Geology Week 5 Notes
Geology Week 5 Notes 80176 - GEOL 1010 - 001
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80176 - GEOL 1010 - 001
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This 9 page Class Notes was uploaded by Sarah Canterbury on Friday February 12, 2016. The Class Notes belongs to 80176 - GEOL 1010 - 001 at Clemson University taught by Alan B Coulson in Fall 2015. Since its upload, it has received 69 views. For similar materials see Physical Geology in Environmental Science at Clemson University.
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Date Created: 02/12/16
Lecture 8—2/9/16 Geology in the News Sudden volcanic erupting found to trigger from gas bubble rapidly formed in magma underneath the volcano This study could help discover why sudden volcanic eruptions happen Part 1- Structural Geology What is structural geology? Focuses on how rocks get deformed after rocks are created Deformation: when rocks don’t look like how they’re expected to look; something must have happened to them to make them that way Topographic features vs geologic structures Topographic features: landscape features that appear on the earth’s surface Ex) Canyons Topographic features are an aspect f geology, but not what we are talking about Tectonic Forces 1- Tensional Force: stretching an object; pulling an object apart in two different directions 2- Compressional Forces: squeezing an object from both sides; compacting the object 3- Shearing: object is being slid in two different directions at once These 3 types of forces line up with the different types of plate boundaries Convergent—compressionsal; Divergent—tensional; Transform—shearing Response to Stress Brittle: when apply a lot of forces, the object will snap and break Ductile: when apply a lot of forces, the object will bend Response can vary based on: -Rock type -Temperature and pressure -Speed of deformation: applying force suddenly or gradually? The faster force is applied, the more likely the rock is to be brittle and vice versa Part 2- Types of Structures Fold: ductile response to a compressional force When a compressional force is applied to a rock and the rock bends (ductile response) Difficult to have just one fold; will end up having a wavy pattern within the rock’s layers Limb: sections of the rock layers that are still fairly straight Hinge: center point where the rock layers twist into a different direction Classifying Folds Classified on 3 things: 1- Shape of fold (in cross-section (aka road-cut or cliff) view) If fold has an arch or rainbow shape, known as antiform If fold has ‘u’ or unpside down arch shape, known as synform Sideways tipped when can’t tell if previously antiform or synform, just known as overturned Altered antiform: overturned antiform (same rule applies to synform) Never name it unless sure of what it is 2- Age of the layers relative to each other Anticline Fold: oldest layer is in between two limbs Shape is not important Syncline Fold: oldest layer is on the outside of the other limbs Why can older rocks be on the outside? Sometimes tectonic forces can cause an entire stack of layers to get turned upside down, which is why you can have an antiformal syncline/synclinal antiform 3- Geometry Horizontal Folds: from birds’ eye view or from the side view (not cross sectional view), all you see are horizontal layers Plunging Folds: while layers are squeezed, there’s another force applied, which makes layers appear like the layers are tipping up or down Joints: a brittle response of rock with no more movement No more movement means that when enough pressure is applied, the rock will snap, and that will be the end of movement Doesn’t mean the rock can’t break on multiple occassions Usually end up with a lot of cracks in the rock Usually happen in sets: a set of rock can break on multiple occasions and crack in more than one direction Faults: brittle response, but with movement Once the rock is cracked, one side will slide down or up Motion sense is relative; we don’t know which side of the rock moved or by how much, but the pattern will appear the same Dip-Slip fault: inclined fault plane; more vertical movement Hanging wall: side of the crack that is on top Foot wall: side of the crack on the bottom. Way to remember: if you draw a line on the crack and a stick person walking down the line, the side in contact with the person’s feet is the foot wall and the side that’s hanging over his head is the hanging wall 1-Normal dip slip fault: hanging wall appears to have moved down and foot wall appear to have moved up 2-Reverse dip slip fault: hanging wall appears to have moved up and foot wall appears to have moved down ***Study Hint: You MUST identify the hanging wall versus the foot wall BEFORE determining if it’s a normal or reverse fault*** 3-Thrust fault: same movement as reverse dip slip, but the fault appears almost horizontal Difficult to identify in the real world Subduction zones have a lot of thrust faults associated with them Strike Slip faults: horizontal movement (map or birds’ eye view); fault cuts straight down, so don’t need to work about the hanging wall or foot wall, because both sides look the same Left-lateral fault: both blocks seem to have moved to the left relative to each other To identify, put yourself on one side of the fault and see in what direction the other side seems to have moved, and the other side will think you moved to the left also Right-lateral fault: both blocks seem to have moved to the right relative to each other To identify, same process as left lateral, but instead of left, use right ***Study Hint: Make sure to get the orientation before trying to identify the fault*** Faults and Forces Different faults form depending on the type of force applied Compression—Reverse dip slip; Tensional—Normal Dip Slip; Shearing—Strike slip Lecture 9—2/11/16 Geology in the News 6.4 magnitude earthquake in Southern Taiwan Apartment buildings fell over Several dead and 100 missing Chinese New Year celebrations cancelled—Big Deal Part 1—Introduction to Earthquakes Why do we care? Cause a lot of damage, loss of life, gives us an incentive to try and prevent What causes earthquakes? Earthquake: build-up of energy along a fault as the rocks try to move past one another Motion along faults Tectonic force try to push past each other, but friction prevents, so energy builds up so that there’s enough energy to move the rocks. Before the rocks are able to move, the bend and become deformed, and finally, the energy has built up enough to overcome the friction and move past one another Elastic Deformation: rocks start to bend or deform some Then the process starts over Explains why some places have several earthquakes How frequent are Earthquakes? Small ones are quite common Ex) About 1 million magnitude 2 earthquakes happen each year (~2700 per day) Larger ones (magnitude 7) are not very frequent- about 10 per year Largest ones (magnitude 9) only less than _____ per year How powerful are Large Earthquakes? Ex) Magnitude 9 earthquake gives off about the same energy as Mount St. Helen eruption or the Annual US energy use The Point of Movement Focus: point on the fault where the movement occurs Always underground Many foci are only 2 – 20 km deep in continental crust Can be deeper, but it’s uncommon because past 20 km, rocks will start acting more ductile. For an earthquake to occur, need brittle motion and breakage Epicenter: point on the earth’s surface directly above focus Movements Before and After Foreshocks: small movements that occur before the actual earthquake occurs Attempt by the earth to release some of the energy that has built up along the fault Aftershocks: small movement that occur after the earthquakes Attempt to release some of the residual energy Part 2—Seismic Waves 3 Different types produced: 1- P (primary) waves: compressional motion; push-pull motion; as one part of the material is compressed, another part has to be stretched Fastest waves, moving about 6km/s (20 times faster than the speed of sound) Can move through both solids and liquids 2- S (secondary, shear) waves: have vertical motion, so not only does it move to the right, but they also moves up and down (appear more wavy) Slower (about half the speed) than a P wave Can’t move through liquids S wave shadow zone: S waves move away from the center in all directions, but when they come in contact with outer core, they stop because they can’t move through the liquid; therefore, they won’t go through to the other side of the earth, the area we call the S wave shadow zone 3- L (long, surface) waves: have both horizontal and vertical motion, leading to a somewhat circular motion Slower than S waves Restricted to moving along the surface Part 3—Measurement and Detection Seismometer (Seismograph [outdated term]): instruments to measure and detect earthquakes 3 Myths to Debunk 1- Solo Machines: they aren’t just one machine that can give all details about an earthquake 3 machines: need one calibrated to east and west; one calibrated to north and south; and one calibrated to up and down 2- Old-fashioned machines: today, we use computers and newer data 3- Swinging needles: the needle is held by a pendulum; so what actually happens is that the machine is shaking. Since it’s hanging from a pendulum, the needle doesn’t move along with the machine—don’t get accurate readings Seismometer Data Amplitude vs Travel time 3 sets of data because of the 3 different machines P waves are the fastest, so they arrive to the seismometer first Then the S waves will arrive L waves arrive last because they’re the slowest Where did it happen? Knowing where the focus lies is important because need to know where to send rescue Key to find the focus is that the different waves travel at different speeds Each station will record how much time is between P waves and S waves, and plug that information into a formula that will tell how far away the focus of the earthquake is Then each station will draw a circle with a radius of the distance between station and focus, and where every circle intersects is where the focus is How big was the Earthquake? 1- Mercalli Index: measures how much damage Use Roman numerals (small Roman numerals means little damage; big Roman numerals means a lot of damage) Not typically used by scientists because you’ll have a different index depending on where you are. Depends on the amount of infrastructure or people in the area Good for who lives in the area, such as insurance people and construction workers, so they know what they need to do 2- Richter Scale: measures the amount of shaking and how much movement occurred Developed in 1935 by Charles Richter Logarithmic (ex- magnitude 3 will experience 10 times more shaking than magnitude 2) 3- Moment magnitude: easiest to calculate with seismometer data Most used by scientists Quakes and Plates Faults and earthquakes occur in sets Earthquakes usually occur along tectonic plates Deepest foci occur along subduction zones because at those points, the temperature is low but high pressure. So since the temperature is low, the rocks aren’t turning ductile yet Risk Assessment Note: some earthquakes occur far from plate margins Predicting earthquakes is difficult because every fault is different Doesn’t mean we shouldn’t try Some scientists are working to measure elastic deformation to see if that can help to predict earthquakes Damage Control Land Use Policies Ex) 1972 California Law: can’t build on a fault Building codes Use reinforced concrete for foundation of homes and buildings Shouldn’t build skyscrapers near faults Myth: Rocks will crack open and swallow your entire house Site Selection Best location to build anything is on solid rock because waves will move through the rock very quickly Don’t build on loose sediments Liquefaction: loose sand often has a lot of water in it, so building on it will fall over or sink because the seismic waves move the loose sediments around
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