Geology Lectures 6&7
Geology Lectures 6&7 Geol101
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This 98 page Class Notes was uploaded by Shelby Green on Thursday February 11, 2016. The Class Notes belongs to Geol101 at Clemson University taught by Dr. Coulson in Spring 2016. Since its upload, it has received 64 views. For similar materials see Physical Geology in Geology at Clemson University.
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Date Created: 02/11/16
Announcements Geology in the News Lecture 6- Metamorphism & Structural Geology Metamorphism • PT 1- Causes of Metamorphism • PT 2- Metamorphic Rocks & Environments Structural Geology • PT 1- Introduction & Tectonic Forces • PT 2- Types of Structures Intro to Metamorphism • Why do we care? • Metamorphic rocks contain many exotic materials/helps to understand the geographic history of an area • Speed – Metamorphism is a very slow process (can occur over millions of years) Temperature • Geothermal gradient: how fast does the temp go up with depth in the earth? – Varies from place to place Temperature • Avg: 30 C/km • Typical range: 20 - 60 C/km Metamorphism via Heat • Contact metamorphism: rock undergoes metamorphism due to direct heart from magma coming towards the surface – Primarily temp driven – Limited in scope/only occurs close to magma Pressure • Pascals and bars – 1 bar = atmospheric press @ surface Pressure • Pressure gradient: ~300 bar / km depth Pressure • Confining pressure: rock is feeling equal pressure from all directions – Ex: swimming underwater See file 6c Pressure • Directed pressure (aka Differential): the majority of the pressure is being applied in a specific direction How Much Pressure is Needed? • Most metamorphic rocks form at 10-30 km depth (mid-lower crust) • Q: how many miles is that? (1 mile = 1.6 km) See file 6b Exposure • How do metamorphic rocks get back to the surface? • Faults rocks on either side move/broken down through weathering and erosion Metamorphism via Pressure • Regional metamorphism-large scale process/apposite of contact metamorphism Other Types of Metamorphism • Fault Metamorphism: occurs on a small scale right along the fault line/ pressure is still the primary driver Metamorphism via Fluid • Metasomatism- rock comes in contact with very hot ground water/water flushes things out of the rock or deposits ne mienrals that changes the mineralogy • Ores-high concentration of a specific mineral Metamorphism via Fluid • Seafloor metamorphism: almost always forms basalt/specialized in environment where the cold seawater interacts with the rock PT 2- Met- Rks & Environments • Q: What metamorphic rock forms? • A: Parent rock’s composition is key Same T/P increase Metamorphic Change • Metamorphic Grade: measures metamorphic change due to change in temp & pressure – Low, intermediate, high – Don’t tell you anything specific/needs to be better differentiated Metamorphic Change • Index minerals: a mineral cant help identify the correct met. grade – Each index mineral is formed in very specific envi. Metamorphic Change • Metamorphic Facies: group of index minerals that form at the same temp and pressure • Ex- Blueschist facies includes the minerals glaucophane, lawsonite, epidote 7 Major Metamorphic Facies Interpreting Facies • Use facies info 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 Length of Metamorphism • Changes w/in minerals can record Pressure-Temp changes – Un-uniformity allows for tracking – Crystal grows outward w/ rings like a tree trunk younger Types of Metamorphic Rocks • 1- Foliated Metamorphic Rocks – Slate – Schist: hard to identify (samples can vary drastically) – Gneiss (“nice”) * These rocks are formed from different pressures Types of Met Rocks • 2- Non-foliated: large crystals – Hornfels – Quartzite – Marble • High concentration of one specific mineral • Appearance is very similar so you cannot eyeball the identity Review- Things to Know • Temp & press gradients • Types of metamorphism • Grade vs facies • Foliated vs nonfoliated • Hornfels & blueschist facies examples Announcements Geology in the News • “sudden” volcanic eruptions found to trigger from gas bubbles in magma Structural Geology • PT 1- Structural Geology & Tectonic Forces • PT 2- Types of Structures PT 1- Structural Geology • What is structural geology? Rock deformation – Focuses on how rocks get deformed after they’ve already been created Geologic Structures • Topographic (landscape) features vs geologic structures • There’s a diff between the two Tectonic Forces • #1 Tensional-stretching an object – Pulling away from the center in two directions Tectonic Forces • #2 Compressional-the area is getting squeezed or compacted Tectonic Forces • #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 Responses to Stress • Response can vary based on – rock type – Temp/Pressure – speed of deformation: faster the application the more brittle the response PT 2- Types of Structures • Folds: formed with a ductile response to a compression force Time 1 Time 2- apply compressional force Time 3 Classifying Folds • Classified based on 3 things: • 1- Shape (in Road Cut or Cross-Section view) • Antiform: the rainbow shape below Classifying Folds • Synform: the smile shape below Classifying Folds • Overturned: – Overturned antiform (blue) – Overturned (purple: not enough info whether it is anti or syn) Classifying Folds • 2- Age of the layers relative to each other • Anticline: oldest later in the center fold between the limbs Oldest rocks Classifying Folds • Also an anticline! Oldest rocks Classifying Folds • Syncline: the center fold layer is the youngest rocks (opposite of anticline) Oldest rocks Classifying Folds • Also a syncline Oldest rocks Confused? • Sometimes tectonic forces can cause an entire stack of layers to get turned upside down. • This is why you can have an antiformal syncline and an synclinal antiform Classifying Folds • 3- Geometry • Horizontal: squeezed in from two sides and nothing else – Lines on top look horizontal See file 6d Classifying Folds • Plunging: *more common – Squeeze from sides and tipped up or down What kind of folds are these? Youngest layers Ex: • Age or layers = Oldest layers Syncline • Shape = synform • Geometry = not Ex: enough info • Age or layers = Anticline Youngest layers • Geometry= Plunging Oldest layers • Shape = Antiform Types of Structures • Joints: brittle response – Most common type of geologic structure – Tend to occur in sets – Can have more than one joint set w/in rock body Types of Structures • Faults-brittle response of cracking and then move/shift in different directions • Different sizes: couple of inches to 100s of miles • Classified by slip direction – Which direction is it moving 1- Dip-Slip faults • Inclined fault plane, vertical motion, one side up, one side down Hanging Wall – Picture yourself walking down the fault line • The side to your feet would be touching = foot wall Foot Wall Dip-Slip faults • 1a- Normal (dip slip) fault: when hanging wall is lower and the footwall is higher Dip-Slip Faults • 1b- Reverse (dip slip) fault: the foot wall is lower and the hanging wall is higher *Study Hint* • You must ID the hanging wall vs foot wall before determining if it’s a normal or reverse fault • Both these are reverse faults Dip-Slip Faults • 1c- Thrust fault – Occur at subduction zones – Special type of reverse fault • Hanging wall higher than foot wall – Not a sleep incline *purple layer has slid up on top of white layer 2- Strike-Slip Faults • Horizontal movement (parallel to the fault plane) • 2a- Left-lateral – No hanging/foot wall – Each side thinks the other moved to the left Map views rather than cross-sections!!! 2b- Right-lateral (strike-slip) fault: Each sides thinks the other side moved to the right What type of faults are these? (bird’s-eye view) (cross-sectional view) Left lateral strike slip Normal dip slip *Study Hint* • Make sure to get your orientation before trying to identify the fault • Q: is this a road-cut view or an aerial view? Aerial view See file 6e Faults & Forces • Diff fault types form depending on the type of force applied Compressional force: hanging wall slides up wall Tensional force: hanging wall slides down Shearing force: hanging wall moves to side Practice: Use the Force! Identify the tectonic setting, the structure, and the tectonic force applied Review- Things to Know • Types of tectonic forces • Types of geologic structures • Be able to ID structures • Know what force forms each type of structure Announcements 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 Lecture 7- Earthquakes • PT 1- Introduction to Earthquakes • PT 2- Seismic Waves • PT 3- Detection, Measuring, and Responding to Earthquakes PT 1- Introduction to Earthquakes • Why do we care? Prevention, responding, precautions, etc What Causes Earthquakes? • Earthquake: build up of energy along a fault or plate margin • Motion along faults TIME 1: Stress < friction What Causes Earthquakes? • TIME 2: Stress ~ friction – Energy has built up, but still no movement • Elastic deformation: when rocks start to bend What Causes Earthquakes? • TIME 3: Stress > friction – Plates move • Process starts over Stress > friction How Frequent Are Earthquakes? • Small ones are quite common • Ex: aprox 1 million magnitude 2 quakes a yr • Q: how many is that per day? 2,740 *only about 10 mag 7 per yr How Powerful Are Large Earthquakes? • Compared to some other things: The Point of Movement • Focus-underground point where the movement occurs on the fault in the earthquake – The enter fault may not move • Epicenter-geographic point on the surface of the earth directly about the focus Many foci are only 2-20 km deep in continental crust *not usually any deeper b/c of the rock has to be brittle for earthquakes to occur Movements Before & After • Foreshocks-small movement that occur before earthquake to try to relieve some of the built of energy • Aftershocks-small movements that relief the rest of the energy after earthquake PT 2- Seismic Waves • Seismic waves: waves in which the energy moves away from focus • 3 different types produced • 1- P (Primary) waves- push/pull waves, also called compressional waves are the fastest waves (6 km per second, 20x faster than the speed of sound through air), can move thru both solids/liquids – Ex: slinky Seismic Waves • 2- S (secondary, shear) waves: vertical motion, slower (about half speed of p waves), cannot move through liquid Seismic Waves • S wave shadow zone: area where s waves can’t get to See file 8b Seismic Waves • 3 - L (long, surface) waves: vertical & lateral motion, restricted to the surface of the earth PT 3- Measurement & Detection • Seismometer (aka Seismograph, outdated): tool to detect earthquakes • 3 Myths to debunk: • 1- solo machines: – Need 3 sesmometers at Minimum, one calibrate for east west, one calibrated for north south, one for up/down (z axis) PT 3- Measurement & Detection • 2- old-fashioned machines: everything is done digitally See file 7c PT 3- Measurement & Detection • 3 swinging needles: the pendulum does not move, but the machine itself does **completely digitally now Seismometer Data **L wave slowest,the so it will arrive last Where Did It Happen? • Key to finding the focus is that the different waves travel at different speeds -p waves arrive first -s waves arrive last = focus point Station P arrival SArrival Dist Cola 1:02:11 1:02:13 40 mi Flor 1:02:12 1:02:15 50 mi Char 1:02:13 1:02:17 68 mi How Big Was It? • 1- Mercalli Index: scale that measures how much damage/destruction the earthquake caused (uses roman numerals low roman numeral=little damage, high roman numeral=lots of damage) * Used by insurance companies to get more for damage *not typically used by scientists – damage will be in the city than in the desertd hits desert more Magnitude continued • 2- Richter Scale: measures the amount of shaking (Charles Richter 1935), logarithmic scale * Not typically used my scientist Magnitude con’t • 3- Moment magnitude: the amount of slip that occurs on the fault – Easier to calculate – Can be measured based on field data – Best for scientific use Quakes & Plates • Fault occur in sets – Earthquakes occur near plate boundaries – Deep earthquakes occur along subduction zones Risk Assessment b/c the trends we found in slide before, we can make risk assessment maps *Note- some quakes occur far from plate margins Predicting Earthquakes • Difficult because every fault is different – Diff rocks/plate motion/forces – No magic formula Damage Control • Land Use Policies – ex: 1972 California Law • made it illegal to build on the fault • Building codes – make more stable so there is less damage • Ex: Don’t allow tall buildings on fault line Damage Control • Myth- Rocks will crack open and swallow your entire house Damage Control • Site Selection: solid rock best to build on b/c of stability Damage Control continued • Liquefaction: loose sediment contains a lot of water – Waves go through it and create mud Review- Things to Know • Types of seismic waves • Seismometers • Scales used to measure quakes • Hazards and how to minimize them
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