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Plate Tectonics http://www.scotese.com/earth.htm Continental Drift Theory Who: When: What: Theory suggests that … Evidence: Weakness: Alfred Wegener & Continental Drift Who: Alfred Wegener When: Early 1900s What: Theory suggests that continents were once joined together in a supercontinent, called Pangaea and have since drifted apart Weakness: Couldn’t explain the MECHANISM … HOW the continents moved. Continental Drift: Supporting Evidence 1. Continents seem to fit together like puzzle pieces 2. Fossil records show similar plant and animal fossils on continents separated by oceans … suggesting that continents were once one land mass (mesosaur; lystrosaur) on different continents Fossil Evidence Supports Continental Drift Continental Drift: Evidence 3. Similar Geological Formations (Mt. Chains and rock formations ) on the margins Different Continents match. • Same minerals • Same age Continental Drift: Evidence 4. Evidence of glacial ice sheets in tropical/subtropical locations like India • Glaciers did not reach equator in last ice age. • Therefore … India must have been located closer to the poles! Pangaea…200 mya •Harry Hess’s Builds on Wegener’s C. Drift: 1940s-50s Uses echo location (sonar) to map the sea floor Discovers that the sea floor is not flat. It has…. A. Oceanic Ridges (Volcanic Mt. Chains) B. Fissures – cracks where lava exits. C. Trenches – deep canyons D. Seamounts – underwater mountains. http://www.youtube.com/watch?v=6CsTTmvX6mc Harry Hess’s: SEAFLOOR SPREADING Theory •The sea floor is continually remade at Spreading Zones (M.O.R) and destroyed at subduction zones. •Ocean crust is NEW at the M.O.R. and then gets pushed away. OLD crust is subducted. •SFS provides the HOW for C.Drift. The continents can move as the Sea floor spreads apart. Movies: http://ocean-ridge.ldeo.columbia.edu/Other_stuff/PAR_Movie/parmovie.html Theory of Plate Tectonics = Earth is made up of a system of rigid plates that move slowly due to convection currents of rock in the mantle. • 1960s • Built upon Wegener’s C.Drift evidence (fossils & mt. chains); Hess’s S.F.S. evidence (age of rocks); and seismic data Evidence for Plate Tectonics: 1. Isotope dating of rock reveals that rock near M.O.R. is younger than ocean floor far away from plate boundary. Old crust gets pushed away as new crust is made. The seafloor moves! Red = < 20 ma Blue = 200 ma Oldest seafloor ~ 200 ma Evidence for Plate Tectonics: 2.Magnetic “stripes” of sea floor Magnetic banding on sea floor show that the sea floor moves. Iron in basalt lines up w. E’s mag. Poles Pattern of iron FLIPS as E’s mag. pole flips = Ocean floor made CONTINOUSLY http://www.youtube.com/watch?v=YIAXiE8RedA Evidence for Plate Tectonics: Magnetic banding on sea floor show that the sea floor moves. If sea floor did not move … the entire ocean floor would have the same polarity. Geomagnetic Data from NOAA: http://www.ngdc.noaa.gov/geomag/ subchrons: Evidence for Plate Tectonics: 3. Earthquake & Volcano Data • Locations of Eqs & Volanoes outline plate boundaries • • http://earthquake.usgs.gov/earthquakes/recenteqsww/ http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/quakes_all.php Why do the plates move? • Convection currents of soft rock in the mantle cause the plates floating on top to move. • Convection currents are powered by differences in the temperatures within Earth’s interior – Magma near core heats up, expands, and rises because it is less dense – Magma closer to crust, cools, becomes denser, and sinks • Heat needed for convection comes from: 1. Heat trapped in Earth’s interior from its formation 2. Radioactive decay of heavy elements in the mantle Why do the plates move? Convection Currents http://www.youtube.com/watch?v=IpnHAj4R-Z8&feature=related Why do the plates move? Ridge Push Slab Pull @ spreading centers where .. Hot, less dense rock rises pushing plates apart. Ridge forms from lava that builds volcanic mts. @ subduction zones where... Cooler, denser rock sink due to gravity Sinking rock in convection current pulls the overriding plate with it. Ocean Depth and Age Younger crust is warmer (less time cooling) … and more bouyant (less dense). Older crust is cooler (more cooling time) and has accumulated more sediments = DENSER Ridge: Higher elevation (younger, hotter rock, less dense rock) Basins (away fr. Ridge): Lower elevation (older, cooler rock, denser, sinks) The ocean floor gets recycled by sea-floor spreading and subduction Old ocean crust gets destroyed / partially melted Takes 200 million yrs to recycle ocean floor New ocean crust forms at fissures at the Mid-Ocean Ridge Plate Movements Diverging Plate Boundaries: 2 plates move apart. New crust form btw. plates Sea floor Spreading (Ocean Plates separate) •Results: New ocean crust forms @ fissure & old crust pushed away •.Shallow Eqs, Underwater volcanoes Locations: M.O.R.s & Iceland Rifting (Land Plates Separate) •Results: Crust thins Land plates pull apart –> Rift valley forms new ocean crust narrow sea develops • Shallow Eqs •Volcanoes •Locations: Great Rift Valley (E. Africa) Rifting and Extensional Tectonics “unsuccessful” rifts “successful” rifts Converging Plate Boundaries: 2 plates collide SUBDUCTION (ocean-land) •Results: Dense (iron rich basalt) ocean plate sinks back into mantle • Ocean crust destroyed • Volcanoes on over-riding slab = Coastal volcanoes & Mts. • Deep focus EQs •Locations: Andes (S.Ame); Washington, US (Mt. St. Helens); Alaska •SuBDUCTION (ocean-ocean)= Older (cooler & denser) ocean plate subducts Results: same as above + volcanic island arcs Location = Aleutian islands (Alaska); Japan; Indonesian Islands; Caribbean Converging Plate Boundaries: 2 plates collide Mt. Building (Land & Land collide) •Results: Two land plates collide & crumple – uplift •.EQs •Large Mt. Chains Locations: Himalayas (Nepal, India) Transform Plate Boundaries Transformation Plates slide past one another. • Results: EQs, faults, fault-block Mts. • Locations: San Andreas Fault, CA; along MORs Movie: http://oceanridge.ldeo.columbia.edu/Other_stuff/PAR_Movie/parmovie.html Basics: http://geoinformatics.sdsc.edu/doe/student/tectonics/sea_floor_spreading_i.ht ml 1. 2. 3 Eurasian Plate Indian Plate 4. 5. C B A D Use this diagram to help you EXPLAIN HOW the seafloor gets Which recycled. (5pts) ~ Use a topic sentence; vocabulary terms; & supporting details! ~ You can include a diagram to help you. ~ Be sure to include the following terms in your explanation: mantle; plates or lithosphere; convection currents; fissure; Midocean ridge; sea-floor spreading; trench; subduction; volcanic activity, ridge push, slab pull, less or more dense, new/old crust, ocean crust, continental crust