TEST 2 Geology StudyGuide
TEST 2 Geology StudyGuide Geol101
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This 22 page Study Guide was uploaded by Shelby Green on Monday February 22, 2016. The Study Guide belongs to Geol101 at Clemson University taught by Dr. Coulson in Spring 2016. Since its upload, it has received 177 views. For similar materials see Physical Geology in Geology at Clemson University.
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Date Created: 02/22/16
Study Guide Test 2 Lecture 5 Sediment: most common type of rock Sedimentary Processes 1. Weathering a. Parent rock breaks down into particles called sediment Two types of weathering: a. Physical Plant roots: split rock as the plant grows Frost wedging: water gets into crack freezes ice expands splits rock repeat b. Chemical More common (ex) feldspar +H2O+H2CO3 kaolinite + dissolved ions Saprolite Formation: substance that has undergone extensive chemical weathering (translates to “rotten rock” because it falls apart easily) 2. Erosion a. Requires energy to transport from A to B (ex: water, wind, glaciers, gravity like landslides) 3. Deposition a. Basin: any place that can be filled with sediment b. Accommodation space: measurement of how much sediment can fit in a basin c. Subsidence: when the ground level in a basin sinks downward because of weight from sediment thus creating room up top for more sediment 4. Lithification a. Compaction: sediment gets compact as layers are added to basin **Layers also referred to as strata or beds b. Compaction fills the caps between the stratum of sediment c. Cementation: when the constant compaction pushes absorbed water out of the sediment layers and leaves behind little crystals that act as natural cement Classification 1. Detrital (aka clastic) sediment created by physical weathering a. Gravel b. Sand c. Silt d. Clay Poorly sorted Traits of Detrital Material Sorting: how uniform the grain size is indicates how long the erosion process was o Long erosion = well sorted (close to same size) o Short erosion =poorly sorted (all different sizes) Rounding: how round the grain is indicates how long the erosion process was o Long erosion = well rounded o Short erosion = jagged, poorly rounded Identifying Clastic Sedimentary Rocks Well Grain size is key pick out the dominate grain rounded size o Ex: sand = sandstone o Ex: silt = siltstone 2. Chemical Sediments a. Form via chemical reactions Dissolution and re-precipitation Saltwater evaporation b. Crystal structures c. Usually comprised of 1 major mineral type: Ex: halite = salt rock Ex: quartz = chert d. Economically viable: high concentration of one mineral so easy to find if looking for that specific mineral without the processing costs 3. Biogenic Sediments: a. Sediment particles come from living organisms (ex: shells, partially decayed plants, microscopic organisms) i. Chalk: poorly cemented ii. Limestone: well cemented, tiny shell fragments iii. Coal: compressed plant material Mass Wasting (landslides) Slope Destabilization Factors Angle of repose: max avg. number of steepness before whatever is added to the top just falls back down Lack of Moisture: low water content/dry Excessive moisture: too much moisture turns sediment into mud with landslides easily Lack of vegetation: plant roots stabilize slopes and hold sediment in place Excessive vegetation: too much vegetation can (1) add too much mass to a steep slope, (2) absorb too much water and make soil dry, (3) roots make channels for water to travel down Types of Mass Wasting/Landslides Categories based on: o Material: sediment or mud or rock or snow or ice o Type of movement: rolling/sliding/flowing/falling downhill o Speed: rapidly or gradually 1. Rockslides: a. Fast, but not as fast as other materials b/c of friction between the ground and the rock 2. Creep: a. Sediment moving slowly downhill b. Barely noticeable Ex: fence on hill starts to lean downward because sediment is moving downhill Causes of Mass Wasting Thunderstorms/heavy rains Earthquakes Human Landscaping Clear cutting **Risk Management Maps are updated frequently to determine how at risk certain areas are for mass wasting Preventing Mass wasting Drainage control: adding pipe systems to prevent over saturation of ground Decrease slope grades: flattening hills to decrease the speed of run off Building codes: like not allowing big houses to be build in steep hills Retaining walls : concrete barriers to block falling rock from reaching roads Rock Bolts: drive bold through rock into rock behind it to hold the outside rock in place **Prevention Cost: expensive to build/implement changes, but damage is more costly estimated return is $10-$2,000 per $1 spent on prevention **Case Study 1983 slide caused $200 million in damage, but could have been preventable if $0.5 million had been spent on drainage systems Lecture 6 Metamorphism (very slow process) Metamorphic rocks help understand the geographic history of an area Temperature Geothermal gradient: how fast the temp go up with depth in the earth (varies) Avg: changes 30°C/km Typical range: 20-60°C/km Metamorphism via Heat Contact metamorphism: rock undergoes metamorphism due to direct heat from magma coming towards the surface o Primarily temp driven o Limited in scope/only occurs close to magma Pressure ** Note: 1 bar= 1 atm at surface of the earth Pressure gradient: ~300 bar/km depth Types of Pressure 1. Confining pressure: rock is feeling equal pressure from all directions a. (Ex) swimming underwater 2. Directed pressure (aka differential): the majority of the pressure is being applied in a specific direction a. Compressional How much pressure is needed? Most metamorphic rocks from at 10-30 km (aprox. 1- 1.6 mi) depth (mid-lower crust) Exposure How metamorphic rocks get back to the surface o Faults rocks on either side of fault move/broken down thorough weathering and erosion Types of Metamorphism Via Pressure o Regional metamorphism: large scale process/opposite of contact metamorphism o Contact metamorphism: INSERT DEFINITION o Fault metamorphism: occurs on a small scale along the fault line. Pressure is still the primary driver Via Fluid o Metasomatism: rock comes in contact with very hot ground water. Water flushes things out of the rock or deposits new minerals that change the mineralogy Ores: high concentration of a specific mineral o Seafloor metamorphism: almost always forms basalt. specialized in environment where the cold seawater interacts with the rock Metamorphic Change Metamorphic grade: measures metamorphic change due to change in temp and pressure o Low, intermediate, high o Doesn't tell you anything specific/needs to be better differentiated Index minerals: a mineral can help identify the correct metamorphic grade o Each index mineral is formed in very specific environment Metamorphic Facies: group of index minerals that form at the same temp and pressure o Ex) blueschists facies includes the minerals glaucophane, lawsonite, epidote 7 Major Metamorphic Facies **Facies used to reconstruct how metamorphism occurred Length of Metamorphism Prograde: temp and pressure are climbing towards peak or metamorphic grade in rock history Retrograde: temp and pressure are decreasing after they have already peaked in rock history (usually due to reaching the surface) ** We need to figure out the time frame of each period Changes within minerals can record Pressure-Temp changes o Un-uniformity allows for tracking o Crystal grows outward with rings like a tree trunk Types of Metamorphic Rocks 1. Foliated Metamorphic Rocks a. Slate b. Schists: hard to identify (samples can vary drastically) c. Gneiss (pronounced “nice”) *these rocks are formed from different pressures 2. Non-foliated: a. large crystals b. high concentration of one specific mineral c. appearance is very similar so you cannot eyeball the identify i. Hornfels ii. Quartzite iii. Marble Geology in the News: sudden volcanic eruptions found to trigger from gas bubbles in magma Structural Geology: Focuses on how rocks deformed after they've been created = rock deformation Note: Topographic (landscape) features and geologic structures are very different Tectonic Forces 1. Tensional: stretching an object (pulling away from the center in two directions) 2. Compressional: the area is getting squeezed or compacted 3. Shearing: material getting slid in two different directions at the same time Responses to stress: Brittle: break with enough force Ductile (aka plastic): bend away from force Response can vary based on: o Rock type, temp/pressure, speed of deformation (faster the application, the more brittle the response) Types of Structures Folds: formed with a ductile response to a compression force *Know limbs and hinge (where all the motion seems to be occurring) Classifying Folds 1. Shape (in road cut or cross-section view) a. Antiform: anti-smilie face (frownie Anti face) b. Synform: smilie face Syn c. Overturned: i. Overturned Antiform ii. Overturned Synform iii. Overturned can’t tell if Antiform or Synform Overturned synform Overturne d antiform Overturned 2. Age of Layers relative to each other a. Anticline: oldest layer in the center fold between the limbs b. Syncline: the center fold layer is the youngest rock **This happens because tectonic forces can cause an entire stack of layers to get turned upside down 3. Geometry: a. Horizontal: squeezed in from both sides and nothing else (looks horizontal in birds eye view) b. Plunging: squeeze from both sides and tipped up or down (more common) Synform, Antiform, Syncline, Anticline, Horizont Types of Structures al Plunging Joints: brittle response o Most common type of geologic structure o Tend to occur in sets o Can have more than one joint set within rock body Faults: brittle response of cracking and then move/shift in different directions o Different sizes: from a couple of inches to 100s of miles o Classified by slip direction Types of Faults: 1. Dip Slip Fault: inclined fault plane, vertical motion, one side up and one side down **Picture yourself walking down the fault line the side your feet would touch = foot wall a. Normal Dip Slip: when the hanging wall is lower, and the footwall is higher b. Reverse Dip Slip: when the foot wall is lower, and the hanging wall is higher Reverse Normal Reverse c. Thrust Fault: hanging wall higher than foot wall (not a steep incline, occur at subduction zones) 2. Strike-Slip Faults: horizontal movement (parallel to the fault plane) a. Left-lateral: no hanging/foot wall, each side thinks the other Top layer has slid up moved to the left on top of bottom layer b. Right lateral: each side thinks the other side moved to the right Faults and Forces Compressional: hanging wall slides up line Tensional: hanging wall slides down line Shearing force: hanging wall moves to side Lecture 7 Geology in the news: 6.4 magnitude earthquake in Southern Taiwan Apartment building fell over Several dead and 100 missing Chinese new year celebrations cancelled Earthquakes: build up of energy along a fault or plate margin 1. Stress < Friction 2. Stress ~ Friction (energy has built up, but still no movement, elastic deformation=rock starts to bend) 3. Stress > Friction (plates move) Earthquake Frequency Small earthquakes are quite common o Aprox. 1 million quakes with a magnitude of 2 per year ~ 2,740 per day o Only about 10 quakes with a magnitude of 7per year Power of Earthquakes Earthquake Movement Focus: underground point where the movement occurs on the fault in the earthquake o The center fault may not move o Many foci are only 2-20 km deep in continental crust (usually not any deeper b/c the rock has to be brittle for earthquakes to occur) Epicenter: geographic point on the surface of the earth directly about the focus Foreshocks: small movement that occurs before earthquake to try to relieve some of the built of energy Aftershocks: small movements that relief the rest of the energy after earthquake Seismic Waves Seismic waves: waves which the energy of an earthquake moves away from focus Three Types 1. Primary Waves (P waves): push/pull or compressional waves, fastest wave (6 km/s, 20x faster than the speed of sound), can move through both solids and liquids a. Ex) moves like a slinky or like a caterpillar 2. Secondary Waves (S waves): shear waves, vertical motion, slower (about half the speed of p waves), cannot move through liquid a. S wave shadow zone: area where s waves can’t get to (where the red lines don't reach in diagram) 3. Long Surface Waves (L waves): vertical and lateral motion, restricted to the surface of the earth Measurement and Detection Seismometer (aka seismograph, the outdated name): tool to detect earthquakes Three myths about seismometer: 1. It’s a solo machine: acutally needs 3 seismometers minimum (one calibrated for E/W, one calibrated for N/S, one calibrated for Up/Down or z axis) 2. Old fashioned looking machines in movies: those are outdated everything is done digitally now 3. Swinging needles: the pendulum used for drawing waves doesn’t actually move, but the machine itself does (pendulum isn’t used anymore, everything digital) Finding the focus: P waves arrive , then S waves arrive *Understand how this chart is used Measuring Damage 1. Mercalli Index: scale that measures how much damage/destruction the earthquake caused a. Uses roman numerals (I=lowest grade of damage) b. Not used by scientists b/c doesn't account for how populated the area hit was c. Easier to manipulate for insurance companies 2. Richter scale: measures the amount of shaking (Charles Richter 1935), logarithmic scale a. Not typically used by scientists 3. Moment magnitude: the amount of slip that occurs on the fault a. Easier to calculate b. Can be measured based on field data c. Best for scientific use Quakes and Plates Faults are along plate boundaries o So, earthquakes occur near plate boundaries o Deep earthquakes occur along subduction zones Because of this trend risk assessment maps are made *Some quakes can still occur far from plate margins But predicting earthquakes is difficult because every fault is different o Diff rocks/plate motion/forces o No magic formula to calculate when an earthquake will occur Damage Control Land Use Policies: o Ex) 1972 California Law made it illegal to build on the fault Building Codes: o Make sure buildings are stable so there is less damage Ex) Tall buildings can’t be built along fault lines Site Selection: o Solid rock is best to build on b/c of stability Liquefaction: o Loose sediment contains a lot of water, when seismic waves goes through the sediment, it creates mud Earthquake myth: o Rocks will crack open and swallow your entire house Lecture 8 Geology in the news: tin cans found within the walls of collapsed buildings after Taiwan earthquake Dating Methods Relative dating: putting together a sequence to know what came 1 , 2 , andnd 3 rd o Ex) Dinosaurs existed before humans o Cost effective/essentially free/easier Absolute dating: putting numbers on actual events o Ex) dinosaurs went extinct x million years ago o Can be expensive/more difficult/not always important Relative Dating Fossils: any evidence of past life forms o Footprint, plant impression, sea shell, etc o Mainly found in sedimentary rock Stratigraphy: subset of geology (predates) study of strata or layers of rocks Unconformities: breaks or gaps in time records where not all the time periods are represented in the layers of rocks (example on right) o Gap occurs when… Sediment deposits runs out or no other layers are formed during that time Run out of accommodation space (basin becomes full) Eroding sediment faster than it is being deposited Three types of Unconformities 1. Disconformity: sedimentary rock on one side of gap and a diff type of sedimentary rock on the other (black line=gap) 2. Nonconformity: sedimentary rock on one side of the gap and igneous or metamorphic on the other **study tip: nonconformity = not the same 3. Angular unconformity: layers underneath gap come up and strike gap at an angle (takes a long time to develop) Two Problems with Unconformities 1. Identification: hard to find the gap 2. Duration: hard to find out how much time is past Stratigraphic Principles 1. Principle of Original Horizontality: layers of sediment originally form as a horizontal line 2. Principle of Superposition: when looking at a stack of layers, the one on the bottom is the oldest and layers above get younger 3. Principle of Cross-Cutting Relationship: whatever did the cutting is the youngest 4. Principle of Faunal Succession: fossils seen in different layers are always going to occur in a set pattern (the oldest rocks contain oldest fossils) a. Correlation: linking two places together to see how old the rocks are in relation to one another Example: correlation found via faunal succession Correlation with Fossils Not all fossils are great for correlation o Want to identify short spans of time (more precision=stronger correlation) o Smaller, inconspicuous fossils usually better Index (or guide) fossils: ideal fossil to use for correlation Four Qualifications to be index fossil: 1. Were numerous: a. a lot of them don’t get fossilized, so a bigger population increases chances of fossils 2. Widespread: a. The more places it lives in, the more places that can be correlated together 3. Went extinct quickly: a. “Die as fast as you can.” b. Precision = stronger correlation (cockroaches are a bad index fossil b/c they’ve been around forever) 4. Easy to identify: to insure you’re comparing the same fossils Other Correlation Tools: Some places don’t have index fossils to use, so there are other dating methods Lithostratigraphy: correlations based on rock types o Decent at starting correlation but too many underlying factors to finalize correlation Sequence Stratigraphy: correlation based on pattern/sequence of unconformities o Beneficial to oil and gas industry b/c costal areas are ideal for sequence stratigraphy Chemostratigraphy: chemically correlate areas o Ex) Iridium anomaly at the Cretaceous-Tertiary boundary: meters and asteroids have a massive amount of iridium so there must be a connection there Magnetostratigraphy: magnetic signature that get recorded in rock layers o A little rough on the eyes: like trying to take two bar codes and slide them around until you can find matching black and white stripes Geologic Time Scale Originally built via stratigraphy Fossils were the key for defining boundaries Eons 1. Hadean: 1 billion years of the earths existence a. 4.5 Ga-4.0 Ga b. Density stratification period c. Earth’s earliest atmospheres/oceans form 2. Archean: continent building a. 4 Ga – 2.5 Ga (about ½ billion years long) 3. Proterozoic: oxygen build up in the atmosphere for the first time a. 2.5 Ga - 550 Ma 4. Phanerozoic: curretnt eon we’re living in where Fossil records are at its best Three Phanerozoic Eras 1. Paleozoic (550-200 Ma): translates to “ancient life” a. Cambrian Explosion: a sudden burst of new organisms on the scene 2. Mesozoic (200 Ma – 65 Ma): dinosaurs 3. Cenozoic (65 Ma – now): includes the ice age a. Mammoths became dominate species Geology in the News: new info on why quake occur deep in subduction zones water released from mineral called lawsonite reduces friction and enables the fault to move despite the high pressure environment Absolute Aging Two Approaches: 1. Non-radiometric: involve zero radiation 2. Radiometric: involve radioactive materials Non-radiometric Methods 1. Varves: type of sediment deposits a. Thin layers that alternate light and dark bands that only form under certain conditions (counting bands tells how long the climate in the area has met the proper conditions) i. Light color normal sediment deposits during warmer months ii. Dark color when lake ices over and additional sediment not added to water, but sediment floating in water settles and gets deposited ***Note: Varves only form if nothing disturbs the sediment (crabs, snails, creatures of that nature) 2. Dendrochronology: a. Counting the growth ring in trees (1 ring = yr, dark rings= tree had stopped growing) i. Note: some species of trees don't form rings yearly (some more than one ring per year or some less than one ring per year) ii. Forest fires can stop the trees from growing b. Used to learn about area and its environment i. Climate can affect if the tree grows quickly or slowly ii. Cross Reference trees with each other to get an extended time lie Radiometric Dating Isotopes: atoms of exact same element with different number of neutrons present o Radioactive isotopes: neutrons that are weighed unstable Radioactive Decay: radioactive atoms will break down to become more stable o Parent Isotope: before decay o Daughter Isotope: after decay (***can have multiple daughter isotopes) ***Note: there is no way to predict when an atom will start decaying, but the rate of decay is known so once decay starts, can predict when the process will end. Radiation: emitted energy Decay Series (aka chain): multiple steps/daughters of decay to become stable Half-life: the time it takes for half the current atoms to decay (# of parents divided by 2) Half-lives don't vary with any environment factor. o Th-234 = 24.1 days o Pb-210 = 22.3 years o U-238 = 4.4 billion years Radiometric techniques are created/improved by first dating things that we already know the age of (ex: Egyptian mummies) Requirements for Radiometric Dating: 1. Radioactive isotopes must be present in the specimen 2. Must have both parent and daughter atoms in specimen a. If only parent atoms decay hasn't started b. If only daughter atoms decay is already finished 3. Can only go so far back in time a. Usually can go max 5 isotopes back until parents atoms eventually runs out 4. Closed System: not interactive with its surroundings a. No parents or daughter isotopes to be added or subtracted i. Add parents/lose daughters: looks like little time has passed ii. Add daughters/lose parents: looks like more time has passed Case Study of Carbon 14 C-14 N-14 + particle + energy (carbon 14 decays to nitrogen 14) Half life of C14 = 5,730 years o Can track back to 10 half-lives o Extremely sensitive Earth hasn't run out of C-14 because this reaction… 0 14 14 + o Cosmogenic ray + n + N C + p o C-14 globally distributed because atmospheric reaction all around the world o Amount decaying = amount produced Some isotopes do go extinct because they’re not reproduced Carbon Dating Carbon dating can only go back so many year and can only work for certain materials (doesn't work for rocks/minerals) o Animals are exposed to C-14 through the food chain C14 + O2 = 14CO2 in atmosphere, plant filters the 1CO2, the animal eats the plant, and from there after, everything in the food chain is exposed to C14 When the animal dies, it becomes a closed system because no parent atoms added by eating Some organisms don't get carbons from the atmosphere or food chain Decay Equation Age = [ln(N /f 0/-0.693]*half-life o N /N = % 1C in the sample relative to amount found in living tissue f 0 (i.e., how much parent is left) Ex: fossil still has 10% of its4C Age = [ln(0.10) / -0.693) * 5,730 Age = 18,940 years old Assumptions for Carbone-14 1. System remains closed after death a. Not true extra tests are required to ensure specimen was a closed system 2. Amount of C-14 in living tissue doesn't vary through time a. Not true… i. C-14 production varies based on solar cycles but variations are small/on short timescales ii. Fossil fuel burning changes the amount of C-14 in the atmosphere (scientists already took into account this change and fixed their methods)
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