Directions: Answer the following questions in your own words using your text (ch 7-10 and 12). Several of the questions refer to multiple chapters. Please keep in mind that because this is a take-home, open book test, your answers should demonstrate that you understand the material. Your answers should be well thought out, complete yet succinct, well written, with references cited. This is not a group effort; please make sure your work is your own. Please be careful with the use of images, if you do not explain your images you will receive no credit for them. Please e-mail me the test completed, including the multiple choice questions that follow the essay/shot answer questions, as a doc, docx or pdf file. Good Luck!
1. Differentiate between relative and absolute dating. List, define and discuss the principles used to define relative age. Discuss isotopic dating: what atomic particles are involved, what are some common isotopes that are used, and what are some uses of isotopic dating? Be sure to explain the calculation of the age of a rock.
Relative dating is telling us the age of something compared to that of the substances around it, more plainly stated the sequence of events. Absolute dating tells how old something is, an amount of time can be associated with an object unlike in relative dating. There are four stratigraphy principles used to determine geologic history of a locality or a region: 1) original horizontally, 2) superposition, 3) lateral continuity, and 4) cross-cutting relationships. Original horizontally states that beds of sediment (sedimentary strata) deposited in water forms as horizontal or nearly horizontal layers. If rocks that are at inclined/folded have moved from their original position. The principal of superposition says that within deposition of sediment the oldest layer is at the bottom and the layers progressively get younger as they move upward. This can also applied to lava flow. The lateral continuity principle states the original sedimentary layer continues laterally until it tapers or things at the edges. The cross- cutting principal states that disrupted pattern/established pattern is older than the cause of the disruption. Disruptions in the pattern of the rock prove the cross cutting principle. Another principle is that of inclusion which states that fragment in the host rock are older than the host rock. To find the absolute date we can use isotopic dating which is used to determine the age of the rock through radioactive elements. In isotopes the number of neutrons are varied but the number of protons are the same. In radioactive decay the nuclear change of isotopes is spontaneous with unstable nuclei (energy is produced during this). The nuclei of these radioactive isotopes change in three ways: alpha decay, beta decay, and electron capture. Alpha decay is when 2 protons and 2 atoms are rejected from the nucleus consequently the atomic number reduces by two and the atomic mass number is reduces by four. The original isotope is called the parent isotope and the new changed isotope is called the daughter isotope. In beta decay an electron is released from a nucleus and it becomes a proton with the atomic number increasing by one to the daughter nucleus and no change to the atomic number in the parent nucleus. Finally in electron capture the proton in the nucleus captures a surrounding electron, which consequently becomes a neutron. This atom is now different for the daughter nucleus has an atomic number one less than the parent nucleus (isotope). In isotopic dating the larger the number of atoms present in a radioactive isotope in a given rock, the percentage of those atoms radioactively decaying are constant over a given span of time. The rate of this proportional decay is known as half-life or the time it decays for one-half of the given number of radioactive atoms to decay. The common parent isotopes that are used for determining ages of earth’s materials are K-40, U-238 U-235, Th-232, Rb-87, C-14; the daughter products of these isotopes are Ar-40, Pb-206, Pb-207, Pb-208, Sr-87, N-14. To determine the half- life of a rock using K-40, you mush first find the amount of K-40 in the rock through chemical analysis and then the amount of Ar-40 the daughter produced. Add these two values which tells us how much K-40 was present when the rock was formed. Knowing this original amount the half life the age of the rock can be calculated based on the half life by the equation N=N0 e-λt . N is the number of atoms of the isotope at the t time that has elapsed. N0 is the number of atoms of the isotope when the time began. e is a mathematical constant 2.718 and λ is the decay constant that has to do with the number of atoms of the certain isotope that remains. Isotopic dating is typically used to find the how long ago a rock was formed. The type of rock and the isotopes being analyzed are two dependents when it comes to what is being dated. For example in a metamorphic rock geologist are looking for the cooling of the rock rather than the peak high temperature when metamorphism is occurring. This dating could be trying to find the ratios for a whole rock or just one mineral within that rock. Isotopic dating typically determines the closure temperature, or in layman terms when the “clock starts”. This occurs when neither the parent or daughter isotopes can enter or leave the rock. Igneous rocks are where the best dates are normally found unlike a plutonic rock which can take over a million years to solidify making it hard to determine when precisely the rock was formed. For dating to be accurate the rock must show no signs of weather or hydrothermal alternation, one must be able to be certain that there were no daughter isotopes present at the time of closure/make corrections of the amounts before the clock started, and there must be enough parent and daughter atoms to be measured by an instrument
2. What are the effects on metamorphic minerals and textures of temperature, confining pressure and differential stress?
First off, metamporphic rock is the new rock that is left after metamorphism takes place. Metamorphism is when a rock in the Earth’s interior, whether that be new textures, new mineral assemblages, or even both; all occurring in the solid state Heat is imperative for metamorphic reactions to occur and it mainly comes from the outward flow of geothermal energy from Earth’s deep interior. The geothermal gradient determines particular temperature of a rock. All minerals are stable at a given temperature. When a mineral is stable it does not react with another substance or transform to a new mineral/substance. The temperature range in which a mineral is stable is affected by such factors as pressure and absence/presence of another substance. When geologists know the temperature(s) that a mineral is stable, they are able to tell the temperatures of metaphormis for a rock containing that mineral. Minerals that are stable at higher temperatures tend to be less dense compared to the minerals stable at lower temperatures. The higher the temperatures the more the atoms vibrate within their crystal structure. The more open this structure is the great the atoms can vibrate. When the heat and vibration get too much these bonds break become liquid. If partial melting occurs the melting component becomes magma and the remaining solid stays a metamorphic rock. Partial melting occurs when the highest temperature in metamorphism overlaps the temperature of partial melting of a rock. Pressure also affects the characteristics of a metamorphic rock. Typically when we think of pressure we think of a pressure that is applied equally on all surfaces of a certain substance (known as confining pressure); however, for metamorphism pressure is expressed as kilobars. A bar is equal to a thousand times the pressure of the atmosphere at seal level, and 1,000 bars equals one kilobar. For a buried rock the pressure gradient is close to 1 kilobar per 3.3 kilometers of burial in the crustal rock; pressure gradient is the increase in lithostatic pressure within the depth of the earth. A mineral that has crystallized under these high-pressure conditions tends to take up less space than the original mineral or minerals that it was formed from. This new mineral is denser than the lower pressure counterparts, due to the fact that the atoms are closer together and packed more closely because of the pressure forces that occurred. However if the pressure and temperature both increase the new mineral will likely be less dense if the effect of the higher temperature is greater, but if the effect of the higher pressure is greater than the new mineral will be denser. Stress, force per unit, effects the metamorphic rock as well. Differential stress has the effect of making the object take on an oblong or flattened form. This occurs when the forces are stronger or weaker in different directions. Compressive stress contrasts this and is when the object is flattened/compressed due to forces that are applied opposite directions directly. Shearing, which is related to differential stress, causes parts of the object to move or slide in relation to one another across a plane. It often occurs perpendicular to the direction of compressive stress. Shearing of rocks is common during faulting also known as dynamic metamorphism, which is when bedrock moves along a fracture. Mylonite is usually formed when the rocks that come in contact with the fault are broken and crushed.
3. What are the various sources of heat for metamorphism?
There are three sources of heat for metamorphism: contact, regional, and geothermal. Contact metamorphism takes place less than 10 kilometers below the earths surface which is why confining pressure is generally low and the why high temperatures is the main factor in contact metamorphisms. It occurs adjacent to a pluton when magma encroaches on a cooler country rock. This magma “bakes” this rock adjacent to intrusive contact, which is how contact metamorphism got its name. This contact zone is rather narrow and most rocks found are nonfoliated due to the lack of impact from differential stress. Regional metamorphism takes place greater than 5 kilometers underground and these forks are typically foliated and show that differential stress has occurred due to recrystallization. The temperatures are anywhere from 300-800 degrees Celsius depending on depth and gradient of the region. Hydrothermal process is when hot water creates new rocks and minerals form precipitation of ions form hydrothermal solutions.
4. Differentiate between meandering streams and braided streams? What are the processes responsible for the development of each stream? What depositional and erosional features are associated with each stream?
In a very basic manner a braided stream carries larger sediment particles, while meandering carries more fine-grained particles. A braided stream occurs when the stream fills its main channel with sediment, which flows in a network of interconnected rivulets around bars. These deposition of a bar is the middle of a stream and it diverts water toward the side where eroding of the banks occurs along with the widening of the stream due to the force exerted. If a stream is loaded with a lot of sediment it may deposit multiple bars in a channel, which ultimately causes the stream to widen and more bars to be deposited. A stream like this goes through many stages of deposition and erosion. It becomes braided when the load is heavy with sediment (most likely a bed load) and has banks that are easily eroded. This pattern develops in deserts as sediment loaded stream looses water either my evaporation or percolation or when melt water off of glaciers when the discharge from these glaciers is small compared to that of the large amount sediment that the stream has to carry. A meandering stream occurs when rivers carrying fine-grained silt or clay develop pronounced, sinuous curves called meanders. The sediment typically becomes finer downstream so meandering is most common in the lower reaches of a river. The rivers velocity is higher on the outside of a curve compared to that of the inside and this higher velocity erodes the riverbank rather quickly. The inside of the curve promotes deposition for it is of lower velocity, this is how sandbars are formed also known as a point bar made up of ridges of sand and gravel. The concurrent erosion on the outside of the curve and deposition on the inside tends to deepen a gentle curve creating a hairpin meander. These meanders are not permanent for the deposition and erosion is a continual process causing them to migrate back and forth across flat valley floors and downstream which leaves scars and point bars marking their former position. Meander cutoffs may form during floods, which is a shorter channel across the neck, leaving the old cutoff meander to become a crescent shaped oxbow lake.
5. Suppose you had a radioactive isotope X whose half–life in disintegrating to daughter product Y is 120,000 years. By calculating how much it took to make the present amount of Y, you determine that the rock originally contained 32 grams of isotope X. At present only ¼ gram of X is in the rock. How many half lives have gone by? How old is the rock? (Please show your work). This shows us that the rock is 240,050 years old and therefore two half-lives have gone by (240,050/120,000).
6. What are Unconformities? List, define and discuss the different unconformities given in your text.
Unconformity is a type of contacting for figuring out the geologic history of an area. It represents a gap in the geologic record meaning that the rock above the contact is quite a bit younger than the one beneath it. Uniformities are mainly buried erosion surfaces. There are three types of uniformities—disconformities, angular conformities, and nonconformities. Disconformity represents the missing rock strata that separates beds that are parallel to one another. It is assumed that the older rocks eroded away parallel to the horizontal bedding plane with the renewed deposition burring erosion surface. It is the hardest type of unconformity to detect for it looks like a sedimentary contact in sequence of sedimentary rock. It is rare for weathered zone to have been persevered, which implies that there is a disconformity. Typically the only way a disconformity can be detected is by studying the fossils from the beds in a sequence of sedimentary rock. If there are missing beds this shows that there was likely a disconformity in the sequence; however, it could also be that that neither an erosion nor deposition took place in a significant amount of geological time. An angular conformity is a contact which the younger strata overlies an erosion surface on tilted or folded layered rock. It implies the following sequence of events from oldest to youngest: 1) deposition and lithification of sedimentary rock 2) uplifted along with folding of layers 3) erosion 4) renewed deposition followed by erosion of the surface. Nonconformities are when younger sedimentary or volcanic rock covers an erosion surface on plutonic or metamorphic rock. Because of this, nonconformities typically indicated deep or prolonged erosion before burial, for both metamorphic and plutonic rocks form in considerable depths of the Earth’s surface. The following is the geological history of a nonconformity 1) crystallization of igneous or metamorphic rock at depth 2) several kilometers of erosion of overlaying rock 3) deposition of new sediment ultimately becoming sedimentary rock on an eroded surface.
7. List and define the types of drainage patterns defined in your text. Where might you find each drainage pattern?
Drainage patters are the arrangement of a river and its tributaries in map view. The following are four types of drainage patterns: dendritic, radial, rectangular, and trellis. Dendritic is a pattern resembling branches of a trees. This pattern develops on erodible rock or regolith and is the most common type of all the patterns. Radial pattern is when streams deviate outward like spokes of a wheel and they form on high conical mountains like composite volcanoes and domes. Rectangular pattern is when tributaries have numerous 90 degrees beds that join other streams at right angles, developing on reoccurring fractured rock. Pathways for streams are formed when the right angles meet because fractured erode more easily than unbroken rock. Trellis pattern is made up of parallel main streams with short tributaries meeting at right angles. It forms where tilted layers of resistant rock (sandstone) alternate with nonresistant rock (shale). Erosions like this typically result in surfaces such as parallel ridges and valleys.
8. How does a stream transport sediment and how is this related to a stream’s competence and capacity?
Stream transports sentiment through bed load, suspended load, and dissolved load however; most of the load is carried in suspension and solution. The bed load is the large/heavy particles such as sand or gravel that travel on the streambed; they move either by traction or salutation. With particles such as cobbles and boulders, which rarely loose contact with the streambed as they move along, roll or slide along the stream bottom eroding each other through abrasion. This rolling or sliding down the bottom is known as traction. Sand grains do move by traction but they also move by salutation, which is more of a bouncing movement off the bottom rather than a rolling motion. The salutation process is begun when turbulent waters lifts then up counteracting with the downward force of gravity allowing for the grains to be suspended above the streambed. When the water slows down (due to being an eddy) gravity pulls the particles back to the bed of stream. This process occurs again which is how it salutates downstream. Suspend load is the sediment that is light enough to remain lifted above the water. This is why streams look muddy during or after a flood because the suspend load is large. Coarser bed loads unlike silt and clay moves on the streams bottom. Suspended load has less effect on erosion unlike that of a bed load, however large amounts of sediment are transported in suspension. Dissolved load is made up largely of soluble products of chemical weathering. Most streams contain many ions in solution many of which precipitate out of the water as evaporate minerals if the stream dries up or they will stay until they reach the ocean (bicarbonate, calcium, potassium, sodium, chloride and sulfates are examples). Dissolved load is visible so it is not until the water evaporates that the visible crystals begin to form. We can see that these three ways deal with different particles of different sizes all being carried in different ways. The capacity of stream is the maximum load that a stream can transport which is proportion to the amount of water (discharge); so, the more water the large amount of sediment it can carry. A stream competence is the measure of the largest particles that it can transport. This is directly related to the velocity of a stream, which can vary. This is why a flood can cause more erosion than a normal flow for the increased capacity and competence (cliffnotes).
CliffNotes.com.Sediment Load. 26 Mar 2011http://www.cliffnotes.com/study_guide/topicArticleld-9605,articleld-9512.html
9. What are Milankovitch’s Cycles?
Milankovitch’s Cycles are named after the Serbian astronomer, Milutin Milankovitch, who is credited with calculating the incoming solar radiation cycles of 21,000, 41,000, and 100,000 years, which addresses the question of what caused glacial ages. The variation in the Earth’s orbit and inclination to the sun, the angle of the Sun’s ray (distance to the sun), and the angle of the Earth’s pole in relation to the plane of the Earth’s orbit around the sun all have to do with Milankovithc’s theory. However, the variations in orbital relationships and the “wobble” of the earth’s axis are mainly responsible for the glacial and interglacial episodes.
10. List and define the erosional and depositional features of alpine and continental glaciation. Be sure to indicate whether the feature is erosional or depositional.
The features of a mountain range are largely due to mountain glaciation. The following are the effects of erosional landscapes associated with alpine glaciation A U-shaped valley is a characteristic of glacial erosion and it is typically a deeper valley compared to that of a tributary glaciers for there is more bedrock ground away allowing for large truck glaciers to erode in a downward manner. When the glaciers disappear there is a hanging valley that remains above the valley. Truncated spurs are ridges that have triangular facets produced on the lower end of the mountain by glacial erosion. This is due to the valley curves that are straightened out when the glacier, which is too inflexible and large, carves the slides of the valley ultimately eroding the lower end of the ridges that extend to the valley. Even though the glaciers smooth the sidewalls of the valleys, the bedrock carves into a series of steps due to the variable resistance. The waters will steep into the cracks freeze and enlarges the fractures to make new ones. This grinds and loosens more pieces and after the ice has melted a chain of rock-basin lakes may fill the depressions carved out of the weather rock (sometimes called paternoster lakes). These areas where the bedrock is more resistance to erosion are known as rounded knobs. Cirque is a steep-sided, half-bowl-shaped recess carved into a mountain at the head of the valley. This just doesn’t occur by the glacier but also by the weathering and erosion of the rock walls above the ice. Avalanches and frost wedging steepen the slops of the glacier and break up the rock; this broken rock adds onto the valley glacier and becomes part of the load. The erosional process that make a cirque larger also help create a horn, which is a sharp peak that remains after ciruqes cut back into the mountain. Arêtes are also form, which are adjacent to the glacially carved valley, which are formed from frost wedging. Erosional landscapes with continental glaciation tend to produce rounded topography from the ice sheets. Just like in the valley, the rock beneath the ice sheet becomes eroded however due to the weight of the ice the effects may be more pronounced. While rounded knobs are common, grooved and striated bedrock are also produced. These grooves can be several meters deep and kilometers long, the direction of the grooves/striations show the movement direction of the past ice sheet. It is possible for the ice sheets to bury mountain ranges, round off ridges/summits, and streamline them in the direction of the ice movement due to their thickness. Glacial deposition is centered around the broken rocks from the valley walls that have been carried along at the base of the ice. These rock fragments are mainly angular for they have not been tumbled around enough for the edges to be rounded. This unsorted debris carried and/or deposited by a glacier is called till. Glaciers can carry any size rock, even boulders like an erratic. An erratic is an ice-transported boulder that has not been derived form-underlying bedrock and if it has it shows the movement direction of the glacier carrying it. A feature of an alpine depositional is a moraine. A moraine is when till occurs as body of unsorted/layered debris either on a glacier or left behind by a glacier. There are a few types of moraines: lateral, medial, and end. Lateral moraines are formed when the debris from steep cliffs accumulates along the edges of the ice and typically have an elongated low mound look to them. When the tributary glaciers come together it allows for the adjacent lateral moraines to join and be carried by a medial moraine. The active glacier brings debris to the terminus and if this terminus remains stationary for a while then a ride of till called an end moraine piles up. Valley glaciers build crescent –shaped or horseshoe-shaped end moraines, while an ice sheet takes a similar form but is longer and more irregular than a valley glacier. There are also two types of end moraines, which apply to both alpine and continental glacial deposition, terminal moraine that marks the further advance of a glacier and a recessional moraine that is built when a terminus of a receding glacier is stationary. When the ice melts the rock debris that is deposited forms a fairly thin layer of till which is called ground moraine (continental depositional). Drumlins occur in areas of past continental glaciation where bodies of till shape into streamlined hills. It is shaped like an inverted spoon, which is, aligned parallel to the direction of the ice movement. Outwash is another example of continental glaciation. Outwash is the materials deposited by the debris filled melt water that is part of continental deposition. It can be distinguished from till because it has characteristics of stream deposited sediment which are layered and sorted. A feature of outwash is an esker, which is a ride of water-deposited sediment. They are formed of cross-bedded and swell-sorted sediment and can be up to 10 meters high. These eskers are deposited through tunnels running within or under the glaciers by meltwater. As this meltwater helps build thick deposits of outwash on the side of the glacier, it surrounds and buries blocks of stagnant ice by the sediment. When this block of ice melts a depression is formed known as a kettle. Many smaller lakes form from these kettles. When sediment builds up and the ice melts, this sediment becomes an irregular ridge or a lower mound with is known as a kame. For alpine glaciation rock flour from the streams that drain these glaciers with heavy sediment form a braided pattern.
11. Who was James Hutton and what is the Principle of Uniformitarianism?
James Hutton was an eighteenth century Scotsman, commonly referred to as the father of geology, who realized that geological features could be explained through present day processes. He recognized that mountains were the shapes they are because they had been carved by erosion. He also made the realization that the sedimentary rocks we find on continents are products of sediment from the land which were deposited as mud and sand. Hutton’s ideas became the foundation for both life science and geology. His concept that the present operating processes are the same as those in the past is known as the principal of uniformitarianism (it ultimately stated that “the present is the key to the past”). Although Hutton’s principle makes it seem that changes in the earth take place at a uniform rate, Hutton was aware of the fact that violent, short-lived events affected the earth as well.
12. Compare and contrast the movement of sediment in a glacier to the movement of sediment in a river.
Glaciers can move sediment of any size ranging from boulders to small and particles. Rock fragments are dragged on the base of the glacier. These rock fragments come from water that has seeped into the cracks, which freezes and breaks up the bedrock. Valley glaciers move down slope by gravity and tend to flow faster on steeper and upper parts of the glacier. They also tend to move faster in temperatures that are near or at the melting point then were temperatures below freezing. Velocity also varies within a glacier just like in a stream, they both move faster at the central portion than on the sides. Basal sliding and plastic flow are both parts of the movement of the sediment. Basal sliding is when the glaciers as one unit slides over the underlying rock. Meltwater forms on the base form the pressure of the overlying glacier, which allows for basal sliding to occur. Plastic flow is movement with the glacier that is do to the natural make up of ice. The pipe is more sharply bent at the base due to the pressure from the overlying ice that flows more when the depth increases. In streams sediment is transported mainly by suspension and solution through bed load, suspended load, and dissolved load. Bed load carries larger heavier sediment particles that for the bed load of streams and are moved by traction or salutation. Traction is when the sediment rolls or slides along the stream bottom eroding it and the other rocks (abrasion). Abrasion also occurs in glacier movement of sediment. The forces of water along with the forces of gravity carry the sediments down stream, which is known as salutation. Suspended load carries lighter particles that remain lifted form the bottom of the stream during the entire movement downstream, little erosion occurs here. Like mentioned early the velocity moves the sediment quicker in both glaciers and rivers at the central portion rather than the sides. The movements in both also cause erosion to the both the bottom of the river and to the other rocks, although the abrasion is greater in rivers.
