Week 3 Lecture: Plate Tectonics
Week 3 Lecture: Plate Tectonics GEO 101
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This 6 page Class Notes was uploaded by Emma Foster on Tuesday January 27, 2015. The Class Notes belongs to GEO 101 at University of Alabama - Tuscaloosa taught by Keene in Spring2015. Since its upload, it has received 165 views.
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Date Created: 01/27/15
Plate Tectonics What is a tectonic plate A piece of lithosphere Tectonic Plates Some oceanic Some oceaniccontinental Move 1 to 15 comyear Continental Margins Active margin two plates moving toward or away from each other moving 2 different directions Passive margin one turns into another no motion How do we know they move 9 6 major pieces of evidence 1 Shape of continents 2 Paleozoic Glacier Deposits a Extent b Direction of movement 3 Ancient climate belts Fig 22b 4 Distribution of Fossils Fig 22c 5 Distribution of ancient rocks Fig 23 aampb GUEST LECTURER Earths magnetic field Earths magnetic field resembles the field produced by a bar magnet o Earths field lines curve so the tilt of a magnetic needle changes with latitude This tilt is the magnetic inclination o The magnetic pole and the geographic pole do not coincide so in most locations a compass does not point exactly to geographic north The difference is declination o The declination observed today varies with location Fig 24 Paleomagnetism Rock magnetism can be measured in the laboratory The study of fossil magnetism is called paleomagnetism Rocks can become magnetized during formation Different rock layers show different magnetic north 0 Does the North magnetic pole move that much NO the continents move Iron minerals archive the magnetic signal at formation Hot magma High temp no magnetization Thermal energy of atom is very high Magnetic dipoles are randomly oriented Iron materials archive the magnetic signal at formation Cooled magma Low temp permanent magnetization Thermal energy of atoms slow Polar wandering Layered basaltic record magnetic changes over time Inclination and declination indicate change in position Apparent polar wandering Polar wandering paths were initially misinterpreted Not the signature of a wandering pole on a fixed continent The signature of a fixed pole on a wandering continent Polar wandering Each continent had a separate polar wandering path Now understood to represent that 9 The location of the magnetic pole is fixed 9 The continents themselves have moved 9 These curves align when continents are reassembled Sea oor bathymetry Before World War II we knew little about the sea oor Echosounding sonar allowed rapid sea oor mapping Sea oor maps created by ships crossing the oceans Bathymetry maps are now produced using satellite data Fig 27a The Ocean Floor Oceanographers were surprised to discover that A midocean mountain range runs through every ocean Deepocean trenches occur near volcanic island chains Submarine volcanoes poke up from the ocean oor Huge fracture zones segment the midocean ridge These observations are all explained by plate tectonics l L39and U39vl C Sonar mapping delineated bathymetric features Midocean ridges Deepocean trenches Volcanic Islands Seamounts Fracture Zones Today s View of the ocean oor reveals the location of Midocean ridges Deepocean trenches Oceanic fracture zones The Oceanic Crust By 1950 we had learned much about oceanic crust Oceanic crust is covered by sediment Thickest near the continents Thinnest or absent at the midocean ridge Oceanic crust consists primarily of basalt Lacks variety of continental rock types No metamorphic rocks Heat ow is much greater at the midocean ridges Earthquakes occur in distinct belts in oceanic regions The earthquakes were surprising They were limited to Parts of oceanic fracture zones Midocean ridge areas Deep ocean trenches Geologists realized that earthquakes defined zones of movement SeaFloor Spreading In 1960 Harry Hess published his quotEssay in Geopoetryquot Sediment thickens away from ridges Earthquakes at midocean ridges indicate cracking Cracked crust splits apart Highheat ow form molten rock rises into the cracked crust New ocean oor forming at midocean ridges Increasingly Transform Mldocean Increasingly older fault ridge older Volcanoes 45 n oil W Y I l 40000 I 60000 I 80000 100000 years REVIEW How do we know plates move 9 Shape of continents 9 Glacier deposits 9 Climate belts 9 Distribution of fossils 9 Distribution of rocks 9 Paleomagnetism Also 9 Reconstruct movement What makes them move 7 pieces of evidence 1 midocean ridge 2 sediment changes thickness seamount chain trench fracture zones earthquake distribution Magnetic reversals NEDquot What does this mean 1 Midocean ridges Sediment thickness Volcanic island chains Trenches Fracture zones around midocean ridge Earthquake distribution along ridges Magnetic reversal seen in rock N991990N SeaFloor Spreading and Continental Drift 9 New sea oor created in midocean ridge 9 Lithosphere plate moves with continent 9 Plate gets recycled into trench How do we know where the plate boundaries are Plate boundaries gt 3 major types Divergent Convergent Transform Divergent boundaries gt Two plates move apart Midocean ridge Continental rift What makes midocean ridge move apart Heats rises above mantle and melts rock and it spills out and creates new ocean oor Midocean ridge Cooling creates topography Cools gets denser and sinks a little bit Continental rift Divergent boundary in continent Heat from asthenosphere and heat rises and pulls the land apart Volcanoes begin to form and volcanoes push out lava to create new ocean oor Heat is making the whole continent rise up and separate Convergent boundaries gt two plates moving toward each other Subduction zone Collision Subduction zone Reason for earthquakes at plate boundaries Continental crust can39t be subducted only oceanic crust can because it39s denser Subduction Ocean continent No subduction boundary no volcanoes Subduction ocean ocean Older one will subduct because it39s cooler Collision continent continent Continental crust can not sink When continental crust collides it creates a giant mountain range Transform Boundaries fault 9 two plates sliding past one another Mostly oceanocean Sometimes continentcontinent No new plate forms and no old plate is consumed 9 fracture zones Active is transform boundary Inactive is no longer boundary part of plate Other features Triple junctions Place where three plates intersect Hot spots Random volcano that does not have anything to do with plate boundaries Single volcano Hawaii and emperor seamount island chains The hot spot stays in place and the plate moves over it Big picture What makes plates move Heat rising GRAVITY Ridge push Slab pull In uence of convection disputed
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