Test 2 Cheat Sheet

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Clemson University *

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1010

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Geology

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Oct 30, 2023

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Lesson 2.1 Continental drift—hypothesis by Alfred Wegener that continents are mobile (Pangaea) Hypothesis was not taken seriously because missing good explanation of WHY and HOW Important lines of evidence: 1. Fit of continents 2. Location of glaciations a. Glacial sediments of the same age are found on four continents b. Striations (scratches in rocks carved by glaciers c. Distribution best explained if all continents were part of Pangaea centered at South Pole 3. Paleoclimates a. Wegener’s model predicts the distribution of sed. rocks of same age which formed in tropical climates (coal) in northern Pangaea. 4. Fossil distribution a. Mesosaur (freshwater reptile) and glossopteris (plant) 5. Rock types, mountain belts and faults Marie Tharp revealed existence of crevices and valleys, and a long V-shaped cleft that ran through each profile. These rift valleys offered support to Wegener’s continental drift theory. If two land masses were moving apart, they’d split the ocean floor in two (forming valley). She recognized v-shaped valleys, which ran down the center of the ocean’s MOR and earthquake locations lined up with certain features. Discoveries from mapping: MOR , trenches (SKETCH as well as abyssal plain ) Important discoveries from sampling: 1. Ocean crust is made of basalt 2. Oceanic crust is covered in sediment a. Thickest near the continents b. Thinnest (or absent) at MOR c. High heat flow at MOR—hot water spewing out of rocks d. Deep trenches along edges of ocean basins e. MOR ran like seams down the middle of each ocean basin None of the features made sense until examining seafloor with magnetic detectors. 1. Molten rocks too hot; magnetic particles randomly aligned 2. As lava cools/solidifies, Fe-minerals lock in magnetic signal

Lesson 2.2 When new basaltic ocean crust forms magnetic minerals in the rock align with Earth’s magnetic field Lines of evidence: 1. Ocean sediments thicken away from MOR 2. There is high heat flow near MOR 3. The ocean crust is youngest near MOR 4. Striped pattern of magnetic anomalies is symmetric across MOR Normal polarity = north After reversal = south Magnetic anomalies caused by polarity reversals… + anomaly: seafloor polarity = Earth’s polarity - anomaly: seafloor polarity DOES NOT EQUAL Earth’s polarity A – anomaly can be observed if the N magnetic poles of magnetic minerals in a section of basaltic crust points towards the SOUTH GEOGRAPHIC POLE of Earth. New seafloor must be MADE at MOR (seafloor spreading) Rocks close to every MOR record NORMAL magnetic polarity (point north), but moving farther from MOR the rocks began ALTERNATING btwn NORMAL and REVERSE polarity (symmetric stripes). The thickness of magnetic “stipe” can tell the duration of a magnetic reversal period SKETCH MOR POLARITY DIAGRAM AND OLDEST/YOUNGEST OCEANIC CRUST!!! Seafloor spreading formation of new areas of oceanic crust, which occurs through the upwelling of magma at MOR and its outward movement on either side.

Lesson 2.3 Theory of plate tectonics: The outer layer of the Earth (lithosphere) consists of separate plates that move with respect to one another Convergent Ocean-ocean / ocean-continent: subduction Subduction: Oceanic plate dives back into mantle, oceanic lithosphere is destroyed (DRAW) Subduction Zones: 1. Trench (Mariana trench is deepest spot-on Earth) (Oceanic lithosphere destroyed here(subduction)) 2. Volcanic Arc (Aleutian Islands) a. A row of volcanoes in the overriding plate, that parallels the sub zone b. Sub zones are always curved, thus so is the row of volcanoes 3. Accretionary Prism/Wedge: seafloor sediment scraped off sub plate 4. Deep, large earthquakes (occur up to depths of ~600 km… no lower because subducting slab heats up and becomes a ductile solid at ~600 km deep) Why do volcanic arcs form along sub zones?  As descending plate heats up, wet ocean crust is “dehydrated”  Water moves up into the mantle above the descending slab  This water causes the mantle to melt—magma rises to surface and erupts forming volcanoes P-waves = cold rock s-waves = hot rock Continent-continent: Collision DRAW SOUTH AMERICA WITH ACTIVE? / PASSIVE MARGINS Active margins: earthquakes, volcanoes, and mountain belts (lack a continental rise and abyssal plain) Passive margins: continental slope ends in an oceanic trench, and beyond the trench, the topography is hilly and irregular, often dotted with rugged volcanic seamounts. SKETCH 3 PLATE BOUNDARIES!!!

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Lesson 2.4 Collision Zone—Why no significant volcanism? 1. No decompression (VERY high pressure) 2. No water added 3. No heat transfer Conveyer Belt: subduction zones often evolve into collisions (Ex. Himalayas). Supercontinent cycle: supercontinents form and then split up about every 500 million years. The weight of mountain ranges causes lithosphere to “sag”, creating basins (low points in topography) in areas next to the mountain range where sediment accumulation occurs. Lesson 2.5 MOR  Rising asthenosphere under ridge o Pushes plates apart o Asthenosphere melts (via decompression melting ) under MOR. This magma wells up to fill gap and cools. SKETCH MOR WITH ROCK SIZES SKETCH CROSS SECTIONAL VIEW OF MOR Continental rift example: The Basin and Range Providence (in US, CA) Transform—crust is neither created or destroyed Continental Transform—San Andreas Fault, CA (Strike-slip fault, right-lateral fault) What drives plate motion? 1. Plates dragged by convection currents in asthenosphereric mantle ( convection traction ) 2. Plates slide away from elevated ridge axis under the influence of gravity ( ridge-push ) 3. Plates pulled by heavy slab ( slab-pull ) (most important) (SKETCH ABOVE) MOR abundant underwater vents (black smokers)

Lesson 2.6 Hot Spots: volcanoes that form independent of plate tectonics. Form from mantle plumes. (ex. Hawaiian Islands) Mantle plumes are spots where hot mantle rises (via convection) in one isolated spot. That rising rock causes decompression melting. Stress: force applied over an area of rock (force/area) Strain (aka deformation): change in shape that occurs when stress exceeds strength of rock Elastic deformation: material will return to its original shape when stress is removed (potential energy) Permanent deformation: material remains deformed even when stress is removed a. Brittle: material is broken into pieces (includes faults and joints) (high stress rates) (favored by high speed of deformation) b. Ductile: material is stretched or bent (includes folds) (high temps/pressure favor) Compressional stress: convergent Tensional stress: divergent Shearing stress: transform Lesson 2.7 Folds = bending of strata (ductile response to compressional stress) Anticline—oldest (SKETCH) Syncline—youngest Fault—a fracture where the two sides move (or slip) relative to each other Dip-slip fault (SKETCH) a. Hanging wall—is ABOVE the fault b. Footwall—Below the fault c. Two types: a. Thrust faults: hanging wall moves up the fault plane (occur in cont-cont collision zone) (ex. Appalachian) i. Subduction zones = big trust ii. Accretionary prism = small trust b. Normal fault: hanging wall moves down the fault plane (SKETCH) Strike-slip fault (transform) a. Left-lateral b. Right-lateral

Normal Faults a. MOR b. Continental rift Lesson 2.8 Elastic rebound theory explains EQs Earthquake: Sudden slip on fault  Releases elastic potential energy  causes ground motion When part of a fault slips… energy is released in the form of seismic waves Body waves: 1. Primary Waves (P-waves) a. Travel the most quickly, 1 st waves to arrive (can travel through liquids) 2. Secondary Waves (S-waves) a. Travel not as quick as p-waves, 2 nd to arrive (cannot travel through liquids) Surface Waves (travel more slowly, but cause more damage): 1. Love waves (slithering snake) Rayleigh waves (ripples on pond) “size” of EQ can be described in two ways (both are logarithmic) intensity (modified Mercalli intensity scale, aka damage to people/buildings) magnitude (Richter (old), moment magnitude(new))

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