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This 0 page Class Notes was uploaded by Em Dorothy on Friday November 20, 2015. The Class Notes belongs to EEOS 120 at a university taught by Robert F. Chen in Fall 2013. Since its upload, it has received 9 views.
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Date Created: 11/20/15
Plate Tectonics Last time we took a look at the formation of the earth and the interior of the earth and now we want to concentrate on the very surface of the Earth and explain its shape 1 Observations slide of earth from space Fig 29 A Earth is mostly covered in water sometimes called the blue Planet 708 earth under water Pacific52 Atlantic25 Indian20 Ice2 B Mountain ranges are long lines 1900global contraction hypothesiss1ide of ocean oorFig 2l3 C MidOcean Ridges Worldencircling Fracture zones are perpendicular to the ridge lines D Trenches are long lines E Seamounts and island chains Also in lines F Flat portions of earthocean bottom 30 of earth is 4000m sea level for continents continental shelves II Hypsographic Curve Depth vs earth39s area A Made possible by the Challenger expedition 18721876 crew of 243 6 scientists 68000 miles 492 soundingsl33 dredgings 362 hydgrographic stations T bottom sample net tows depth 4700 new spp third largest ship at the time wanted to know ocean bottom so they could lay telegraph cables thought there were a lot of fossils of past life 8195m Challenger Deep was deepest sounding annexed Christmas Island which was mined for phosphates and paid for it Debunked Thomas Huxley s Bathybius haekelii deep sea slime thought to be protoplasmic lifeit was CaSO4 precipitated from alcohol B BiModal Distribution Fig 214 Mt Everest8848 km Marianas11035 km 21 of Earth39s surface is near sealevel average land elevation is 840m average ocean depth is 3800m Average altitude of whole earth is 2430m Why bimodal distribution C Theories to explain observations 1 Erosion and precipitation tend towards sealevel In the past 15000 years agothe last ice age sealevel was about 120m lower because the ice caps trapped the water gt shelf break Will readjust for sealevel changedependent on how much water there is in the oceans 4000 m because two types of crust oating on the mantle Continental crust averages 800 m above sea level The difference in heights is determined by density 4800m Sea level depends on how much water there is ice ages 2 IsostasyTwo types of crust oating on the liquid mantle Like ice cubes the density of the crust determines where they will lie Continental crust 26728 gcm3 Oceanic crust is 30 gcm3 Mantle is 453 gcm3 liquid Core for comparison is 1072 gcm3 Seawater is 103 gcm3 Thus the continents have deep roots extending into the denser mantle GeoidAn equipotential gravity surface Spheroid with few anomalies Gravity is not greater at the top of a mountain than at the ocean We39ve explained the hypsographic curve how about processes how were the mountains built Contintental crust is 3560 km Oceanic crust is 410 km III Uniformitarianism Hutton late 18th century the present observable processes are sufficient to explain the geologic past IV Evidence for Plate Tectonics A Ring of Fire Pacific is bounded by ridges trenches rifts and active areasvolcanism and earthquake epicenters p140 p154 KennettLittle activity inside the plate B Magnetic Anomalies 1963Vine and Matthews Iron minerals such as magnetite are formed when basalt cools below 5500C the Curie Temp These are oriented with the Earth39s magnetic field 0 If you measure magnetic anomalies away from a spreading ridge you see reversing stripes every million years or so like a tape recorder Also from deep sea drilling falling iron minerals align themselves with the magnetic field as they settle These agree worldwide and with spreading center data V Plate tectonics A Plates 7 major plates 12 main plates p78 Thurman p132 Kennett Rigid undeformable thin in motion llO cmyr How fast do plates move OVine and Matthews 1963Northeast Pacific Juan de Fuca Can get a time scale of 125 cmyr OToday NASA has a program that uses lasers radio telescopes etc to measure plate motions to the fraction of a cm gtAtlantic moving at 15 cmyr Two types of crust can make up a plate Oldest crust is in North Pacific at 220 my most younger than 200my B Convection of the mantle C Plate Boundaries There are three types of plate boundaries 1 DivergentLike the Atlantic which opened 180 my p191 Kennett p38 Berger History of the Atlantic Just like Red Sea today Salt from evaporitesgtsalt diapirs and oil open and close yields Scottish Highlands and Appalachians Hydrothermal Vents found on the East Pacific Rise MidAtlantic Ridge Juan de Fuca spreading zone Galapagos 3000C water circulates through vents every 10 million years Chemosynthetic bacteria 2 ConvergentLike the Andes or the Himalyas Benioff Zoneearthquakes that tilt to 700km below continentAndesitic volcanoes occur above 100 km quakes Three types of convergent margin Lithosphere is rigid rock sphere including the crust and the upper mantle Asthenosphere is the plasic sphere of the mantle which can ow slowly OceanOcean Marianas Aleutians or Tonga OceanContinent Peru California of old ContinentContinent Himalayas Appalachians 3 TransformLike the San Andreas Here Baja California will be off Alaska in 200 my Arizona will be beach front property D Hot Spots are holes in the mantle that are stationary with respect to each other and to the African Plate Plate motions can be seen in them Hawaiian chainEmperor Seamount chain for instance shows the plate took and turn 40my agop164 Kennett Kure atoll 30my Oldest seamount65 my 00ther chains Cook austral Line Islands Marshall Islands Guadalupe all follow the same direction VI Mineral Resources Made in geological timescales of millions of years Used in decades Economic political ramifications of redistributing mineral resources VII Erosion Constantly battling mountain building to form landscape Increased due to human activity Hurts agriculture water clarity