Midterm 1 Study Guide

Midterm 1 Study Guide - Lecture 1 Radioactive isotope —...

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Unformatted text preview: Lecture 1 Radioactive isotope — occurs at constant rate independent of temperature or pressure Half life — divide by 2 Age of the Earth — 4.6 billion years old, determined by dating meteorites Age of rocks — use isotope that has a big enough half life (uranium) Oldest continental rocks — occur at the center of continents, Greenland Oldest oceanic rocks - .2 billion years old Power of ten notation — 109 = 1,000,000,000 Ages of sun and other planets — all 4.6 billion years Age of universe — 13.6! 15 billion, we know that because universe is expanding Lecture 2 What is an earthquake — sudden slip in fault plane Fault plane — place of weakness around a fault trace Fault trace — surface line, intersecan between the fault plane and the earth’s surface Fault scarp — a cliff or steep slope formed by displacement of the ground surface Hypocenter (focus) — point where earthquake starts (lat, long, depth] Epicenter — point directly above hypocenter on the earth’s surface (lat, long) Where eanhquakes occur — at plate boundaries 3 types of plate boundary diagrams divergent — two plates moving apart, mid-ocean ridges convergent — one plate moving up, one moving down, against each other, trencha'island- arc structures transform — sliding against each other, San Andreas Fault, fracture zones linking sections of mid—ocean ridges. Plate tectonics — anvthina that raises [DDOEIaDl'lV (earthquakes. volcanoes. mountain building} Lithosphere — the outer, rigid shell of the earth above the asthenosphere, contains the crust, continents, plates, and uppermost mantle Asthenosphere — layer below the lithosphere, it is the weaker, hotter, deeper part of the upper mantle Mantle COnvection — slow inotiOn of earth’s mantle in resp0nse to its variatiOns in density, causes earthquakes and tectonics Hot spots — place on earth’s surface where hot mantle is stirring below, where active volcanism has occurred for a while where magma comes up, ex — Hawaii Continental Collisions — subduction zone is destroyed, mountains are produced, two continents are sutured together, ex — Himalayas Nature of seismicity at each type of plate boundary. Divergent — two tectonic plates moving away from each other, mid—ocean ridges Convergent — two tectonic plates moving toward one another, Trench Island arc structures Transform — tectonic plates slide and grind against each other, Fraction zones linking mid-ocean ridge segments Deep-focus earthquakes occur in subduction zones [two tectonic plates meet and move towards one another, with one sliding underneath the other and moving dOWn into the mantle], as deep as 700 km Lecture 3 Wegener’s evidence for plate tectonics — father of continental drift, plate tectonics Fit of continents (Pangaea) — everything was one continent prior to 2 MY Evidence found - 300 MY old southern carboniferous ice sheet, 250 million year old deserts = only make sense it Pangaea existed Migration of species from paleontological record — canit explain why certain animals exist On other continents without the existence of Pangaea Modern Evidence Computer fit of continental margins at 100 fathoms deep — fit is unique if you fit the 100 fathom contour on continental shelf instead of fitting shore lines (different from Wegener) Earth’s magnetic field Dipole field — north and south Inclination i — angle between the magnetic field vector and the surface of the earth, tells paleo latitude at time of rock’s creation, tells how far you are from pole Declination 5 — angle between the horizontal component of the magnetic field vector and geographic north, gives direction to paleo pole, tells which direction is to the pole Cannot determine longitude based on information given Remnant magnetization — when a rock cools through its Curie temperature, it becomes magnetized in the direction of die earth’s magnetic field at that time, fields facing one direction Ferromagnetic minerals — magnetite F9304 Magnetic domain — region within a material which has uniform magnetization Paleomagnetic polar wander curves — transition of the magnetic pole thru time, different continents have different polar wander curves because continents have changed positions How measured? Why are they evidence for plate tectonics? Because pre 20!] MY ago, all polar positions of all the continents were identical, meaning they were one (Pangaea) tani = 2tan-8, E} = paleolatitude at which rock cooled thru Curie Temp Linear magnetic anomalies — are parallel to mid-ocean ridges and are caused by reversals of the polarity of earth’s magnetic dipole, the form a symmetric striped pattern parallel to the ridge axis Measured by dragging a magnometer along with a ship How are they formed? Reversal of magnetic pole — can measure the spreading rate Reversal time scales — knowing how long ago the poles switched by measuring how old the rocks are Spreading rate —the distance of the anomaly from the ridge axis divided by the age of the anomaly Lecture 4 Strike (angle between north and fault trace) and dip (angle between horizontal and fault plane) Faulting Strike slip — a fault whose relative displacement is purely horizontal, transform fault, left lateral, right lateral, slip during earthquake is parallel to the strike, SAF — right lateral Dip slip — a fault in which the relative displacement is parallel the direction of dip of the fault plane (perpendicular to the strike); the offset is either normal or reverse Stress = FIA Normal stress (perpendicular, shoe on ground}, shear stress (parallel) Principal stresses: 01 > 02 RIB Type of faulting when: 61 is vertical, 02 is vertical, I53 is vertical. Elastic rebound theory of earthquakes — elastic energy builds up, the plates slowly deform until ground breaks and energy forms earthquake H.F. Reid measured ground motion before and after 1960 SFO EQ Geodesy — study of the measurement and representation of the earth, including crustal motion Lecture 5 Fault plane solutions from first motion of P waves Wave fronts and ray paths Body waves P and 8 (particle motion relative to ray path) Surface waves. First P motion on a vertical seismorneter Up = away from source, Down = toward source. Pattern of P first motions on a focal sphere for: Strike—slip, normal, and reverse faults. Lecture 6 The San Andreas fault. Right lateral strike—slip fault Transform fault linking spreading center beneath the Salton Sea with a spreading Center off the Oregon coast. Plate boundary between the Pacific and North American plates. Slip rate = 35 mmfyear, 3 cmfyear Age 30 million years — before that California coast was a subduction zone like South America today. Sierra Nevada Mountains are granite batholiths that cooled below the now eroded away andesite volcanoes. Coastal ranges are the old trench sediments that popped up when subduction stopped. Big bend north of Los Angeles and compression in the LA basin LA is on die Pacific Point Lecture .7 Volcanoes mostly occur at plate boundaries — exceptions are hot spots. Convergent boundaries Andesite magma — contains high Si02 (high viscosity — traps magma) = grey color, explosive eruptions, steep-sided composite (strato) volcanoes, ex — Cascade Mts Divergent boundaries (and at hot—spots] Basalt magma — contains low Si02 (low viscosity) = black color, quiet eruptions, shallow sloped shield volcanoes Tsunami - a seismic sea wave usually generated by dip-slip faulting on the ocean floor. Very long wavelength {say 2UUkm), very long period (say 15min) V: wavelengthftime or frequencywavelength Amplitude increases as the wave approaches shore since front of wave slows first when it hits shallow water near shore. The back of the wave catches up, shortening the wavelength but increasing the amplitude, This is why Ordinary sea waves break when they reach shOre. Tsunami early warning system — seismological stations Amplitude, wavelength, period, frequency, velocity and relations between them: 3‘ = 1/1" (cycles/second) I = M” (seconds/cycle) v = Lecture 3. Western US Tectonics: San Andreas Fault Big bend (lots of compression] — San Gabriel Mountains (created by Big Bend, lots of compression) and Transverse Ranges. Salton Sea — buried spreading centers and transform faults. Cascadia subduction zone — in Oregon and Washington, used to be on coast of California, chain of Andesite volcanoes Basin and Range province = Crustal extension behind former subduction ZOne, causing valleysf trenches Yellowstone — sits on hot spots trace Seismic threat to Los Angeles, San Francisco, Seattle, Salt Lake City ...
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This note was uploaded on 09/04/2009 for the course GEOL 240Lxg taught by Professor 12:30-01:50pm during the Fall '07 term at USC.

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Midterm 1 Study Guide - Lecture 1 Radioactive isotope —...

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