Structure and composition of the Earth

The overall composition of the Earth is very similar to that of meteorites, and because of this, it is thought that the Earth originally formed from Planetesimals composed largely of metallic iron and silicates.

What makes Earth unique?
Soon after the Earth formed, unique processes occurred - division into metallic core, silicate mantle and crust - which, along with surface water, made it different from the other planets in our Solar System. The formation of the early mantle was important as it consisted primarily of ferromagnesium silicate minerals, some of which contained water as an essential component (e.g. amphibole group minerals). Water-bearing magmas (molten rock) from deep in the lower mantle then rose towards the surface (being liquid, they were lighter than the surrounding solid rock) and emerged as volcanic eruptions.

The Earth's atmosphere and hydrosphere developed from the degassing (loss of gaseous elements such as carbon, hydrogen and oxygen) of the early-formed core and mantle during this volcanic activity. In the present, abundant gases are still released from the Earth during volcanic eruptions and these are mainly composed of water (77%), carbon dioxide (12%), sulfur dioxide (7%), and nitrogen (3%), with minor amounts of hydrogen, carbon monoxide, sulfur, chlorine and argon.

The Earth can be divided into two main parts.
Atmosphere: measured from the surface of the Earth upwards to 150 km (anything above this is called space)
Solid Earth: measured from the surface of the Earth downwards to the core
The atmosphere
The atmosphere makes up less than one millionth of the total mass of the Earth, and contains mainly nitrogen and oxygen (99% of the total) as gases. Other important components of the atmosphere are hydrogen, carbon dioxide, and inert gases such as argon and helium.

The atmosphere is divided (measured from the surface of the Earth) into:

Troposphere (0 km - 13 km)
Ozone Layer (13 km - 25 km)
Stratosphere (25 km - 50 km)
Mesosphere (50 km - 75 km)
Thermosphere (75 km - 150 km)

The solid Earth
This can be divided into the:
Biosphere (water, organic substances and skeletal matter) - solid and liquid - and includes all forms of life (e.g. plants and animals) and their products (e.g. skeletons) both on land and in the sea
Hydrosphere (fresh and salt water, snow and ice) - mainly liquid, some solid - includes all forms of water
Internal structure of the Earth, which includes:
Crust (normal silicate rocks such as granite and basalt) - solid
Mantle (ferromagnesium-rich silicate rocks) - solid
Core (iron-nickel alloy) - liquid upper part and solid lower part
Structure of the Earth
Although geologists generally don't study the atmosphere, biosphere or hydrosphere, all three are vitally important in understanding geological processes, particularly weathering and erosion, and the formation of many sedimentary rocks (e.g. formation of coral reefs in shallow warm seas).

Properties of Minerals
The Physical properties of minerals are used by Mineralogists to help determine the identity of a specimen. Some of the tests can be performed easily in the field, while others require laboratory equipment. For the beginning student of geology, there are a number of simple tests that can be used with a good degree of accuracy. The list of tests is in a suggested order, progressing from simple experimentation and observation to more complicated, either in procedure or concept. |

Properties of Minerals
The following physical properties of minerals can be easily used to identify a mineral: 1. Color 2. Streak 3. Hardness 4. Cleavage or Fracture 5. Crystalline Structure 6. Diaphaneity or Amount of Transparency 7. Tenacity 8. Magnetism 9. Luster 10. Odor 11. Taste 12. Specific Gravity
Below is a detailed description of each of these properties of minerals.
Properties of Minerals- A Detailed Description
Color
Most minerals have a distinctive color that can be used for identification. In opaque minerals, the color tends to be more consistent, so learning the colors associated with these minerals can be very helpful in identification. Translucent to transparent minerals have a much more varied degree of color due to the presence of trace minerals. Therefore, color alone is not reliable as a single identifying characteristic.

Streak
Streak is the color of the mineral in powdered form. Streak shows the true color of the mineral. In large solid form, trace minerals can change the color appearance of a mineral by reflecting the light in a certain way. Trace minerals have little influence on the reflection of the small powdery particles of the streak.
The streak of metallic minerals tends to appear dark because the small particles of the streak absorb the light hitting them. Non-metallic particles tend to reflect most of the light so they appear lighter in color or almost white.
Because streak is a more accurate illustration of the mineral’s color, streak is a more reliable property of minerals than color for identification.
Hardness
Hardness is one of the better properties of minerals to use for identifying a mineral. Hardness is a measure of the mineral’s resistance to scratching. The Mohs scale is a set of 10 minerals whose hardness is known. The softest mineral, talc, has a Mohs scale rating of one. Diamond is the hardest mineral and has a rating of ten. Softer minerals can be scratched by harder minerals because the forces that hold the crystals together are weaker and can be broken by the harder mineral.
The following is a listing of the minerals of the Mohs scale and their rating: 1. Talc 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Orthoclase Feldspar 7. Quartz 8. Topaz 9. Corundum 10. Diamond
Cleavage & Fracture
Minerals tend to break along lines or smooth surfaces when hit sharply. Different minerals break in different ways showing different types of cleavage.
Cleavage is defined using two sets of criteria. The first set of criteria describes how easily the cleavage is obtained. Cleavage is considered perfect if it is easily obtained and the cleavage planes are easily distinguished. It is considered good if the cleavage is produced with some difficulty but has obvious cleavage planes. Finally it is considered imperfect if cleavage is obtained with difficulty and some of the planes are difficult to distinguish.
The second set of criteria is the direction of the cleavage surfaces. The names correspond to the shape formed by the cleavage surfaces: Cubic, rhombohedral, octahedral, dodecahedral, basal or prismatic. These criteria are defined specifically by the angles of the cleavage lines as indicated in the chart below:
Cleavage Type Angles
Cubic Cleaves in three directions @ 90o to one another Rhombohedral cleaves in three directions but not @ 90o to one another Octahedral Cleaves in four directions Dodecahedral Cleaves in six directions Basal Cleaves in one direction Prismatic Cleaves in two directions
Fracture describes the quality of the cleavage surface. Most minerals display either uneven or grainy fracture, conchoidal (curved, shell-like lines) fracture, or hackly (rough, jagged) fracture.
Crystalline Structure
Mineral crystals occur in various shapes and sizes. The particular shape is determined by the arrangement of the atoms, molecules or ions that make up the crystal and how they are joined. This is called the crystal lattice. There are degrees of crystalline structure, in which the fibers of the crystal become increasingly difficult or impossible to see with the naked eye or the use of a hand lens. Microcrystalline and cryptocrystalline structures can only be viewed using high magnification. If there is no crystalline structure, it is called amorphous. However, there are very few amorphous crystals and these are only observed under extremely high magnification.
Transparency or Diaphaneity
Diaphaneity is a mineral’s degree of transparency or ability to allow light to pass through it. The degree of transparency may also depend on the thickness of the mineral.
Tenacity
Tenacity is the characteristic that describes how the particles of a mineral hold together or resist separation. The chart below gives the list of terms used to describe tenacity and a description of each term.
Magnetism
Magnetism is the characteristic that allows a mineral to attract or repel other magnetic materials. It can be difficult to determine the differences between the various types of magnetism, but it is worth knowing that there are distinctions made.
Luster
Luster is the property of minerals that indicates how much the surface of a mineral reflects light. The luster of a mineral is affected by the brilliance of the light used to observe the mineral surface. Luster of a mineral is described in the following terms:
Metallic mineral is opaque and reflects light as a metal would. Sub metallic the mineral is opaque and dull. The mineral is dark colored. Non-metallic The mineral does not reflect light like a metal.
Non-metallic minerals are described using modifiers that refer to commonly known qualities.
Waxy, the mineral looks like paraffin or wax. Vitreous, the mineral looks like broken glass. Pearly, the mineral appears iridescent, like a pearl. Silky, the mineral looks fibrous, like silk. Greasy, the mineral looks like oil on water. Resinous, the mineral looks like hardened tree sap (resin). Adamantine, the mineral looks brilliant, like a diamond.

Odor
Most minerals have no odor unless they are acted upon in one of the following ways: moistened, heated, breathed upon, or rubbed.
Taste
Only soluble minerals have a taste, but it is very important that minerals not be placed in the mouth or on the tongue. You should not test for this property in the classroom.
Specific Gravity
Specific Gravity of a mineral is a comparison or ratio of the weight of the mineral to the weight of an equal amount of water. The weight of the equal amount of water is found by finding the difference between the weight of the mineral in air and the weight of the mineral in water.
Knowing the properties of minerals will help you to identify minerals in the field.

Types of Rocks What are the three types of rocks? First we have igneous rocks, then, metamorphic rocks and lastly sedimentary rocks.

What are Igneous Rocks?

Igneous rocks are formed from the solidification of molten rock material. There are two basic types: 1) intrusive igneous rocks such as diorite, gabbros, granite and pegmatite that solidify below Earth's surface.
2) Extrusive igneous rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface.
What are Metamorphic Rocks?

Metamorphic rocks have been modified by heat, pressure and chemical process usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks.
There are two basic types of metamorphic rocks:
1) Foliated metamorphic rocks such as gneiss, phyllite, schist and slate which have a layered or banded appearance that is produced by exposure to heat and directed pressure.
2) Non-foliated metamorphic rocks such as marble and quartzite which do not have a layered or banded appearance.

What are Sedimentary Rocks?

Sedimentary rocks are formed by the accumulation of sediments.
There are three basic types of sedimentary rocks:
1) Clastic sedimentary rocks such as breccia, conglomerate, sandstone and shale that are formed from mechanical weathering debris;
2) Chemical sedimentary rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and, 3) Organic sedimentary rocks such as coal and some limestones which form from the accumulation of plant or animal debris.

PLATE TECTONIC

Plate Tectonics is a theory developed in the late 1960s, to explain how the outer layers of the Earth move and deform. The theory has caused a revolution in the way we think about the Earth. Since the development of the theory, geologists have had to reexamine almost every aspect of Geology. Plate tectonics has proven to be so useful that it can predict geologic events and explain almost all aspects of what we see on the Earth.

Tectonic Theories
Tectonic theories attempt to explain why mountains, earthquakes, and volcanoes occur where they do, the ages of deformational events, and the ages and shapes of continents and ocean basins. * Late 19th Century Theories

* Contraction of the Earth due to cooling. This is analogous to what happens to the skin of an apple as the interior shrinks as it dehydrates. It could explain compressional features, like fold/thrust mountain belts, but could not explain extensional features, such as rift valleys and ocean basins. Nor could it explain the shapes and positions of the continents. * Expansion of the Earth due to heating. This was suggested after radioactivity was discovered. This could explain why the continents are broken up, and could easily explain extensional features, but did not do well at explaining compressional features.

* Wegner's Theory of Continental Drift
Alfred Wegner was a German Meteorologist in the early 1900s who studied ancient climates. Like most people, the jigsaw puzzle appearance of the Atlantic continental margins caught his attention. He put together the evidence of ancient glaciations and the distribution of fossil to formulate a theory that the continents have moved over the surface of the Earth, sometimes forming large supercontinents and other times forming separate continental masses. He proposed that prior to about 200 million years ago all of the continents formed one large land mass that he called Pangaea.
The weakness of Wegner's theory, and the reason it was not readily accepted by geologists was that he proposed that the continents slide over ocean floor. Geophysicists disagreed, stating the ocean floor did not have enough strength to hold the continents and too much frictional resistance would be encountered.

In 1950s and 1960s, studies of the Earth's magnetic field and how it varied through time
(paleomagnetism) provided new evidence that would prove that the continents do indeed drift. In order to understand these developments, we must first discuss the Earth's magnetic field and the study of Paleomagnetism.

The Earth's Magnetic Field and Paleomagnetism

The Earth has a magnetic field that causes a compass needle to always point toward the North magnetic pole, currently located near the rotation pole. The
Earth's magnetic field is what would be expected if there were a large bar magnet located at the center of the Earth (we now know that this is not what causes the magnetic field, but the analogy is still good). The magnetic field is composed of lines of force as shown in the diagram here.
A compass needle or a magnetic weight suspended from a string, points along these lines of force. Note that the lines of force intersect the surface of the Earth at various angles that depend on position on the Earth's surface. This angle is called the magnetic inclination. The inclination is 0o at the magnetic equator and 90o at the magnetic poles. Thus, by measuring the inclination and the angle to the magnetic pole, one can tell position on the Earth relative to the magnetic poles.

In the 1950s it was discovered that when magnetic minerals cool below a temperature called the Curie
Temperature, domains within the magnetic mineral take on an orientation parallel to any external magnetic field present at the time they cooled below this temperature.

At temperatures above the Curie Temperature, permanent magnetization of materials is not possible. Since the magnetic minerals take on the orientation of the magnetic field present during cooling, we can determine the orientation of the magnetic field present at the time the rock containing the mineral cooled below the Curie Temperature, and thus, be able to determine the position of the magnetic pole at that time. This made possible the study of
Paleomagnetism (the history of the Earth's magnetic field). Magnetite is the most common magnetic mineral in the Earth's crust and has a Curie Temperature of 580oC

Initial studies of the how the position of the Earth's magnetic pole varied with time were conducted in Europe. These studies showed that the magnetic pole had apparently moved through time. When similar measurements were made on rocks of various ages in North America, however, a different path of the magnetic pole was found.

This either suggested that (1) the Earth has had more than one magnetic pole at various times in the past (not likely), or (2) that the different continents have moved relative to each other over time. Studies of ancient pole positions for other continents confirmed the latter hypothesis, and seemed to confirm the theory of Continental Drift.

Sea-Floor Spreading
During World War II, geologists employed by the military carried out studies of the sea floor, a part of the Earth that had received little scientific study. The purpose of these studies was to understand the topography of the sea floor to find hiding places for both Allied and enemy submarines. The topographic studies involved measuring the depth to the sea floor. These studies revealed the presence of two important topographic features of the ocean floor:

* Oceanic Ridges - long sinuous ridges that occupy the middle of the Atlantic Ocean and the eastern part of the Pacific Ocean.

* Oceanic Trenches - deep trenches along the margins of continents, particularly surrounding the Pacific Ocean.

Another type of study involved towing a magnetometer (for measuring magnetic materials) behind ships to detect submarines. The records from the magnetometers, however, revealed that there were magnetic anomalies on the sea floor, with magnetic high areas running along the oceanic ridges, and parallel bands of alternating high and low magnetism on either side of the oceanic ridges. Before these features can be understood, we need to first discuss another development in the field of Paleomagnetism - the discovery of reversals of the Earth's magnetic field and the magnetic time scale.

* Reversals of the Earth's Magnetic Field. Studying piles of lava flows on the continents geophysicists found that over short time scales the Earth's magnetic field undergoes polarity reversals (The north magnetic pole becomes the south magnetic pole) By dating the rocks using radiometric dating techniques and correlating the reversals throughout the world they were able to establish the magnetic time scale.

Vine, Matthews, and Morely put this information together with the bands of magnetic stripes on the sea floor and postulated that the bands represents oppositely polarized rocks on either side of the oceanic ridges, and that new oceanic crust and lithosphere was created at the oceanic ridge by eruption and intrusion of magma. As this magma cooled it took on the magnetism of the magnetic field at the time. When the polarity of the field changed new crust and lithosphere created at the ridge would take on the different polarity. This hypothesis led to the theory of sea floor spreading.

If new oceanic crust and lithosphere is continually being created at the oceanic ridges, the oceans should be expanding indefinitely, unless there were mechanisms to destroy the oceanic lithosphere. Benioff zones and the oceanic trenches provided the answer: Oceanic lithosphere returns to the mantle by sliding downward at the oceanic trenches (subducting). Because oceanic lithosphere is cold and brittle, it fractures as it descends back into the mantle. As it fractures it produces earthquakes that get progressively deeper.

Plate Tectonics

By combining the sea floor spreading theory with continental drift and information on global seismicity, the new theory of Plate Tectonics became a coherent theory to explain crustal movements. Plates are composed of lithosphere, about 100 km thick that "float" on the ductile asthenosphere.

While the continents do indeed appear to drift, they do so only because they are part of larger plates that float and move horizontally on the upper mantle asthenosphere. The plates behave as rigid bodies with some ability to flex, but deformation occurs mainly along the boundaries between plates.
The plate boundaries can be identified because they are zones along which earthquakes occur.

Plate interiors have much fewer earthquakes.

Types of Plate Boundaries

There are three types of plate boundaries:
1. Divergent Plate boundaries, where plates move away from each other.
2. Convergent Plate Boundaries, where plates move toward each other.
3. Transform Plate Boundaries, where plates slide past one another.

Divergent Plate Boundaries
These are oceanic ridges where new oceanic lithosphere is created by upwelling mantle that melts, resulting in basaltic magmas which intrude and erupt at the oceanic ridge to create new oceanic lithosphere and crust. As new oceanic lithosphere is created, it is pushed aside in opposite directions. Thus, the age of the oceanic crust becomes progressively older in both directions away from the ridge. Because oceanic lithosphere may get subducted, the age of the ocean basins is relatively young. The oldest oceanic crust occurs farthest away from a ridge. In the Atlantic Ocean, the oldest oceanic crust occurs next to the North American and African continents and is about 160 million years old (Jurassic) In the Pacific Ocean, the oldest crust is also Jurassic in age, and occurs off the coast of Japan.
Because the oceanic ridges are areas of young crust, there is very little sediment accumulation on the ridges. Sediment thickness increases in both directions away of the ridge, and is thickest where the oceanic crust is the oldest.

* Knowing the age of the crust and the distance from the ridge, the relative velocity of the plates can be determined. (Absolute velocity requires further information to be discussed later). * Relative plate velocities vary both for individual plates and for different plates. * Sea floor topography is controlled by the age of the oceanic lithosphere and the rate of spreading. * If the spreading rate (relative velocity) is high, magma must be rising rapidly and the lithosphere is relatively hot beneath the ridge. Thus for fast spreading centers the ridge stands at higher elevations than for slow spreading centers. The rift valley at fast spreading centers is narrower than at slow spreading centers.

As oceanic lithosphere moves away from the ridge, it cools and sinks deeper into the asthenosphere. Thus, the depth to the sea floor increases with increasing age away from the ridge.

Convergent Plate Boundaries

* When a plate of dense oceanic lithosphere moving in one direction collides with a plate moving in the opposite direction, one of the plates subducts beneath the other. Where this occurs an oceanic trench forms on the sea floor and the sinking plate becomes a subduction zone. The Wadati-Benioff Zone, a zone of earthquakes located along the subduction zone, identifies a subduction zone. The earthquakes may extend down to depths of 700 km before the subducting plate heats up and loses its ability to deform in a brittle fashion.

* As the oceanic plate subducts, it begins to heat up causing the release water of water into the overlying mantle asthenosphere. The water reduces the melting temperature and results in the production of magmas. These magmas rise to the surface and create a volcanic arc parallel to the trench.

* If the subduction occurs beneath oceanic lithosphere, an island arc is produced at the surface (such as the Japanese islands, the Aleutian Islands, the Philippine islands, or the Caribbean islands

* If the subduction occurs beneath continental crust, a continental volcanic arc is produced (such as the Cascades of the western U.S., or the Andes mountains of the South America)

* If one of the plates has continental lithosphere on its margin, the oceanic plate will subduct because oceanic lithosphere has a higher density than continental lithosphere.

* Sediment deposited along the convergent margin, and particularly that in the trench will be deformed by thrust faulting. This will break the rocks up into a chaotic mixture of broken, jumbled, and thrust faulted rock know as an accretionary prism.

Transform Plate Boundaries

Where lithospheric plates slide past one another in a horizontal manner, a transform fault is created. Earthquakes along such transform faults are shallow focus earthquakes.

Most transform faults occur where oceanic ridges are offset on the sea floor.
Such offset occurs because spreading takes place on the spherical surface of the Earth, and some parts of a plate must be moving at a higher relative velocity than other parts One of the largest such transform boundaries occurs along the boundary of the North American and Pacific plates and is known as the San Andreas Fault. Here the transform fault cuts through continental lithosphere.
Triple Junctions occur at points where three plates meet.

Hot Spots
Areas where rising plumes of hot mantle reach the surface, usually at locations far removed from plate boundaries are called hot spots. Because plates move relative to the underlying mantle, hot spots beneath oceanic lithosphere produce a chain of volcanoes. A volcano is active while it is over the vicinity of the hot spot, but eventually plate motion results in the volcano moving away from the plume and the volcano becomes extinct and begins to erode

Because the Pacific Plate is one of the faster moving plates, this type of volcanism produces linear chains of islands and seamounts, such as the Hawaiian – Emperor chain, the Line Islands, the Marshall-Ellice Islands, and the Austral seamount chain. In this case the hot spot is currently located beneath the Big Island of Hawaii (the active volcanoes) at the south eastern end of the Hawaiian Ridge.

Many other hot spots are known, most in the ocean basins. Where hot spots occur beneath continental lithosphere, large volumes of rhyolite are produced.

Hot Spots and Absolute Plate Velocities
Plate velocities determined from the rate of sea floor spreading or by making measurements across a plate boundary are only relative velocities. That is we know the velocity of one plate only if we can assume that the adjacent plate is not moving. In order to determine absolute plate velocities, we need some fixed reference point that we know is not moving. One place where this might be possible is in the Pacific Ocean, where the Hawaiian Islands are part of a chain of islands, far removed from any plate boundary, where islands and seamounts in the chain increase in age from the southeast to the northwest Furthermore, the island at the southeast end of the chain, the Big Island of Hawaii, is the only island with currently active volcanoes. The island chain appears to have formed as the Pacific plate moved over a Hot Spot, an area in the Earth's mantle where hot material from the Earth's interior is moving upward. If we can assume that such a hot spot is stationary, then we can calculate the absolute velocity of the Pacific Plate as it has moved over the hot spot.

By using hot spots and, more recently, the Global Positioning System (GPS), to determine absolute velocities, we find that the African Plate is almost stationary (expected because the African Plate is surrounded by oceanic ridges, and the Mid Atlantic Ridge is moving toward the west. Furthermore, the Atlantic Ocean is getting bigger and the Pacific Ocean is getting smaller.

Evolving Plate Boundaries
Plate boundaries can evolve. New plate boundaries can form where mantle upwelling results in creating a rift in the crust and plate boundaries can die when two plates of continental lithosphere collide.

Continental Rifting - A new divergent plate boundary can form when continental lithosphere stretches, and thins to form a rift valley. As the rift widens and thins, upwelling asthenosphere can melt to produce magmas that start to create new oceanic lithosphere and spread the new plates apart.
An example of where a rifting may be forming a future diverging plate margin is an area of northeastern Africa, called the East African Rift Valley. Another area where this is apparently occurring is the Basin and Range Province of the Western U.S.

Continental Collisions - When two plates that have low density continental lithosphere collide with one another subduction ceases because the continental lithosphere has too low of a density to be subducted. As the plates continue to collide fold – thrust mountain belts that develop along the zone of collision.
Currently the highest mountains in the world, the Himalayas represent this kind of event. The Himalayas resulted from a collision of the plate containing India with the plate containing Eurasia. This collision is still taking place and results in joining the two formerly separate plates. The occurrence of ancient fold -thrust mountain belts such as the Appalachian Mountains of the Eastern U.S., the Urals of Central Russia, and the Alps of southern Europe, are evidence of ancient continental collision margins.

What Causes Plate Tectonics?
From seismic wave velocities we know that the asthenosphere behaves in ductile manner, that is even though it is solid it can flow under stress and behave like a liquid. If this is the case, then it can also convect. Convection is a mode of heat transfer wherein the heat moves with the material. Convection is caused when material that occurs at a deeper level is heated to the point where it expands and becomes less dense than the material above it. When this occurs, the hot less dense material rises. In a confined space, rising hot material will eventually cool and become denser than its surroundings. This cool dense material must then sink. This gives rise to convection cells, with hot rising currents and cool descending currents. If the asthenosphere is in fact moving as a result of convection, then convection could be the mechanism responsible for plate tectonics. Hot rising currents would occur beneath oceanic ridges.

Magma intruding into the ridge would push lithosphere apart at the ridge. As the new lithosphere cools, it will slide off the topographic high that results from the upwelling of the mantle and will eventually become cold and dense. This dense lithosphere will tend to pull the rest of the lithosphere downward. A combination of dragging the lithosphere along the top of the convection cell, ridge push, sliding, and slab pull all appear to be contributing factors to the cause of plate tectonics.

There is still some debates as to whether asthenospheric convection drives the plates or the plates themselves drive plate tectonics. Until we have a better idea of what is happening in the mantle this debate will not likely be resolved. At least for now it appears that both convection and slab pull are the major factors.

SUMMARY AND GENERAL TERMINOLOGY FOR:

PLATE TECTONICS
I. Introduction
A. General
1. The theory of plate tectonics is a recent development in the geological sciences, really accepted by scientific community since the early 1960's.
2. Earlier in the century geologic paradigm was dominated by the belief that ocean basins and continental land masses were permanent and fixed on the surface of the earth.
3. The theory of Plate Tectonics now recognizes that the positions of land masses are not fixed and that they have moved about the earth's surface over geologic history
a. Ocean basins/oceanic crust are continually being created and destroyed through tectonic processes.

B. Terminology
1. "Tectonics" - is a term that refers to the deformation of the earth's crust.
2. "Plate" - refers to the subdivision of the earth's crust and lithosphere into a number of tectonically coherent blocks
3. "Plate Tectonics" - refers to the formation and migration of these lithospheric plates,

II. Overview of Earth Interior

A. Crust- a relatively thin outer layer
1. Oceanic crust- thin on order of several km's thick
a. volcanic / basalt in composition
2. Continental crust-thicker on order of 10's of kms thick
a. plutonic/sedimentary/"granitic" in composition
B. Mantle- rocky layer located below the crust and having a thickness of 2885 km
1. Mantle- dense, iron-magnesium silicate rocks
2. "Moho"
a. Mohorovicic discontinuity or Moho-seismic discontinuity in which velocity of earthquake waves increases abruptly below a depth of 50 km
b. now known to be boundary between crust and upper mantle
3. Asthenosphere- soft zone of partially melted rock

C. Lithosphere- outer solid portion of the earth which includes the upper mantle above the aesthenosphere and the crust.
D. outer core- 2270 km thick, possesses characteristics of mobile liquid
1. Liquid, iron-rich,
E. inner core- 1216 km thick, solid metallic sphere
1. Core- thought to be composed of iron and nickel, very speculative, based on study of meteorites and speculation that they represent the interior composition of earth.

III. Historical Perspective on the Evolution of Plate Tectonic Theory: Continental Drift a Precursor to Tectonic Theory

A. Continental Drift - Alfred Wegner (German earth Scientist) proposed a hypothesis in early 1900's that the world continents have been drifting about on the earth's surface
1. Supercontinent of "Pangaea" existed 200 M.Y. ago in which all of major worlds continents were once amalgamated together, and have since broken apart and migrated or drifted to their present positions/configurations.
2. Evidence for Wegner's hypothesis of Continental Drift
a. Jig-saw puzzle fit of the Continents:
b. Fossil Evidence
(1) Mesosaurus which is only found on east coast of South America and west coast of Africa.
(2) How did these critters migrate across the ocean basins?
c. Similar Rock Types and Structural Rock Deformation across ocean basin
d. Paleoclimatic Evidence
(1) Evidence for glacial conditions 250 m.y. ago are found in similar aged rocks from southern Africa, South America, India and Australia 3. Problem with Wegner's ideas
a. not widely accepted
b. suggested on the continents were "drifting" not ocean basins
c. did not have a viable mechanical explanation as to how continents would "drift"

IV. Modern Plate Tectonic Theory
A. Basic Model - Based on early work by Wegner, more recent mapping of seafloor, magnetic surveys of earth's magnetic field, and observation of earth's seismic activity or earthquake activity.
1. Plates- Plate tectonics model suggests that the outer, rigid lithosphere of the earth consists of about twenty rigid segments known as "plates".
a. Plate Mobility - it is recognized that each moves as a distinct rigid unit in relation to other plates.
(1) These plates move on top of a semi-plastic aesthenosphere, and interact with one another along their boundaries.
2. Plate Boundaries and Nature of Interaction between Plates
a. 3 types of plate boundary interaction: Divergent, Convergent, or Transform fault boundaries
b. Divergent Boundaries - boundary condition in which tectonic plates move apart, resulting in upwelling of magma and volcanic material to create new seafloor: i.e. creation of new crust.
(1) Located at crests of mid-oceanic ridges, where plates move apart and molten rock is injected and cooled to form new seafloor.
(2) Seafloor spreading- process of plate divergence and injection of magma. (3) Continental Rifting
(a) Pulling apart of continental crust by faulting
(b) Incipient seafloor spreading center
c. Convergent Boundaries- plate boundaries in which two plates move toward one another or collide.

(1) Collision of one plate into another results in downbending of one plate and descent of that plate beneath the other,
(2) Subduction zone- a zone of plate convergence in which where an oceanic plate descends into the upper mantle beneath the overriding plate.
(3) Trenches - zone where subducting slab dives beneath over-riding plate
(4) Volcanic arc - an arcuate chain of volcanoes on continental crust that result from subduction of oceanic crust beneath continental crust.
(5) Types of convergent boundaries
(a) Oceanic-Continental Plate Convergence
(b) Oceanic-Oceanic Plate convergence
(c) Continent-Continent Plate Convergence

d. Transform Fault Boundaries: condition where plates slide horizontally past one another along a fault (or fracture along which there is movement)
(1) Crust is neither consumed nor destroyed.
(2) Transform faults connect convergent and divergent boundaries into a worldwide network of interconnected plate boundaries.

V. Evidence to Support Modern Plate Tectonic Theory
A. Magnetism and Paleomagnetism (result of search for German submarines in WWII)
1. Earth has a magnetic field about it with a magnetic north pole and south pole similar to a bar magnet with lines of magnetic force flowing from North to south.
2. Paleomagnetism - iron-rich minerals such as magnetite (Fe3O4) act as tiny magnets, when these minerals cool from a magma there is a temperature at which they align with the magnetic field of the earth (curie point),
a. Polar Wandering -
b. Polar Reversals
(1) Normal Polarity -So rocks have been found with paleomagnetism similar to today’s polar arrangement termed "normal" polarity
(2) Reversed Polarity- rocks which indicate magnetic north pole at current position of south magnetic pole
c. Seafloor Stripes

B. Evidence from Seismic Records of Earthquakes
1. The distribution of earthquake focii or origination points of earthquakes was examined around the world and at convergent plate boundaries or subduction zones.
C. Evidence from Ocean Drilling
D. Hot spots
1. Hawaiian Islands

VI. Driving Mechanism for Plate Tectonics: what force causes the plates to move about the earth's surface?
A. Heat Transfer/Convection within Mantle
1. Model: the lower or inner portion of the mantle, near the core, is hotter than the upper mantle; this unequal distribution of heat results in circulation of heated, semiplastic mantle material...warm, less dense material of lower mantle rises very slowly in regions of spreading centers, spreads laterally, cools, and slowly sinks back into the mantle and reheating process repeats, these mantle convection currents result in shear force being applied to overriding crustal plate and drive plate tectonic motion.
B. Other Ideas
1. active subduction pull
a. cold, dense subducting slab pulls plate into interior of Earth
2. ridge push
a. active spreading centers push slab into interior of Earth