ated with the northward motion of the Pacific plate along th dge
Nort
n plat earthquak the edge of the North American plate. The earthquakes gene ed ary sh generated along this plate boundary tend to have very shalw foci lom rs low foci, less than 15 kilometers deep (Figure NC. 53), and ver ensely popul can be very powerful. In the densely populated urbanized aroun any b area around San Francisco Bay, many buildings have been bed ck th constructed on soils or bedrock that are not very resistant ons. he 1 to seismic vibrations. In both the 1906 San Francisco and a Prieta earthquak the 1989 Loma Prieta earthquakes, intense ground shaking,
,
e c liquefaction, and fire all contributed to scope of the disaster elated e po and were related to the poor response of weak soil or rock thqua vibra to earthquake vibrations. rea o
The second area of notable seismic activity is associated h Mendo with the Mendocino triple junction along the northern coas eac coast, where each year 80 to 100 earthquakes greater than magn ude magnitude 3 occur. Here, forces generated as the Juan de
(G
Fuca (Gorda) plate descends into the Cascadia subduction ca zone can produce powerful earthquakes in a zone that dips eastward beneath the continent. In this plate tectonic sett ti ting, earthquakes occur in both the oceanic plate and on land, including some that can be powerful. In July and
August of 1991, four major earthquakes, ranging from magnitude 6.3 to 7.1, occurred near the Mendocino triple junction, and there is good geologic evidence for even larger prehistoric events, possibly as great as magnitude 9. In addition to the impacts of local communities, large coastal earthquakes in northern California have the potential to produce tsunamis that could affect distant regions bordering the Pacific Ocean.
Along the eastern side of the Sierra Nevada, active normal faulting associated with the continuing uplift of the mountain block produces a third zone of seismic activity from the Lake Tahoe area south to Owens Valley. The eastern Sierra earthquakes are mostly generated along normal faults, but several prominent strike-slip faults in the region indicate that both tension and shear stresses exist in this area. Although the majority of eastern Sierra earthquakes are small, several greater than magnitude 6 have occurred

Northern California.9 Northern California Earthquakes

35

Figure NC.53 Distribution of earthquake foci along the San Andreas Fault from San Francisco to Parkfield. Note that most earthquakes occur within 15 km of the surface. The seismic gap filled by the 1989 Loma Prieta earthquake is represented on the bottom panel.

ᮣ

Epicenter of
Loma Prieta earthquake San Francisco

fault
San Andreas

San Juan
Bautista

Parkfield

Los An
Angeles

Depth (km)

field fie ie
Parkfield

Sa
San Juan
Bautista

Parkfield eld gap

Lom
Loma
P
Prieta

Portola
Valley

San
Francisco

Southern
San Francisco
Santa Cruz
Peninsula gap Mountains gap ap 0
5
10
0

100

0

100

2
200

300
0

400

300

400

0
5
10
200
Distance (km) ance in the past 40 years. In Lone Pine, along the southern porn alo tion of the Sierra Nevada fault system, an 1872 earthquake da system earthq ke wer 7 destroyed all but seven houses and killed no fewer than 27 people. This earthquake occurred before seismometers peop eismometers dev , rnia’s mos were developed, but it may have been California’s most powerful historic temblor. A recurrence of such an earthhi ic temb nce quake along the Sierra Nevada fault system would have alon he Sierr ystem woul r im s th major impacts on the resort communities in the RenoTahoe area, and regions farther south.
T
d region
Earthquake Forecasts in Northern California: Even ts i though scientists have been intensely studying earthquakes in California for more than a century, no reliable means to predict their occurrence with precision has been developed. pred rence w
Instead, geologists can forecast the probabilities of future
Inste
eologi earthquake occurrences based on measurements of ground earth ke occurren motion and strain, historic patterns of seismic activity, and mot d h geologic evidence of fault activity. In recent years, the analysis of seismic gaps, seismically “quiet” segments along active faults, has helped to identify places most prone to future earthquakes. Because no two faults are exactly the same, the forecasts for major earthquake potential vary across the state, from very unlikely in some places to almost certain in others.
The time frame is a key consideration in making and using earthquakes forecasts. Over the long span of geologic

time, the probability of a major earthquake in California is time 100%, but the likelihood of such an event tomorrow is very
100
lo low. Geologists generally try to forecast earthquakes over periods of 20 to 50 years to coincide with the time frames most widely used by planning commissions, disaster response agencies, and the insurance industry. In 1989, the
U.S. Geological Survey developed the first widely accepted earthquake forecast for the San Andreas fault system. This forecast estimated the probabilities for major earthquakes between 1988 and 2018 within the entire fault zone. The probabilities varied from 10% to 90% at various locations along the fault system. Interestingly, the Loma Prieta earthquake occurred in an area where a seismic gap had been identified in this study (see Figure NC.53).
In 2007, an interdisciplinary team of scientists known as the Working Group on California Earthquake Probabilities developed a new updated earthquake forecast called the
Uniform California Earthquake Rupture Forecast, Version
2 (UCERF). The new forecasts involve a broader array of data and techniques for estimating probabilities than did earlier assessments. Statewide, the UCREF estimates the probability of a magnitude 6.7 earthquake in the next 30 years at more than 99%. Californians also face a 46% chance of experiencing a magnitude 7.5 or larger earthquake in the same time frame.

36

Geology of Northern California

Figure NC.54 Major faults of the San Andreas system in the San Francisco Bay Area.
This map from the United States Geological Survey also identifies features resulting from recent offset along these faults by the circled numbers.

Ma
Map courtesy of the U.S.Geological Survey. sy U.S.G

ᮣ

duration of such an induced earthquake, and even a moderate temblor could have disastrous consequences in densely popuenc lated areas. In the absence of any safe seis mechanism to control seismic events in anning California, careful planning and preparaod sense tion for them makes good sense. ornia G gical The California Geological Survey, the es Geologica rvey, an
United States Geological Survey, and the
Em
California
Office
of
Emergency
nt collabor Management have collaboratively develst oped forecast models and seismic hazard th p maps that are used to plan communities nd pr and make preparations for coordinated ponses earth addi responses to earthquakes. In addition, in s gen ent the most sensitive areas, stringent building co hav ed th codes have been developed that result in mor arth nt structu more earthquake-resistant structures. Local oning any zoning ordinances in many areas limit residenti mmerc l dential and commercial development in loc g nd locations where strong ground shaking or acti anticipat d. Pers liquefaction are anticipated. Personal ion importa d preparation is also important, and checkavaila t lists are available at the aforementioned ncies th provi agencies that provide guidelines for preparng an apa ing homes and apartments for a potential
Th
earthquake. The combination of continuearth ing earthquake research, careful planning, ent b stringent building codes, wise land use, and pe personal preparation is the best way to limit the property damage and loss of life in future northern California earthquakes.

Sectio
Section Northern California.9 Summary
In northern California, the probabilities are lower than hern Califor he prob ate whole CERF e here for the state as a whole. UCERF estimates that there is a mag de quake 93% chance of a magnitude 6.7 or greater earthquake in the northern part of the state, with a 63% chance par he stat nce emblo n S of the temblor occurring in the San Francisco area (Figure NC.54). The faults most likely to produce this large
C.54). T y earthquake are the Hayward-Rogers Creek (32% probauake a
-Rogers Cr hance), bility), San Andreas (21% chance), and Calaveras faults
(18% probability). The UCERF estimates only a 2% ability RF e probability for an earthquake of magnitude 8 in northern hquake California, illustrating that very large earthquakes are rare, illustr g l even along the San Andreas fault system. This may seem he S dreas fau d comforting, but we should not forget how disastrous earthquakes in the magnitude 6 to 7 range have been in
California. Earthquakes in this range of magnitude have resulted in more than $65 billion in property losses in
California since 1971.
Living with Earthquakes in Northern California:
Although human activities such as underground fluid injection can initiate earthquakes, such practices to reduce the earthquake threat in northern California would be extremely risky. There is no way to guarantee the size, location, or

● Northern California earthquakes occur as a conseNo

quence of convergent plate interactions in the Cascadia subduction zone, in relation to the transform boundary between the Pacific and North American plates to the south, and along the normal fault zone at the base of the eastern escarpment of the Sierra Nevada. The San
Andreas fault zone, part of the transform plate boundary, has historically produced the most large earthquakes, but destructive earthquakes could occur in the other plate tectonic settings in northern California as well.
● Earthquakes

in populated regions of northern
California have resulted in intense ground shaking leading to collapse of structures, liquefaction, landslides, and fires that have claimed many lives and caused billions of dollars of property damage. The 1906 San Francisco and the 1989 Loma Prieta earthquakes were the two most destructive earthquakes in northern California history.

● Geologists can formulate earthquake forecasts along

active faults on the basis of ground motion and strain measurements, historic seismicity including “quiet”

Northern California.10 Living on the Edge: Coastal Hazards in Northern California

segments known as seismic gaps, and geological evidence of prehistoric fault activity. The most recent forecasts estimate the probability of a magnitude 6.7 or greater earthquake in northern California at 67%. Faults in the
San Francisco Bay area within the San Andreas fault zone are most likely to produce this future earthquake.
● Because it is currently impossible for humans to pre-

vent or weaken earthquakes, preparation for them is essential in reducing their impacts on people. In northern California, continued monitoring of active faults coupled with wise land use regulations, stringent building codes, and effective response plans will substantially reduce the impacts of future earthquakes.

Northern California.10
Living on the Edge: Coastal al Hazards in Northern California
With 1,800 kilometers (1,120 miles) of ocean frontage,
California has more coastline than all but two other states, n o oth ern e Cali
Florida and Alaska. The northern portion of the California coast trends northwest, directly facing the enormous and y th mou tectonically active Pacific Ocean basin. Winter storms approach the exposed northern coast from the open ocean, bringing with them powerful wind-generated waves. In lon addition, normal wind swells, longshore currents, and occahe no sional tsunamis also strike the north coast with full force. In contrast, the east-to-west alignment of much of the southy ern California coast partially protects the shoreline from the experi north vigorous erosion experienced in the north. Consequently, alifornia coas the northern California coast is rugged and rocky rather
N
) than broad and sandy (Figure NC.55). Some locations along the north coast, such as Big Sur and Mendocino, are world coas famous for their spectacular coastal scenery. he Cali
The California coast also is unique in its recent emergence and in the variety of rock types exposed along the eashore. Recal earl steep seashore. Recall from our earlier discussions that m ns st Ra
R
mountains of the northern Coast Ranges have been eled r ju al m vated over just the past several million years and, in fact, are still rising. The coast of northern California is therere ing f northe fore an emergent coast, where the land is being lifted by em here nic f e ea bed tectonic forces relative to sea level. As bedrock rises from the seafloor, it is exposed to the various agents of coastal sea xposed e ag n m u ift expo erosion. In most places, the uplift exposes rocks of the heterogeneous Franciscan complex or the Salinian terus F nciscan com rane. These rock assemblages include both easily eroded ock a mbla ls suc s m materials such as marine shale or sandstone and more durable igneous and metamorphic rocks such as granite, ble d m basalt, and greenstone. The erosion of such mixed asalt, green

Figure NC.55 The steep and rugged shore in Point Reyes Nationa Seashore is typical of the scenic coast of northern California. Note the nearly ep a ore t th flat marine terrace above the sea cliffs.

Photo courtesy of the U.S. Geological Survey.

ᮣ

37

38

Geology of Northern California

Figure NC.56 A pocket beach developed between projecting headlands on the Monterey Peninsula.

Figure NC.58 Sea stacks along the northern Califonria coast at
Shell Beach.

ᮣ

bedrock assemblages produces an irregular coastline, with coves and pocket beaches developing where soft rocks exist and headlands (Figure NC.56) projecting where harder rocks are exposed.
Coastal Landforms of Northern California: Because of use the pervasive coastal emergence, the shoreline landforms rms in northern California are mostly of erosional origin. Sea al or a cliffs and wave-cut platforms are actively forming in many orming m locations, and elevated marine terraces are commonly commo observed above the coastal bluffs (Figure NC.57). Because gure 57). the marine terraces originated as wave-cut platforms near waverms sea level, geologists can estimate the rate of uplift along alon the coast by establishing the age of the terraces using fosus sils preserved on their surfaces. Radiometric ages for fossil diometric f material from the terraces suggest that the most prominent at p surfaces rose to their present elevation at rates varying from about 28 to 45 centimeters per thousand years in the
4
Santa Cruz and Monterey Bay areas. Farther north, in the uz Monte ay area orth, Mendocino area, the rate of emergence is even higher. no r eme high
This helps explain why the coast of California shows such plain o strong emergent features, even though sea level has been feat tho een Figure NC.57 A marine terrace along the northern California ure NC.5 ng nor coast with remnant sea stacks. Note the younger sea stacks forming ith e s along the modern coast.

James S. Monroe

ᮣ

Sue Monroe

Photo courtesy of the U.S. Geological Survey, Center for Coastal Geology.

ᮣ

rising for the past 20,000 years. The land simply is rising
20,0
nd ris fas han faster than the sea.
Sea cliffs along the north coast are continuously underffs oast cont uously und cut by waves generated during powerful winter storms. w ed ng w nter st w ing proje Because waves approaching a projecting headland are refracted around it, the wave-cut notches in some areas can e-cut tche develop into sea caves on opposite sides of the bedrock d aves posit exposure. Continued wave erosion can deepen the caves exp nued ero until they connect, forming a sea arch (see Figure NC.26). nnec , se (Fig
NC.58
Sea stacks (Figure NC.58), isolated exposures of bedrock, represent remnants of sea arches that have collapsed as a mnan ea consequence of a continued wave erosion. The long-term equence f co effect of the formation of wave-cut platforms, sea caves, ct fo mat sea arches, and sea stacks along an emergent coast is the hes, s steady landward retreat of the cliffs. This has placed many landwar coastal developments at risk along the northern California coast shoreline. shorelin In addition to erosional features, depositional landforms a have also developed in some areas along California’s north ha coast. In places where streams discharge into bays or coves, spits (Figure NC.59) and baymouth bars (Figure NC.60) can develop from the accumulation of sediment transported by both rivers and longshore currents. Tombolos, sand spits that extend from the shore to a sea stack, can also be seen in several places along the northern California coast
(Figure NC. 61). Tombolos are generally rare, but the abundance of sea stacks along the emergent northern
California coast has created many places where the conditions exist under which tombolos form.
Coastal Hazards in Northern California: The majority of
California’s population lives within 100 kilometers (60 miles) of the coast. In northern California, the rugged beauty of the seashore has resulted in extensive residential and recreational development along the rapidly eroding emergent coast. Geologists have determined that coastal erosion is causing some sea cliffs in northern California to retreat at rates as high as 1.1 meters (3 feet) per year. In spite of recent regulatory action to restrict coastal development, there are

Northern California.10 Living on the Edge: Coastal Hazards in Northern California

ᮣ

Figure NC.59 A spit at the mouth of the Russian River.

39

Figure NC.61 A small tombolo connecting the shore to a sea stack remnant in northern California

ᮣ

Spit

James S. Monroe

Tombolo

ᮣ

Figure NC.60 A baymouth bar in Marin County, California. nia. Sue Monroe

Baymouth bar

Figure NC.62 Mass wasting in Bodega Bay resulted from undergur cutting of the sea cliff by waves generated during winter storms of cutt 1997–1998.
19

still hundr l hundreds of miles of coastline in northern California rthern Ca where people and property are threatened by retreating p pro cli cliffs and unstable ground.
The principal hazard along the northern California coast ard alon is mass wasting triggered by wave undercutting, heavy g rainfall, or earthquakes. Rock slides and slumps have repeatuakes. edly caused extensive damage to structures perched above d ex ve c
N
sea cliffs (Figure NC.62). These problems are most severe where the bedrock is soft, sheared, or water saturated. wher e
Unfortunately these conditions are common along the tecly thes tonically active north coast, which is underlain by the highly deformed rocks such as those comprising the Franciscan complex. Mass wasting activity along the northern coast also extends offshore and down the continental slope. Turbidity currents initiated by earthquakes or storm events have scoured several impressive submarine. Monterey Canyon, the largest submarine canyon on the Pacific Coast, is deeper than the Grand Canyon and extends offshore for more than
400 kilometers (250 miles).

James S. Monroe

James S. Monroe

ᮣ

Because the northern California coast faces the Pacific
Ocean basin where strong earthquakes occur frequently, tsunamis pose an additional hazard. Tsunamis can travel thousands of miles from their source, so even earthquakes in Japan or South America could affect people in northern
California. In fact, the principal tsunami threat to northern
California may be distant, not local, earthquakes. This is

40

Geology of Northern California

because the dominant strike-slip motion along the San
Andreas fault system, much of which is on land, would not likely produce large vertical displacements on the seafloor.
Subduction-related earthquakes generated in convergent plate boundaries more commonly result in the type of displacement that initiates tsunamis.
In April 1964, a 7-meter (23-foot) tsunami generated by the magnitude 9.2 Alaskan Earthquake struck Crescent
City on the north coast, resulting in 11 deaths and an estimated $15 million in property damage. In the same town, a boat harbor suffered $10 million in damages from a tsunami that struck in November 2006. Other coastal towns in northern California to experience tsunami damages include
Eureka, Half Moon Bay, and Santa Cruz. A major earthquake in the nearby Cascadia subduction zone is a primary concern in these coastal areas because it could potentially produce a much more disastrous tsunami than any experienced thus far. There is good geologic evidence of a major earthquake, perhaps as large as magnitude 9, in this area
300 years ago that may have caused a tsunami in Miho,
Japan. Fortunately, most populated areas along the northern California coast have tsunami warning systems and evacuation plans in place.
Human Activities and Coastal Hazards: Although mass wasting and wave erosion are natural processes along cesse g
California’s north coast, they have been intensified and triggered in places by human activity. Whenever an oceannever oce facing slope or shore is modified for construction of roads cons ion or buildings, there is potential for otherwise stable slopes othe ble to fail. Highway 1, along the north coast, has experienced hc ience repeated closures due to mass wasting events. Property
P

damage associated with mass wasting along this scenic highway has been significant in such coastal communities as Bolinas, Stinson Beach, and Pacifica (Figure NC.63).
Farther south along the same highway at Half Moon Bay, a
Ha
breakwater completed in 1961 to protect the harbor resultct ed in a fourfold increase in the rate of coastal erosion to e the south. This was the natural response to the interrupse tion of the southward flow of sand along the beach and the al he refocusing of wave energy produced by the breakwater. e breakw
Many cities and planning commissions carefully review ca ll proposals for coastal development in an effort to reduce opment eff the negative impacts that result from altering natural al s. How t slopes and shores. However, much of the development along the north coast was approved before the impacts of ast app human activity were fully evaluated or understood. Coastal ood. erosion will continue to be a concern for residents of e b re esi
California’s scenic north coast. rnia’s th c

Secti N
Section Northern California.10 Summary
C fornia.10 S mmary
● The northern part of California’s long coastline n C fornia

directly faces the Pacific Ocean and is vigorously eroded acific cean a torm even , lon by wind waves, storm events, longshore currents, and occasional tsunamis. In addition, geologic forces are still o amis. additio lifting the Coast Ranges faster than sea level is rising so
Coas
faste that new bedrock continuously appears along the emerbedr ntinu gent coast. In many coastal locations, the emerging m bedrock is a mixed assemblage of hard and soft rock such drock ixed a as the Franciscan complex that does not respond uniFrancis formly to the agents of erosion. The combination of y these factors produces an irregular and spectacularly thes rugged coast with many projecting headlands and small coves and pocket beaches. a Figure NC.63 Coastal erosion in northern California threatens thousands of homes such as this one near Bolinas. ome ᮣ

● Erosional landforms such as wave-cut platforms, sea

cliffs, sea caves, sea arches, sea stacks, and marine terraces dominate the scenery along the emergent coast of northern California. In places where rivers deposit sediments along the coast, or where longshore currents are active, sand can accumulate to form depositional features such as spits, baymouth bars, and tombolos.

Dick Hilton

● Many people live near the actively eroding coast in

northern California. Coastal development is commonly at risk because of cliff retreat and mass wasting events that undermine support for structures. In addition, earthquakes in the Pacific Ocean basin can generate tsunamis that have affected several coastal communities. Human modifications, such as breakwaters, to protect portions of the coast can intensify erosion elsewhere and interrupt the transportation of sediment along the coast.

Review Workbook

41

Review Workbook
ESSENTIAL QUESTIONS SUMMARY

ferous stream deposits known as “auriferous” gravels, and in association with zoic-age base quartz veins, or lodes, in Mesozoic-age basement rocks.

Northern California.1 Introduction
Ⅲ What are northern California’s physiographic provinces?
Physiographic provinces are regions of distinctive geology, landforms, climate, geomorphic trends, soils and vegetation, and drainage. In northern California, seven physiographic provinces are recognized: the Sierra Nevada, the Klamath Mountains, the Great Valley, the
Coast Ranges, the Cascade Range and Modoc Plateau, and the Basin and Range. Collectively, these provinces endow northern California with extraordinary geologic diversity.

Ⅲ What is the Mother Lode?

Ⅲ What is the Farallon subduction zone?
The Farallon subduction zone developed at the convergent plate boundary between the North American and Farallon plates about out 200 million years ago. The entire subduction zone persisted until about
30 million years ago, when portions of it were disrupted by the collision ion between North America and the Farallon-Pacific spreading ridge.
Many of the major geologic features of northern California resulted resul ne. from the convergence of plates in the Farallon subduction zone. ern Ⅲ What two types of plate boundaries exist in northern California today?
North of Cape Mendocino, the Juan de Fuca plate, a remnant of the ate, ntinental
Farallon plate, continues to be subducted under continental lithosphere to the east. South of Cape Mendocino, the transform boundocino nsform merica es ary between the Pacific and North American plates shapes the geologic setting of northern California. The San Andreas fault system ia. A ult s ndary, whe is directly related to the transform boundary, whereas northern
California’s volcanic centers are related to the subduction process. rel th d ar
Ⅲ What are terranes, how do they originate, and why are they important in northern California? a? Terranes are large blocks of rock, typically bounded by faults, reprebo prec i senting seamounts, oceanic rises, reefs, island arcs, or other oceanic features accreted to continents at convergent plate boundaries. ts convergen daries. k ached Terranes are comprised of exotic rocks that become detached from a subducting plate and are sutured to the edge of the overriding contiubdu riding n
Califo
s acc er nent. In northern California, many terranes accreted over the past
500 million years have been identified by geologists. ye ologis
Northern Ca orthern California.2 The Sierra Nevada: California’s Geologic nia.2 Th alifornia’s Ge bone Backbone ra Nevad
Ⅲ What is the Sierra Nevada batholith?
Th
s
The Sierra Nevada batholith is a large body of plutonic rock that comprises the core of the Sierra Nevada, California’s best known com e
N
mo mountain system. It consists of more than 100 individual plutons emplaced mostly between 140 million and 80 million years ago. ween mil

Wh s urr
Ⅲ What kinds of rocks surround the Sierra Nevada batholith?

The Mother Lode is a northwest-trending belt of gold mineralization north nding g in the Sierra Nevada foothills. Along this trend modern placer a Alo is m deposits, ancient gold-bearing river sediments, and lode deposits assoold-bearing s ent ciated with quartz veins have all produced significant amounts of gold. produc Northern California.3 The Klamath Mountains
a.3
Klamat
Mo
Ⅲ In what ways are the Klamath Mountains and the Sierra Nevada
?
Similar? he hM
In both the Sierra Nevada and the Klamath Mountains bodies of
M
-age hav ced o d Mesozoic-age granite have been emplaced in older metamorphic sent s ac rocks representing numerous terranes accreted to North America. ld m lizat o are Gold mineralization is common to both areas as well, but slices of oceanic lith phe (ophiolites) are more common in the Klamath re co lithosphere Mountains t
Klam
than in the Sierra. In general, the Klamath Mountains sent ntinuat n geologica represent a northwest continuation of the geological trends of the
Sierra N
Nevada.

ds s oph ites
Ⅲ What kinds of rocks comprise the ophiolites in the Klamath
Mountains?
Ophiolites represent disrupted fragments of oceanic lithosphere consent di pted fr sisting of mafic and ultramafic igneous rocks associated with oceanic a ultram i sediments. During emplacement of ophiolite sequences on land by ts. place plate convergence, the oceanic rocks generally became deformed and metamorphosed to gre meta orphosed greenstone, serpentinite, schists, and other altered materials. mat
Northe
Northern California.4 The Great Valley

f
Ⅲ What factors have led to the formation of fertile soils in the Great
Valley?
Valley
The soils of the Great Valley have developed on alluvium derived from the weathering granitic and volcanic bedrock in the Sierra fr Nevada and adjacent ranges and deposited on a nearly flat surface by the Sacramento and San Joaquin River systems. These factors, along with the warm climate and abundant organic matter of the Central
Valley, promote the formation of nutrient-rich, water-retaining soils that can support high agricultural productivity.

Ⅲ What was the origin of the sedimentary rocks in the Great Valley
Sequence?
The Great Valley Sequence consists of sandstone, mudstone, and shale that accumulated in a forearc basin associated with the Mesozoic
Farallon subduction zone. These rocks include sediments that were transported by turbidity currents to the deep seafloor where they built large submarine fans.

Metam
Metamorphic rocks of Paleozoic and Mesozoic age surround the ic o
Sierra Nevada batholith. Most of these rocks are of oceanic origin and ada batholith represent terranes accreted to North America before the intrusion of pre anes accr magma into the batholith during the Mesozoic Era. h Ⅲ What is the origin of the natural gas produced in the Great Valley?

Ⅲ When and how was the modern Sierra Nevada uplifted?

Ⅲ Why is the Great Valley so prone to chronic flooding?

Though the Sierra Nevada region may have been elevated since the late Mesozoic Era, a pulse of rapid uplift 5 million to 10 million years ago lifted the mountain range to its present height. The recent ascent occurred via normal faulting and westward tilting along the eastern escarpment of the range.

Ⅲ What types of gold deposits occur in the Sierra Nevada?
Gold occurs in the Sierra Nevada regions as particles in modern river sediments (placer deposits), as flakes and nuggets in Eocene-age

The natural gas resources of the northern Great Valley (Sacramento
Valley) originate from organic matter trapped underground in the oceanic sediments of Mesozoic and Cenozoic age.
The Great Valley is remarkably flat, surrounded by elevated terrain that receives abundant rain and snow, and has a climate that can produce both heavy winter rains and rapid spring melt of snow. The broad floodplain adjacent to the Sacramento and other rivers in the
Great Valley has developed over thousands of years of recurring floods. The natural flooding behavior of rivers in the Great Valley is a serious threat to the populous cities, large farms, and industrial centers that have been established in the region.

42

Geology of Northern California

Northern California.5 The Northern Coast Ranges
Ⅲ What is the Franciscan complex and how did it form?
The Franciscan complex is a complex assemblage of deformed rock consisting of oceanic sediments such as graywacke, shale, and chert mixed with metamorphosed igneous rocks. Such a complex rock mixture is known as a mélange, and represents an amalgamation of rock that formed in the Farallon subduction zone.

Ⅲ How did the San Andreas fault system originate?
The San Andreas fault system is the consequence of the development of a transform boundary between the North American and Pacific plates over the past 30 million years. The faults in the San Andreas system are mostly right-lateral strike-slip faults that accommodate the displacement between the Pacific plate (moving to the northwest) and the North American plate.

Ⅲ What is the Salinian block?
The Salinian block is a large terrane in the northern Coast Ranges that includes granitic and metamorphic basement rocks, similar to those of the Sierra Nevada, overlain by Cenozoic-age sediments of oceanic and terrestrial origin. From its original position near the south end of the
Sierra Nevada, the Salinian block has been transported more than
300 kilometers northwest along the San Andreas fault system.

Ⅲ How old are the northern Coast Ranges and what tectonic forces elevated them?
The northern Coast Ranges are relatively young mountains, rising to their present elevations in the past 3 million to 4 million years. The forces that lifted these coastal mountains are probably related to compression between fault slices in the San Andreas system and the norththward migration of the Mendocino triple junction.
Northern California.6 Volcanoes of the Cascade Range and
Modoc Plateau
R
Ⅲ What California volcanoes are part of the Cascade Range?
Mount Shasta (elevation 4,319 meters/14.161 feet), Mount Lassen
.161
assen
(elevation 3,188 meters/10,457 feet), and the Medicine Lake highland th hla are the principal volcanic features of the Cascade Range in northern n California.

Ⅲ What is the Cascadia subduction zone?
The Cascadia subduction zone generates the magma that sustains the volcanic activity in the Cascade Range. This subduction zone is creaty th ed by the downward movement of the Juan de Fuca plate beneath the wnward move f Jua benea northwest edge of North America. The Cascadia subduction zone is
Am
.
Ca
n only partly in northern California, extending northward from near orthern Cali a, extendi
Cape Mendocino to British Columbia. o Britis mbia. d th Ⅲ What kind of volcanic activity typifies the Cascade Range?
Cascade volcanoes are mostly composite cone (or stratovolcanoes)
(o
that alternate between effusive eruptions of viscous andesitic lava and rnate betw s visco more violent explosive events that result in pyroclatic flows. The len esult pyroc explosive phase of activity was most recently demonstrated by the dem 1980 eruption of Mount St. Helens in Washington, but the geologic
Mo
Wash record provides evidence of much more violent prehistoric activity in es evid f several Cascade Range locations. de Ran ations. La in Ⅲ Why is Mount Lassen an especially interesting volcano in the Cascade
Range?
Mount Lassen is noteworthy among Cascade Range volcanoes because it is the most recently active volcano in California (it last erupted in
1914–1921) and is classified as a lava dome, not a composite volcano.
Geothermal activity and ongoing seismic activity in the Mount Lassen region suggest that the underground magma system is still active.
Northern California.7 The Basin and Range of Northeast
California
Ⅲ What tectonic forces are responsible for the pattern of alternating mountains and valleys in the Basin and Range province?
Tensional, or stretching, forces have broken the crust in the Basin and
Range into hundreds of blocks bounded by normal faults. The blocks

displaced downward (the hanging wall) along these faults are represented by the low valleys that separate the elevated ranges.

Ⅲ What mountains and basins in northern California belong to the
Basin and Range province?
The Warner Range, more than 3,000 meters (9,870 feet) high, is the
870 f best example of a Basin and Range mountain system in northeast nor California. The adjacent Surprise Valley and the Lake Tahoe basin to he Taho the south both developed as a consequence of the downward displacence displ ment along normal faults.
Northern California.8 The Northern California Ice Ages rthern thern exper Ⅲ During what time periods did northern California experience Ice Age conditions? Multiple cycles of climate change affected northern California during mat ffected the Pleistocene Epoch, but the Tahoe and Tioga glaciations, about h, 160,000 and 20,000 years ago respectively, are the two best docuars respect mented episodes.

landscap s result ne Epo
Ⅲ What landscape features resulted from the Pleistocene Epoch glaciations in northern California?
Calif
?
Valley glaciers developed in the mountainous regions of northern gions no
California several times during the Pleistocene Ice Ages. Erosional rnia dur ce E features produced by these glaciers are widespread and include polfe s ad pol ished and striated bedrock surfaces, U-shaped glacial troughs, horns, ishe d bedroc shaped gl horn arêtes, and cirques. In addition, sediments released from melting ice rêtes, irques ments m ic at the clos of the glacial episodes left glacial erratics and moraine he close isod ft d morain deposits in many locations. m pe n hern
Ⅲ How did the Pleistocene landscape of northern California differ from the modern setting?
Du
During the periods of maximum glacial advance, the crest of the gla ad
Sierra Nevada was buried under a thick ice cap. From this high mass
Si
as buried thic of ice, valley glaciers extended down the major canyons, in some cases acier tended completely filling them with ice. Valley glaciers also existed in the ling t th ice
Klamath Mountains and in the Cascade Range. In the lowland valleys, tains a pluvial lakes expanded and withered repeatedly in rhythm with the al ex ded an
Pleistocene climatic oscillations. The decrease in sea level that accomstocene clima sci panied the glacial intervals exposed the floor of San Francisco Bay as a d in broad river valley several times. The Pacific shore was more than 30 ver kilometers west of its present location. ilome rs
Northern California.9 Northern California Earthquakes
Ⅲ What plate tectonic settings are associated with northern California earthquakes? ea
Northern California earthquakes occur mostly along the San Andreas
N
fault system (part of a transform plate boundary), in the Cascadia subduction zone (a convergent boundary), or along the eastern escarpment of the Sierra Nevada (a zone of developing plate divergence, or rifting).

Ⅲ What is the likelihood of another major earthquake in northern
California?
Given the tectonic forces currently affecting the crust in northern
California, another major earthquake is certain to occur eventually.
Modern techniques for forecasting earthquake activity, including the analysis of seismic gaps, suggest that there is a 67% probability of an earthquake exceeding magnitude 6.7 in the next 30 years. The faults in the San Francisco Bay area, such as the Hayward-Rogers Creek,
Calaveras, and San Andreas faults, are mostly likely to produce this earthquake. In this densely populated and intensively developed region, such an earthquake could have disastrous consequences.

Ⅲ What might be the effects of a large northern California earthquake?
Depending on the location of the epicenter and the magnitude, the ground shaking of a large earthquake in northern California could result in damaged or destroyed buildings, fires, collapsed bridges, disrupted communications and transportation networks, contaminated drinking water, liquefaction of soils, and tsunamis along the coast.
Millions of people and billions of dollars in property are vulnerable to earthquake hazards in northern California.

Review Workbook

Ⅲ Given the severity of the potential hazards, how can the effects of earthquakes be minimized in northern California?
Experience has shown that earthquakes hazards can be reduced through a combination of wise land use, continuous seismic monitoring, stringent building codes, and personal and public preparedness.
Fortunately, most populated regions in northern California have plans in place for effective responses to a potential earthquake, and the regulated development of communities has reduced the number of people at risk.
Northern California. 10 Living on the Edge: Coastal Hazards in
Northern California
Ⅲ In comparison to the coast of southern California, why is the northern
California shoreline so rugged and scenic?
The northern California coast trends northwest, directly facing the waves, storms, and tsunamis generated in the Pacific Ocean basin. In addition, the emergent coast of northern California continuously exposes new bedrock to the vigorous erosion that results in such landforms as sea cliffs, sea caves, sea arches, and sea stacks. Finally, in most places, the bedrock of the northern California coast is a complex assemblage of hard and soft rock that erodes to an irregular shore with ith projecting headlands separated by recessed coves and pocket beaches.
s.

43

Ⅲ What coastal hazards exist in northern California?
Coastal hazards in northern California are primarily related to erosion, mass wasting, or seismic activity. Erosion of sea cliffs by waves causes the cliffs to retreat at rates as high as a meter per year, potentially affecting structures overlooking the ocean. Mass wasting occurs king descen readily along the steep slopes descending to the sea along the emergent coast, especially during the rainy season or where the rocks are he weak and/or absorb water. Tsunamis generated by earthquakes in the mis gener
Pacific basin have struck the California coast many times, some from uck nia very distant epicenters. ers. coast za Ⅲ How do human activities affect coastal hazards?
Whenever humans modify the natural conditions along the coast, som dynamic responses result that can sometimes intensify coastal hazards.
For example, although breakwaters can protect one location from amp h c rosion, the eflect th wave erosion, they also deflect the energy of approaching waves to other places where erosion increases. When coastal slopes are modiaces erosi he o ings fied for the construction of roads or buildings, mass wasting can result if the balance between gravity and friction on a table stable slope is nce betw on o compromised. Today, in most coastal communities, awareness of these pr ed. T l com coastal hazards has led to restrictions on development, but there are oastal ha ds h ons dev still many places where people and property are at risk. pl d

ESSENTIAL TERMS TO KNOW
NOW
accretionary terranes, terranes – a large block of rock with ck emergent coast – a coast where land has risen with respect to sea st st whe

characteristics different from surrounding blocks. Terranes are typig bl rranes cally bounded by large faults and are thought to represent seamounts, thoug nt seam oceanic rises, reefs, island arcs, or other oceanic features accreted to her fe crete continents at convergent plate boundaries. ries. Farallon plate – an ancient lithospheric plate that separated the lon a ncie

alluvium – a general term for sediment transported and deposited sed ransported by running water.

arête – a narrow jagged ridge separating two glacial valleys or cirques. idg i ques. basalt – a dark-colored fine-grained igneous rock that forms from ne-grained om magma of mafic (45% to 52% silica) composition.
2%
compos

batholith – a large body of plutonic igneous rock with a surface utonic ign exposure exceeding 100 square kilometers. baymou baymouth bar – a spit that extends across the mouth of a bay, closspi ss ing it off from the open ocean. oce blueschist – a foliated metamorphic rock that contains glaucophane, liated me glauc a bluish-colored mineral of the amphibole group. uish-col breccia – a detrital sedimentary rock consisting of angular rock partal sedim gul ticles tic larger than 2 millimeters.

ca caldera – a large circular to oval depression that develops on the lar summit of a volcano in response to the partial evacuation of the t underlying magma chamber. hamber. cher chert – a fine-grained nonclastic sedimentary rock consisting of ne-gr microcrystalline quartz, which may include the skeletons of silicamicro lline quar secreting microorganism such as radiolarian. microorganisms secre

level.
North America plate fr from the Pacific plate during the Mesozoic and h Early Cenozoic Eras. Remnants of the Farallon plate include the enozoic modern Juan de Fuca, Gorda, Cocos, and Rivera plates in the eastern mo an
Pacific
Pacif Ocean basin. b Farallon subduction zone – the east-dipping zone of subduction rallo developed dev ped where the Farallon plate descended beneath the western edge of the North American plate during the Late Mesozoic and
Early
Ea Cenozoic eras.

fault – a fracture in the Earth’s crust that includes displaced rock masses. felsic – a term describing the composition of magma or igneous rock that contains more than 65% silica and is rich in sodium, potassium, and aluminum.

forearc basin – a basin of sediment accumulation located between a volcanic arc mountain system and an offshore oceanic trench. Forearc basins are developed in association with subduction zones at convergent plate boundaries.

fumerole – a vent at the surface that emits volcanic gases. gabbro – a dark-colored intrusive igneous rock of mafic composition. glacial erratic – a rock carried by a glacier from its source to a surface of dissimilar bedrock.

cinder cone – a small steep-sided volcano consisting of a loose sma accumulation of volcanic cinder.

glacial polish – a smooth and glossy bedrock surface produced by

cirque – a generally circular depression that forms in the uppermost

glacial striation – a linear scratch or shallow groove on a bedrock

the movement of a glacier over it.

reaches of glacial troughs occupied by valley glaciers.

surface produced by the movement of a sediment-laden glacier over it.

composite volcano – a volcano composed of lava flows, pyroclastic layers, and mudflow deposits; sometimes referred to as a stratovolcano.

graben – a term for the block of rock displaced downward as the

dacite – an extrusive igneous rock intermediate in composition

granite – a light-colored plutonic igneous rock of felsic composition. greenstone – a metamorphic rock containing the greenish minerals

between rhyolite and andesite.

diorite – an intrusive igneous rock with nearly equal amounts of dark-colored and light-colored minerals. Diorite is intermediate in composition between granite (felsic) and gabbro (mafic).

hanging wall of a normal fault.

epidote, chlorite, or amphibole that results from the metamorphism of mafic igneous materials.

headland – land that projects seaward from an irregular shore line.

44

Geology of Northern California

horn – a steep and jagged peak having the form of a pyramid that developed between glacial cirques.

horst – a term describing the block of rock displaced upward as the footwall of a normal fault.

roof pendant – a mass of metamorphic rock preserved above the top of an underlying pluton. Roof pendants represent remnants of the
“country rock” into which subterranean magma intruded.

inverted topography – a term describing any landscape feature

sea arch – an archlike exposure of coastal bedrock that results from k th dland wave erosion of sea caves along a projecting headland.

that originated at a low elevation, but is currently elevated above the surrounding terrain, or vice versa.

sea stack – a remnant of bedrock on a wave-cut marine terrace.
-cut
te seismic gap – a portion of an active fault that has produced fewer produ lahar – a volcanic mudflow composed of water, ash, and particles of

and smaller earthquakes than other segments of the same fault. egmen e

volcanic rock.

Sierra Nevada batholith – a large composite pluton consisting of rge n consist

limestone – a chemical sedimentary rock composed of calcite, CaCO3. lode – an ore deposit in which the valuable commodity is concentrated

igneou km more than 100 individual masses of intrusive igneous rock making up the basement of the Sierra Nevada.
a.

in a vein or pod within crystalline igneous or metamorphic rock.

serpentinite – a greenish-black metamorphic rock that forms from uch abbro, per mafic igneous rocks such as basalt, gabbro, and peridotite. Sepentinite occurs in the western metamorphic terranes of the Sierra Nevada, in metamorph the Klamath Mountains, and in the northern Coast Ranges. It is the s, nort official California state rock. ock. mafic – a term describing the composition of magma or igneous rock that contains 45% to 52% silica and is rich in calcium, magnesium, and iron.

mass wasting – the downslope movement of material under the influence of gravity including rockslides, mudslides, rockfalls, and debris flows.

spit – a sandy projection of a beach into a body of water such as a bay b water s

mélange – a deformed and sheared mass of metamorphic and sedi-

superterrane – a large fragment of crustal rocks that is comprised rter fr m ks c

mentary rocks that forms in seduction zones.

moraine – a ridge of mound of unsorted and nonstratified glacial sediment (till) deposited by melting glaciers.

mudstone – a fine-grained clastic sedimentary rock consisting of a g mixture of sand, silt, and clay-size particles.

normal faults – a dip-slip fault in which the hanging wall has moved all h d rom t down relative to the footwall. Normal faults develop from tensional stress and are most common in areas of divergent plate boundaries ate ophiolite – a sequence of mafic igneous rock representing oceanic p ting ocea sist p asalt, lithosphere and upper mantle; ophiolites consist of pillow basalt, sheeted basalt dikes, layered and massive gabbro, and mantle periodite. nd m e Pangaea – a supercontinent consisting of all the Earth’s landmasses f es a, y that existed at the end of the Paleozoic Era, about 250 million years ago.

peridotite –an ultramafic igneous rock thought to comprise much hought compri of the upper mantle. physiographic province – a region of unique geology, landforms, adja drainage, soils, climate and flora and fauna distinct from adjacent climate, areas on the basis of these features.
s.

pillow structures – rounded or bulbous structures that develop in ctures round r elop i er lava erupted under water.

placer – a term applied to stream-transported sediment such as sand dt m-transpo d el co and gravel that contains significant quantities of a valuable mineral such as gold, silver, or platinum. silver pluton – an intrusive igneous body that forms when magma cools y w d c and crystallizes below the surface and within the crust.

plutonic – a term describing the origin of an igneous rock that crysd g in n cru tallized from magma within the crust.

pocket beach– a small beach along an irregular coast located in a ch s each roje adlands. recess between projecting headlands.

pyroclastic – a term that describes the fragmental texture or character of volcanic rock such as tuff and volcanic breccia produced during explosive eruptions.

ary. or estuary. ller n of smaller terranes amalgam amalgamated during accretion to a continental margin. ma

syncli syncline – a down-arched or concave-upward fold in a sequence of down-ar ve-upward o k layers. ed ion con rm tha rock layer (Note: the above definition is used to conform to that of
Wincander, and Hazlett)
Monroe, Wi ett) tombolo – a spit that extends outward into the sea or a lake that l utward to a t nd. connects an island or sea stack to land.

tra transform plate boundary – a boundary between plates that b nd li oth r slide past one another and whose cru is neither produced or crust destroyed. The strik lip faults of the San Andreas fault zone e strike-slip fau ounda represent the trans transform boundary between the Pacific and North
America p plates. triple junction – the point at which three lithosphere plates meet. le p tsunami – a large sea wave that is produced when mass on the nami s seaf seafloor is suddenly displaced by earthquakes, volcanic eruptions, or submarine landsl subma ne landslides.

tuff – a fine-grained pyroclastic igneous rock that consists of consolifi dated par particles of volcanic ash. turbid turbidity currents – a dense mixture of sediment and water that flo downslope on the ocean floor. flows ultramafic – a term describing the composition of magma or igneous rock that contains 40% or less silica and abundant magnesium and iron.

volcanic – a term describing the origin of igneous rocks from magma that cools rapidly after its eruption during a volcanic event.

volcanic arc – a chain of volcanoes that develops on the earth’s surface where magma rises from an underlying subduction zone.

welded tuffs – a hard volcanic rock composed of tiny ash articles that are firmly welded together by the heat associated with an explosive volcanic eruption.

xenoliths – a type of inclusion in which a fragment of older rock is incorporated into younger igneous rock.

Review Workbook

45

More on Northern California Geology
California’s stunning scenery, rich geologic heritage, and abundant natural resources have stimulated considerable interest from geologists for more than two centuries. Scientific studies in the state have resulted in a vast body of technical literature on California geology.
Fortunately, there are many excellent summaries available that describe the geologic features of northern California in comprehensible terms and in the context of modern geologic concepts. Your college or local library may have copies of the following books that you may find valuable in learning more about the geology of northern
California:
Assembling California, John McPhee, 2003, Farrar, Straus, and
Giroux, New York, 304 pages.
Geology Of California, Deborah R. Harden, 2004, Pearson-Prentice
Hall, Upper Saddle River, NJ, 552 pages.
Geology of Northern California, E.H. Bailey, editor, 1966, California
Division of Mines and Geology (California Geological Survey)
Bulletin 190, 508 pages.

Roadside Geology of Northern and Central Califorina, by David Alt and Donald Hyndman, 2003, Mountain Press Publishing, ou Missoula, MT, 384 pages.
The California Geological Survey is an excellent source of informarvey exce tion on earthquakes, rocks and minerals, geologic hazards, and mapgeo ping programs in the Golden State. The Web site at olde e. http://www.conservation.ca.gov/cgs contains links to a vast array of tion.ca ontains information on the geology of California. In addition, the U.S.
Cali
a. ad
Geological Survey Web site (http://www.usgs.gov) offers a search fea(http://w ture that allows you to retrieve many USGS publications on specific u U aspects of California geology, water resources, and geologic hazards. a re
Finally, check with the geology department at your local college and y, e dep university about other sources of information and additional opportuity oth nities to explore California geology. Most college geology programs co offer field courses and introductory lecture courses that address the intr ure co re diverse geologic features of the state. d ologic

The Geology of Northern California provides an overview of the physiographic features and geological history of northern California’s magnificent landscape.
Written to accompanying any college-level earth science course, this module explores the rich geological heritage of northern California. The major geological features of the northern Sierra Nevada, the Klamath Mountains, the Cascade
Range region, the northern Great Valley, the Basin and Range, and the northern
Coast Ranges are explored with an emphasis on applying the fundamental concepts of modern geology to the interpretation of the scenic and varied terrain.
This module also surveys the affect of geological processes on humans in northern
California. The origin of the rich mineral and fuel resources of the Golden State are explored in the context of hundreds of million of years of the tectonic evolution.
In addition, the multiple hazards to people and property that result from California’s on-going geological evolution are considered. We learn why future earthquakes, volcanic eruptions, slope failures, and floods will affect northern Californians, and how an understanding of these threats can help reduce their potential impacts.
Use this module to help make any introductory geology course more relevant to egio college students throughout northern California by providing local and regional ith yo examples of the consequences of dynamic geologic processes. Work with your
Cengage Learning representative to learn how you can incorporate this or rse any of the modules from our Regional Geology Series into your course. mo out
Visit http://custom.cengage.com/regional_geology/ to learn more about other bonus content available and how to order it.
About the Author:

Ambient Images/drr.net

Frank DeCourten is Professor of Earth Science at Sierra College in Grass Valley, a V
California, where he teaches courses in geology, geography, and environmental y, geo
,
sciences. He has written four books and numerous research papers and popular merous rese apers a articles on the geology of western North America. erica. Visit Cengage Learning
Custom Solutions online at www.cengagecustom.com For your lifelong learning needs: www.academic.cengage.com