Midterm Study Guide 2
Midterm Study Guide 2 EART 2 - 01
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This 10 page Study Guide was uploaded by Nolan Shapiro on Thursday November 5, 2015. The Study Guide belongs to EART 2 - 01 at University of California - Santa Cruz taught by T. Lay in Fall 2015. Since its upload, it has received 57 views. For similar materials see Earth Catastrophes in Earth Sciences at University of California - Santa Cruz.
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Date Created: 11/05/15
Lecture 10: Lecture Outline: Earth's internal heat Geographic curiosities o Ocean v continent o Puzzlepiece coastlines Continental drift hypothesis Plate tectonic theory Solid iron core, outer core is molten iron eventually all will be frozen from inside out Crust is the lightest elements and common rocks Contents 40 km, ocean 310 km but earth is 6370 km thick Lower mantles solid with silicates and rock like minerals Outer core mostly iron probably with some liquid sulphur Inner core ironnickel alloy solid Land above and below water. Knew little till 1940s. Lighter rocks floating on crust and denser ones sinking to the bottom of ocean. Mountain ranges on earth and bigger on the ocean floor Mountains erode very quickly on land over long periods of time > trenches in the ocean and mountains show these are new and are forming recently because they are not eroded away nor filled with sediments. Geologists couldn't accept that continents move laterally, up and down yes because of mountains. Even tho continents fit perfect like a puzzle (Pangea) geologists still were hesitant to say they moved horizontally. Alfred Wegener 19151930 developed notion of Continental drift Coastlines Paleobotany Fossils Geologic structures Wegner clined the name Pangaea( "all land" in Greek). for a supercontinent that he said began breaking up in the early Mesozoic. For reconstruction he used present day coastlines this was rightly challenged by geologists. Use freshwater mesosaurus fossils (alligators like that were found in both south america and africa). At the time that this dinosaur lived the continents had to be touching and found that for numerous fossils. Geologic evidence Distribution of distinctive rocks across puzzle piece boundaries Orogenic belts which line up when plates are reconstructed Distribution of climatic distinctive units (e.g. tillites) Intriguing but very hard to swallow Schuchert: "The striking similarity of the coastlines of AFrica and Brazil must have been made by Satan" 1928 How can these huge masses of continental Rocks move? wegner had no mechanism. His ideas did not gain wide acceptance. Alternate Ideas Warren Carey 1930's1950's > pull the pin out of the earth and lit all of the "air" come out and the continents will all be touching and close again like they were The expanding earth New information: 1950's seismic waves tell us thickness of continental crust (~40 km) and oceanic crust (~6 km) 1857 mapping of where earthquakes happen By 1940's also mag global seismic belts in oceanic regions. where the ground shakes. Showing earthquakes were happening under oceans in specific places (margin of pacific and all over ocean) 1930s: Some Earthquake in localized regions are very deep 1950s to 1960 began measuring heat coming out of surface IN continents and ocean: large bands of high heat in oceans. Stick a thermometer in different regions of crust. Regions in middle of oceans shallow earthquakes lots of heat being released and volcanic eruptions happening under crust Magnetic field of Earth is like a dipole magnet Earth is like a dipole magnet produced by flow and deformation of iron core. Amazing fact: The magnetic field reverses, staying aligned with the pole of rotation. It does this quincy (few thousand years) stays in line with spin axis of earth. Even more amazing: when rocks from (cooling lava, depositing sediments), magnetic minerals in the rocks align with the magnetic field existing at the time of formation. This locks in a weak rock magnetization that endures with that direction and can be measured. When locks that have been cooled with magma are stacked on top of each other, the polarity reverses and flipped through the time and age of the rocks. Reversal Timescale Earth's barcode is a unique history of magnetic fields through time 1950s: Extensive Mapping of ocean floor bathymetry (for revealed rideparallel magnetic stripes (high/low intensity). Right along the volcanic ride > strong magnetic field and symmetric as you move away parallel bands alternating high and low magnesium. Ocean floor forms laterally farther away from ridge is older rocks. Turned geology on its side literally. Magnetic Anomaly pattern off the NW United States and Canada, also show symmetry of magnetic stripe patterns relative to rides off shore (pink). Key conceptual breakthrough: Oceanic floor is young at ridge and progressively older away. Contrary to "laws" of superposition and horizontality. Sea floor spreads Lecture 11: The oldest ocean floor is only ~200 million years old To figure out how the plates moved more than 200 million years ago, we have to use continental rocks, which go back to 3.9 billion years old. But there are no magnetic stripes; how do we figure out how the continents moved? Remember, The Earth's magnetic Field is like a bar magnet, with very regular variation in the magnetic field line that vary in a systematic way with latitude (they are roughly symmetric with longitude) Inclination indicates paleolatitude. By looking at latitude and magnetic direction you can determine whether the whole continent moved AS the plates move, they produce earthquakes on their margins Some us think earthquakes are cool, and even become seismologists! Role of Time Behavior of Earth materials depends profoundly on time Response to forces with different time durations varies: o ShortImpulsive Force (e.g., fault slip, explosion): Crust/Mantle rocks respons as a "solid" with "elastic behavior" o Longsustained Force (e.g. gravity): Mantle rocks >650 degrees respons as a viscous fluid, flow and convection; crust/mantle <650 elastoplastic "Rheological" Layering Lithosphere behaves rigidly over most timescales brittle in the shallowest depths Temperature and composition control mechanical behavior Plates are stiff rocks that flow on top of molten rocks (higher temp, called asthenosphere) and lithosphere. ONly place where earthquakes occur are in Lithosphere Early Earthquake Explanations Plato: pent up winds Vigil: buried giant Japanese: Namazu under cities But, such ideas didn't really satisfy Earthquakes remained largely mysterious until the 1890s The 1906 san fran earthquake demonstrated a clear correspondence between abrupt slip on the San Andreas fault and earthquake shaking Faults Motions between plates or crustal masses occur on Faults: Surfaces within the crust/ upper mantle lithosphere across which there have been shearing motion offsets California Faults San Andreas fault is the main plate tectonic boundary fault, but there are many faults in the state that are deforming, therefore hazardous Thrust fault hanging wall gets pushed up above the foot wall typical of subduction zone (compression/squeezing). Subduction zones have huge thrust faults where plates rub San Andreas fault sliding past each other > strike slip faulting parallel to surface Earthquakes occur by shearing on faults. Rock masses sliding past each other abruptly Earthquake Explanation An earthquake is the process of sudden, shearing sip on a fault (or creation of new crack) combined with resultant vibrations Earthquakes are Frictional Sliding instabilities. Repeated stickslip behavior is observed. Friction depends on pressure, temperature, fluids, slip velocity, fault history, and material properties in the fault zone Strain accumulates in the volume of rock around the fault called the fault zone. Sliding between the two rock masses is resisted by friction (static friction) When the strain in the rock approaches the limiting value about 1.0x10^4, you will either break the rock (form a new fault) or overcome frictional resistance of the fault, abruptly releasing stored strain energy The sudden change in stress/strain in the source volume generates P and s waves that expand outward from the surface volume. Much of the energy is consumed in heating of the fault surface as it slides. fault doesn't slip all at once. Largescale motions drive repeated failure of faults, as they slip to keep up with total offset between the plates Lecture 13: Earthquakes Seismic Waves Recording Ground Shaking Locating Events Shaking Intensity & Seismic Magnitudes . Largescale plate motions drive repeated failure of faults, as they slip to keep up with total offset between the plates Faults slide again and again Earth abrupt sliding event is an earthquake Energy released into the surrounding rock spread away as vibrational waves that shake the ground We can record and study the vibrations to learn about the earthquake source location, energy release, sense of faulting, and properties of the earthquake Stress Drop Sudden slip of a fault lowers the shear stress, and relaxes strain in the fault zone The balance between stress (force) and strain (accelerations) result in 2 expanding waves, propagating with velocities determined by the medium properties These are Elastic Waves: p waves and S waves P waves P (primary) waves are body waves They are like sounds waves in water/ air They are faster than S waves (613 km/s) They shake the medium in the direction that the wave is traveling Can travel in solids and fluids S waves S (secondary) waves are body waves (shearing) Travel slower than P waves (27.3 km/s) Shake the medium perpendicular to diredtion that wave is traveling Cannot travel in liquids Pwave Deformation propagates. Particle motion consists of alternating compression and dilation. Particle motion is parallel to the direction of propagation (longitudinal). Material returns to its original shape after waves passes. Seismic waves spread in all three directions from source energy release. At a given instant in time t1, the motions generated by a source time at time t0, will have expanded out as spherical waves. There will be a P wave and an S wave, with the P wave having traveled further from the source than the S wave because it was higher velocity. P wave goes at the speed of sound, faster than s wave. Surface waves P and S waves interact with the Earth's surface to make two other types of waves that travel along the surface Love waves s waves reverberating in crust and upper mantle. shake the ground horizontally, perpendicular to direction of propagation. Rayleigh waves P and s waves interfere. shake the ground up and down and back and forth along direction of propagation. elliptical wave type Most damage to structures is from surface waves MOdern Seismometerseismograph In the instrument, there is pendulum with a weight and spring. The delay of the mass or the weight in the instrument, will measure the effect of the waves himself Results from study of seismic waves Locate Earthquake Sourcesbased on the arrival times of p waves at various different instrument locations Compare Size of Events that release seismic waves (magnitude, moment) Determine orientation of faults and sense of slip Determine internal structure of the earth Difference in arrival times between P and S waves reveal the Distance from recording station to the earthquake hypocenter Seismic Wave Amplitudes Are proportional to amount of slip and fault, and to energy released in fault zone Can use to determine how big the vent was As seismic waves spread from the source the wave amplitude decreases. Can correct for this “distance” dependence Measures of Earthquake size Intensity (Qualitative) o Based on shaking, decreases with distance Magnitude (quantitative/ empirical) o based on seismic wave amplitude Seismic movement (Quantitative/ theoretical) o actual movement of slip on fault Richter magnitude scale ~1933 Richter magnitude= Ml= log(amplitude/wave period) + distance correction About 15M>7 events/ear Moment magnitude scale Moment magnitude= Mw= w/3 x log(rock rigidity x displacement X rupture area) 6 "greatest" 1960 chile 9.5 mw Lecture 14: Seismic Tomography By analyzing travel times of many waves, can locate the anomalous paths through the Earth > 3D models of velocity variations, not just radial layering. Not just varying in depth but also laterally And it will only get worse Population increases is concentrated in areas that lie on faults so fatalities will only increase Earthquake Hazards Primary Hazards: o Fault displacement o Ground shaking, ground waves Secondary hazards: o landslides, liquefaction, tsunamis, dam/highway/nuclear/chemical structure failures, fire, induced events EQ Hazard Mitigation Construction standards (building code) Cibil preparation (education, fire control systems, disaster response teams) Land Use Planning (Fault Mapping, Landslide Mapping, Dam/Nuclear Plant Siting) Timely warning (Prediction?, Tsunami Warning System, POstEvent Warning) Earthquake Predictability Longterm Forecasts o Recurrence interval o Seismic GAps/ Characteristic Earthquakes o Hazard Maps Short term predictions o Precursors o PostEarthquake Warning Short Term Prediction Seismicity Patterns o foreshocks (distinctive? no) o quiescence (temporal Pattern) o migration (spatial pattern) o swarms Strain Effects o uplift/ deformation (nucleation?), changes in P velocity in rocks groundwater level changes radon, He gas emissions electrical, magnetics changes Exotic Methods o animal behavior o tides/planetary alignments o pattern recognition Prediction Case Histories 1975 Haicheng, China Precursors: foreshocks, animal behavior, well levels o Prediction: evacuation, within 2 days, 7.3 hits, 90% house collapse o Advantages: Regime awareness, short term 1981 Peru o Major Seismic gap; more than 100 years since last o U.S. scientists; new theory o Prediction: 8.8 on Aug. 10,1981 o 9.8 on Sept. 15, 1981 o U.S. Earthquake Prediction Council condemned scenario o Peruvian public panic/tourism devastated 2004 Parkfield, CA o Prior events 22 years on average (1856,1881,1901,1922,1934,1966) o Prediction USGS 1988 +/ 5 years o Large monitoring experiment o Failed to happen in time window, and finally did, after 38 year.... 200x? Tokai, Japan o Seismic gap offshore of Tokyo. Previous ruptures in 1707, 184; adjacent area in 1944 o Japanese Prediction Program: monitor seismicity, wells, deformation, electric and magnetic fields, must predict. Kobe earthquake upset this strategy Lecture 15: Volcanoes Temperatures are hot enough to melt rock Volcanic eruptions contribute to building of rocks on the crust Subject to erosion b/c conical structure from rain. Only time will not be eroded is when its eruption Explosive eruption of Mt. St. Helens, 1980 Material that comes out some is molten, some is fine particles of rock and debris. Dangerous if close by Thera island near Greece is a remnant of a volcano. caused devastation and collapse of civilization on the island of crete Spawned Legend of Lost Continent Of Atlantis Life on island snuffed out Earthquakes Tsunami which devastated Minoan civilization Legend of Atlantis comes from the coast of Egypt Volcanoes don’t just erupt one. they may regenerate and re upt multiple times Origin of Volcanoes Ascent of MOlten Rock (magma) from deeper in the crust and upper mantle Earth’s interior is only molten in localized regions Special requirement: must locally exceed the melting temperature Why is the Earth Hot inside? Primordial HEat (From Accretion) Gravitational Heat (Core Formation) Radioactive Decay of U, Th, K, Al isotopes Volcanoes Occur in 3 Tectonic Regions: Subduction zones (island arcs and continental arcs) midocean ridges/rifts o farther away from from ridge the colder it is Hotspots Plume of hot (solid) mantle rises. Locally exceeds melting temperature as pressure decreases and temperature remains high Subduction Zone Volcanoes Fuji, Pelee, Krakatau, Vesuvius, Mt. St. helens, Katmai, Pinatubo Arc Volcanoes (Near Subduction Zones) Island ARcs: Aleutians, Japan, Tonga, Lesser Antilles, etc. Continental arcs: Andes, Cascades, Kamchatka Depends on whether hanging wall plate is oceanic or continental. convergent zones Andesite Most common rock type in arc volcanoes has a relatively high content of silica (Si) This make the magma (and lava, when it reaches the surface) very sticky Sticky lavas can pile up to make cones Characteristic of ARc Volcanism Explosive High Si content traps gas, can build up pressure Sticky lava plugs up vents; have long Repose intervals of 1001000 years Source magma is rich in water and gas due to rising through upper plate and origin of melting Eruption Volumes can be Big Mt. St. Helens .5km3 1883 Krakatoa 6km3 1912 katmai 12km3 Long valley (70,000) 5001000 km3 Energy: Mt. St. Helens= 27000 Hiroshima bombs (1/sec for 9 hours) Caldera Formation Calderadepression formed by collapse of magma chamber Lecture 17: Volcanism types Rift/ridge, hotspot, subduction zone/arc If material rises quickly it can melt: decompression melting. If material rises slowly, it cools as it expands, and it say on an adiabat. So, melting under ridges and rifs is due to rapid rise of mantle rock Under rift, magma chambers forms below bc melting due to pressure decrease. Lighter materials will form oceanic crust, denser ones will sink and form ocean mantle How do we know this structure? Seismic reflection work across midocean ridges and continental rifts Dryline near ridges, rock samples Chemistry of rocks in field and laboratory Examples of ocean crust crosssections where chunk of ocean was squeezed up on land Ophiolites (Oman, Cyprus, Greece, WEster U.S., New Guinea) Colder water sinks through shallow cracks in rocks and meets warm water near magma chambers. Through mid ocean ridge black smoker comes out and talked out variety of different metals which makes smoke that is produced dark. Major Rock Type in MIDOCEAN Ridges and Rifts Basaltdark brown/ black krock ich in Mg, FE,Ca, with about 50% Si When you partially melt upper mantle rocks, you produce basalt magma Melting occurs because rock rises quickly under rift; this lowers pressure faster than rock cools; causes temperature to exceed melting temperature (decompression melting) More silica produces rock that are stiffer. Basaltic campbell's soup, AnesticPlaydough Basalt Or Andesite Andesite: mOst common igneous rock in arc volcanoes; high Si (60%); high viscosity magma/ lava; sticky;build up steep cone; explosive eruptions! Basalt: most common igneous rock in spreading ridge and rift volcanoes; low SiO2 (50%); low viscosity magma/lava; runny; makes broad flat mountains (shield volcanoes) explosive eruptions! Summary of Preading Center/ Rift Volcanism 90% of Earth’s annual volcanic activity (15% of subaerial eruptions) Produces 2.5 km2 area of new seafloor yr Produces 12 knm3 of new ocean crust Produces 6070% of Earth's surface 200my About 20 eruptions/yr (not seen) Mostly nonexplosive; “effusive” Release of chemicals/ water circulation controls ocean chemistry Hot spot volcanoes Isolated volcanoes or lines of volcanoes Do not have to be near a Plate boundary (Hawaii, Tahiti, but may be (Iceland) Endure in fixed location as plates move appear to have deeper origins than upwellings under ridges/ rifts May initiate rifting Often basaltic (when making islands), effusive eruptions. IN continents may be explosive Names of lava types from Hawaii AA blocky/chunks Pahoehoe liquid, soupy
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