Exam 1 Study Guide
Exam 1 Study Guide EAPS 10600 - 002
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This 15 page Study Guide was uploaded by Katy Cook on Sunday January 31, 2016. The Study Guide belongs to EAPS 10600 - 002 at Purdue University taught by Andrew M Freed in Fall 2015. Since its upload, it has received 53 views. For similar materials see Geosciences In The Cinema in Environmental Science at Purdue University.
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Date Created: 01/31/16
UNIT 1- Plate Tectonics Earth has 4 major layers: crust (solid) mantle (solid) outer core (liquid) inner core (solid) Internal structure due to how Earth formed Collisions cause motion to be converted into heat Heat from collisions made Earth hot enough to melt interior, allowing heavy minerals (iron & nickel) to sink and form core Sinking of heavier minerals led Earth’s interior to differentiate into: core, mantle, and crust that then froze in place as Earth cooled Today only the outer core remains hotter than its melting temperature only liquid layer Inner core is solid because it is under more pressure (melting temp increases with depth) Plate tectonics: cold strong lithosphere (plates) float over warm weak asthenosphere Layering by physical properties vs layering by chemical composition Lithosphere- strong outer part makes up plates Asthenosphere- weaker layer in which plates float on Earth’s outer shell is broken into 13 major plates that are all in motion Tectonic plates slide past each other in three ways: - Convergent: towards each other - Divergent: away from each other - Transform: slide past each other San Andreas fault- example of a transform plate boundary No spaces between plates Why does earth have plate tectonics? - It is an efficient way for the Earth to cool down Plate tectonics is driven by mantle convection: the process of cooling off Earth’s hot interior Convection- the process by which heat is transferred through a liquid Convection in the earth drives plate tectonics Convection leads to new plate material forming at mid-ocean ridges and old plate material being destroyed at subduction zones Alfred Wegener- argued the continents were drifting - Coastlines fit together - Identical volcanic flows on both sides of the Atlantic - Fossils of non-swimmers found on both sides of the modern oceans Began to measure the age of ocean crust realized ocean floors are moving too Ocean basins started out as rift valleys If it continues to spread for millions of years, the modern day East African Rift will one day be the center of an ocean Reconstruction of plate motions starting from 180 million years ago (the breakup of the supercontinent Pangaea) Tectonic plates move about as fast as your fingernails grow The evolution of whales appears tied to the collision of India into Eurasian and the closing of the Tethys Sea (land animals could migrate farther) Subduction zones are the places where the most tectonic action occurs - Most and biggest earthquakes - Most volcanoes - Highest topography - Deepest trenches Trenches due to crust being pulled down Icebergs and mountains both float within a volume of denser material - Called “Isostasy” Ice is 10% less dense than water, so about 10% of an iceberg floats above the surface Crust is 15% less dense than mantle, so 15% of a mountain floats above the surface Isostasy explains most of Earth’s topography Why does the Earth have a magnetic field? - Earth’s magnetic field arises from motion within the liquid iron outer core - The magnetic field created in the outer core acts like a huge bar magnet is located in the Earth’s core The magnetic field arises from the complex motion associated with convection within the outer core (called a dynamo) Combination of mantle convection with spinning Earth What would happen if the Earth lost its magnetic field? - Protection from harmful solar radiation comes primarily from our atmosphere, not the magnetic field - Large solar flares (massive concentrations of charged particles) can knock out power grids and scramble communications systems. Without the magnetic field, these problems would get worse, but would not be deadly - Birds and planes would not fall from the sky, lightning would not destroy cities, people with pacemakers would not drop dead, no mass extinctions Sometimes the magnetic poles reverse- magnetic north becomes south and magnetic south becomes north Magnetic reversals are recorded in lava flows by magnetic crystals Magnetic reversals have occurred throughout Earth’s history Each magnetic reversal leads to a period of several thousand years when the magnetic field all but disappears No mass extinctions have occurred during magnetic reversals Geodes form in cavities that fill with mineral rich water that leads to crystal growth - Only found close to the Earth’s surface Chapter 2 Reading Fault: fracture along which one plate has moved relative to the other San Andreas fault: Los Angeles to the South and San Francisco to the North Los Angeles is on the Pacific plate and slowly moving toward San Francisco on the North American Plate Internal Structure of the Earth Rigid outer shell, solid center, and thick layer of liquid between the two that moves around as a result of internal processes Solid inner core: temperature as high as the surface of the sun, primarily metallic (iron and minor amounts of sulfur, oxygen, and nickel) Liquid outer core: composition similar to that of the inner core Mantle: surrounds the outer core, composed mostly of solid iron and magnesium-rich silicate rocks Crust: outer rock layer of the earth, the boundary between the mantle and crust is called the Moho Lithosphere- the cool, strong outermost layer, stronger and more rigid than the material underneath Asthenosphere- hot and slowly flowing layer of relatively weak rock Crustal rocks are less dense than the mantle rocks below Oceanic crust is slightly more dense than continental crust Oceanic crust is thinner Convection: temperature-driven circulation (ex: lava lamp) Mantle convection is fueled by Earth’s internal heat Plate Tectonics Tectonics: the large-scale geological process that deform Earth’s lithosphere, producing landforms such as ocean basins, continents, and mountains Plate tectonics: processes associated with the creation, movement, and destruction of these plates All of the major plates include both part of a continent and a part of an ocean basin Plate boundaries are geologically active areas Tectonic cycle: the continuous recycling of tectonic plates Continental drift- based on congruity of shape of the continents, and similarity of fossils found in South America and Africa Seafloor spreading- in seafloor regions (mid-oceanic ridges) or spreading centers, tectonic plates move away from one another causes plates to grow because hot rock rises to fill void spaces and then cools, adding new oceanic lithosphere to the plate edge Subduction zones: Oceanic lithosphere is destroyed where cool, dense lithosphere sinks into the asthenosphere Magma: molten rock When magma rises to the surface, volcanoes form along the subduction zone The path of the descending plate into the upper mantle is marked by earthquakes Wadati-Benioff zones: inclined planes of earthquakes Convection drives plate tectonics Divergent boundaries: neighboring parts of plates are moving away from each other, new lithosphere is being produced - Mid-ocean ridges form – called rift valley or rift because the plates are pulling the lithosphere apart and splitting (rifting) it Convergent boundaries: occur where plates move toward each other - Ocean-continent (subduction) higher-density oceanic plate descends into the mantle beneath, volcanic arc, and deep trenches - Ocean-Ocean (subduction) older, denser oceanic plate sinks beneath younger, less dense oceanic plate, volcanic arc and deep trench - Continent-continent (collision) compression results, mountain chains, earthquakes Transform boundaries: Plates slide past one another, earthquakes Submarine trench: relatively narrow depression on the ocean floor Suture zone: where two plates join Triple junctions: locations where three plates border one another Seafloor spreading Validated by 1. Identification and mapping of oceanic ridges 2. Dating of volcanic rocks on the floor of the ocean 3. Understanding and mapping of the paleomagnetic history of ocean basins Earth has a dipole magnetic field (equal and opposite charges) Convection occurs in the iron-rich fluid, hot outer core of Earth because of compositional changes and heat at the inner-outer core boundary Curie point: temperature in which volcanic rock orients themselves parallel to the magnetic field and becomes magnetized Paleomagnetism: study of the magnetism of rocks Earth’s magnetic field periodically reverses Cause of magnetic reversals is not well known, random Magnetic anomalies can be represented as stripes on maps (black = normal) Merging the magnetic anomalies with the numerical ages of the rocks produced the record of seafloor spreading Youngest volcanic rocks are found along active mid-oceanic ridges Hot spots: volcanic centers resulting from hot materials produced deep in the mantle - The partially molten materials are hot and buoyant enough to move up through the mantle and the overlying tectonic plates Seamounts: submarine volcanoes (islands eroded by waves and landslides) Pangaea and present continents Continental drift started 180 million years ago with breakup of supercontinent “Pangaea” Occurrence of the same fossil plants and animals on different continents Evidence of ancient glaciation on several continents, with inferred directions of ice flow How plate tectonics work Ridge push: gravitational push away from the ridge crest toward the subduction zone Slab pull: results when the lithospheric plate moves farther from the ridge and cools, gradually becoming denser than the asthenosphere beneath it Slab pull more influential than ridge push in moving plates Plate tectonics and hazards At divergent plate boundaries: earthquakes and volcanic eruption Along boundaries where one plate slides past another: earthquakes, hilly or mountainous areas where rainfall may be higher landslides and flooding Convergent plate boundaries: Explosive volcanoes, landslides and flooding, monsoon Chapter 3.1-3.3 Reading Tsunami: a large ocean wave triggered by an earthquake Introduction to Earthquakes Most earthquakes occur along plate boundaries Earthquakes occur along a plane of weakness in Earth’s crust known as a fault Fault: semiplanar fracture or fracture system where rocks have been displaced Footwall: block below the fault plane Hanging wall: block above the fault plane Faulting: process of fault rupture Plates moving past another is slowed by friction along the fault plane Stress is a force that results from plate tectonic movements Strain is the change in shape or location of the rocks due to applied stress Three different types of stress: tensional, compressional, shearing Dip-slip fault: offsets rocks in a vertical motion due to compressional or tensional stresses Strike-slip fault: offsets blocks of crust in a horizontal direction due to shearing stress Normal fault: identified by downward movement of the hanging wall relative to the footwall Reverse fault: upward relative movement of the hanging wall due to compression (convergent plate boundaries) Strike-slip faults are either right-lateral or left-lateral Blind faults: do not extend to the surface (identified by anticlines and synclines which are folds at the surface) The Earthquake Processes Active fault: moved during past 10,000 years of the Holocene Epoch Inactive: not moved during past 2 million years Paleoseismicity: prehistoric record of earthquakes Earthquake cycle: there is a drop in elastic strain after an earthquake and an accumulation of strain before the next event Elastic strain: deformation that is not permanent, provided the stress is eventually released (ex: rubber band returning or breaking) Elastic rebound: rocks deform elastically until a critical point is reached and the fault slips, releasing the stored elastic energy Four stages of earthquake cycle: 1. Long period of inactivity along segment of geologic fault 2. Accumulated elastic strain produces small earthquakes 3. Foreshocks (small-moderate earthquakes before main event) 4. Mainshock- major earthquake and its aftershocks Epicenter: place on the surface of the Earth above where the ruptured rocks broke to produce the earthquake Focus: point of initial breaking or rupturing within the earth Sudden rupture of rocks produces shock waves (seismic waves) Body waves: seismic waves within the body of the Earth Surface waves: travel along surface Two types of body waves- S waves and P waves P waves: compressional or primary waves, faster, can travel through solids, liquids, and gas (fastest through solids) S waves: shear or secondary waves, only travel through solids, slower, produce up-and-down motion Surface waves- slowest, cause much of the damage Tectonic creep- gradual movement along a fault that is not accompanied by perceptible earthquakes Slow earthquakes are also produced by fault rupture, but can last from days to months Earthquake shaking Influenced by magnitude, location in relation to the epicenter and direction of rupture, and local soil and rock conditions The first magnitude estimates are made using the Richter scale Richter magnitude determined by measuring the maximum amount of ground shaking due to the S wave Seismograph records ground motion Richter scale is logarithmic Moment magnitude scale: measures actual energy released during earthquake Earthquakes are given descriptive adjectives based on their magnitude - Major earthquakes (7-7.9) - Strong (6-6.9) - Great earthquakes (above 8) Modified Mercalli Intensity scale- earthquake intensity, qualitative, assigned one of twelve Roman numerals Shake map: shows both intensity of shaking and potential damage Attenuation: loss of energy before waves reach surface Directivity: path of greatest rupture can focus earthquake energy Seismogram- written or digital record of seismic waves Triangulation: process of locating a feature using distances from three points Distance to the epicenter is calculated for each of three seismographs and values used for radius of circle around each station, intersection is the epicenter Supershear: propagation of rupture is faster than the velocity of shear waves, can produce strong ground motion Seismic waves move faster through consolidated bedrock than sediment or soil Material amplification- as P an S waves slow, the energy is transferred to the vertical motion of surface waves Unit 1 Continued The hollow Earth conspiracy: the realm of Aghartha How do we know the Earth is not hollow? - Laboratory experiments show that rocks are too weak at the high temperatures and pressures in Earth’s interior to support voids - Seismic waves show no voids - The magnitude of our gravity requires a dense interior to our plate - No known physics to build a hollow planet with (or without) a tiny sun at its center wrong center of gravity - If the Earth was hollow, everyone standing on the inside shell would fall into the central sun and burn up Diamonds come from am rare type of volcanic eruption that brought them up from great depths at great speed Only get to surface through magma grabbing country rock (with diamonds inside) The last kimberlite eruption is thought to have taken place more than 25 million years ago The cullian diamond is the largest gem-quality diamond ever found Unit 2 Earthquakes Biggest Earthquakes 1. Chile 1960 (9.5) 2. Alaska 1964 (9.2) 3. Sumatra 2004 (9.1) 4. Japan 2011 (9.0) 5. Kamchatka 1952 (9.0) Around 1.5 million earthquakes/year Alaska earthquake equivalent to about 150,000 atom bombs Earthquake in Chile leveled Hilo Hawaii 6,500 miles away Large earthquakes recur on the San Andreas fault about every 150 years Earthquakes occur because of elastic rebound The bending of a stick builds up stress and elastic energy. When the stress exceeds a threshold, the stick breaks and the elastic energy is released, enabling the two halves of the stick to straighten out The same process happens with the Earth’s crust: it bends due to motion of the plates, building up stress and elastic energy. It then breaks at a fault (line of weakness) when the stress exceeds a threshold, releasing the elastic energy. This allows the crust to straighten out (just like the stick) Faults experience stick-slip behavior, the process in which a fault remains stuck for a long time as stresses build, then slips quickly during an earthquake Limited fault strength means fault gets stuck for a while, then break when stresses get too high No fault strength means the fault never gets stuck no earthquakes Earthquakes occur when asperities break Repeated earthquakes allow very long displacements to develop along faults A right-lateral strike-slip fault: if you stand on one side of the fault and look at the other, it moves to the right during an earthquake The San Francisco region is currently building stress (and elastic energy) toward a large earthquake Three types of faults arise from three types of stress states - Thrust faulting (compressional loading, convergent boundaries) - Normal faulting (extensional loading, divergent boundaries) - Strike-slip faulting (shear loading, transform boundaries) San Andreas is right-lateral strike-slip fault An earthquake can deform train tracks Why does an earthquake cause shaking? - The rubbing of the two sides of a fault during an earthquake generates seismic waves, like hands rubbed together make sound waves - Seismic waves propagate outwards like water wave when a stone is dropped into a pond - Seismic wave continue to be generated as slip moves along the fault In addition to surface waves, earthquakes cause body waves that travel through the interior of the Earth Any M5 earthquake or bigger can be recorded anywhere in the world- shaking from smaller earthquakes dissipate too much with distance Two types of body waves - P-waves - S-waves Two types of surface waves - Love waves - Rayleigh waves P-Waves (P for primary pressure) - A body wave that travels through the Earth - Push-pull motion (compresses then expands like a sound wave) - Travels through solids, liquids, and gases - Fastest seismic wave (first to arrive) S-waves (for secondary or shear) - A body wave that travels through the Earth - Up-down or side-to-side motion (shear) - Will not travel through liquids - Slower than P-waves (second to arrive) Love waves - A surface wave that travels along the surface of the Earth - Side-to-side motion - Along with Rayleigh waves, they are last to arrive but cause the most shaking Rayleigh waves - A surface wave that travels along the surface of the Earth - Up-down motion like an ocean wave - Along with love waves, they are the last to arrive but cause the most shaking Seismologists record seismic waves at seismic stations by using seismometers that produce seismograms Seismograms are a measure of ground motion during an earthquake P-waves arrive first (minimum shaking), then S-waves, then surface waves (most shaking) P and W save arrival times can be used to locate an earthquake An earthquake is assigned a magnitude based on the extent of area on the fault that slipped, the amount it slipped, and the strength of the crust Earthquake magnitude is a logarithmic scale: for every whole number increase in magnitude the amplitude of shaking goes up by a factor of 10 Bigger earthquakes occur much less often The world’s largest earthquakes occur along convergent plate boundaries (subduction zones) because rock is strongest in compression Earthquake rupture lengths are proportional to magnitude- there is a limit to how big an earthquake can be (M10) The duration of shaking increases with magnitude 3.4-3.9 Reading Earthquakes are not randomly distributed mostly near plate boundaries Earthquakes occur along all three types of plate boundaries Greatest magnitudes associated with subduction zones Megathrust earthquakes- greater than M9 Intraplate earthquakes can be large and damaging, but less common Recurrence interval- time between events Federal emergency management agency have opted a new building code designed to mitigate earthquake hazards Secondary effects are those that subsequently result from the faulting and shaking - Liquefaction of the ground - Regional changes in land elevation - Landslides - Fires - Tsunamis - Disease Fault scarp- low cliff created by surface rupture along the fault that was the source for the earthquake Resonance- the matching of vibrational frequencies High shaking frequencies damage low buildings Low shaking frequencies damage tall buildings Liquefaction- intense shaking can cause a near-surface layer of water-saturated sand to change rapidly from a solid to a liquid - Vibrations from seismic waves increase water pressure in space between sand grains Earthquakes and landslides are closely linked - Most common in hilly and mountainous areas Fire is another hazard linked to earthquakes - Shaking and surface displacements can break electrical power and natural gas lines, starting fires - Essential water mains can break Outbreaks of disease sometimes associated with large earthquakes - Loss of sanitation and housing - Contaminated water supplies - Disruption of public health services - Sewer and water lines rupture, become polluted by pathogens Faults produced by earthquakes influence the underground flow of water, oil, and natural gas Faulting related to earthquakes may be responsible for the accumulation or exposure of economically valuable minerals Veins- mineral-filled cracks, can be source of precious metals Frequent small earthquakes can help prevent larger ones in the same area Seismic gaps- greatest potential for producing large earthquakes in the future Earthquakes caused by human activity - Loading the Earth’s crust, as in building a dam and reservoir - Injecting liquid waste deep into the ground through disposal wells - Creating underground nuclear explosions Longterm forecasts do not help residents of a seismically active area anticipate and prepare for a specific earthquake Hazard reduction program goals: - Develop an understanding of the earthquake source - Determine earthquake potential - Predict effects of earthquakes - Apply research results Short-term predictions mainly based on precursors - Patterns and frequency of earthquakes - Deformation of ground surface - Seismic gaps along faults - Geophysical and geochemical changes in Earth Unit 2 Continued With no water pressure due to broken water mains, San Francisco burned to the ground following the earthquake To build a new San Francisco, the rubble fro the old one was scraped into the bay creating new shoreline of a very unconsolidated nature Loose sediments amplify seismic shaking like Jell-O The marina district, build on 1906 earthquake rubble, experienced some of the most intense shaking during the 1989 M6.9 Loma Prieta earthquake Sediments in the Santa Rosa Basin greatly magnified the amplitude and duration of shaking during the 1906 earthquake Mercalli intensities: the shaking that people and buildings actually feel - Based on felt reports - Can be used to estimate the size of historic earthquakes Mercalli intensities provide a means of estimating how destructive an earthquake was likely to be Seismic waves from any strong source (ex: shotgun) will be partially reflected by rocks with different densities and returned to the surface where they can be recorded Reflection seismology allows visualization of crustal structure several kilometers down Earthquake seismology uses seismic waves from earthquakes to visualize the Earth’s deep interior Earthquake seismology can reveal the structure of subducting lithosphere deep within the Earth No S waves visible traveling through the outer core S waves cannot travel through liquids The U.S. Geological Survey believes that there is high seismic risk in our own backyard Three of the largest earthquakes to occur in the contiguous U.S. happened in the Midwest For the same magnitude earthquake, shaking is felt farther away in the eastern US compare to the western US - Shaking is more intense in the East because the crust is stronger (less active faults breaking it up) and transmits seismic energy better Earthquakes occur in the Midwest (far from any plate boundaries) due to reactivation of ancient faults New Madrid seismic zone is thought to be the reactivation of an ancient failed rift system Faults are found virtually everywhere, though most tend to be inactive (until they are not) Unit 3 Earthquake mitigation 1976 Tangshan earthquake official death toll of 240,000 Why casualties may have been greater - Population over 5 million - Earthquake occurred at 3:00 am when most asleep inside buildings - Took two days before outside help arrived - No foreign assistance - China has a history of minimizing official casualty rates for political reasons 2010 Haiti earthquake 316,000 people killed, over 1 million homeless Our best chance of predicting an earthquake may come from identifying a reliable precursor: an observation that consistently indicates an earthquake is about to occur Foreshocks are smaller earthquakes that sometimes occur before a much bigger mainshock Aftershocks are smaller earthquakes that always occur after a much bigger mainshock Very sensitive measurements show no surface motion on faults just before earthquakes Water levels can change Earthquakes tend to occur where they have occurred before We can often determine the time since the last earthquake Earthquakes have approximate repeat times We can use this information to estimate earthquake probabilities Seismic gap- a segment of an active fault that has not ruptured as recently as its neighboring segments Future earthquakes are most likely to occur in seismic gaps GPS measurements enable us to measure slip deficit rates, which in turn enable us to predict how big an earthquake is likely to be Knowing the slip deficit rate and the time since the last earthquake enable one to forecast the potential size of the next earthquake The biggest earthquakes are not the deadliest - Largest populations - Bad building practices Biggest earthquakes - Largest stress buildup - Longest faults We can’t stop earthquakes, but we can minimize the death and destruction that results - Educate public about earthquake hazards - Don’t build near active faults, especially on loose soils - Build earthquake resistant structures - Develop earthquake early warning systems
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