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Plate Tectonic Theory Notes CONTINENTAL DRIFT CONTINENTAL)DRIFT) • Alfred Wegener in the early 1900s proposed the hypothesis that continents were once joined together in a single large land mass he called Pangaea (meaning all land in Greek). Continental)Drift)) • He proposed that Pangaea had According to the hypothesis of Evidence(for(( split apart and the continents had continental drift, continents Plate(Tectonics(( moved gradually to their present have moved slowly to their ( positions - a process that became Tie(to(Literacy(–(Predict( known as continental drift. current locations. Pangaea about 200 million years ago, before it began breaking up. The continents about 70 million years ago. Notice that the The position of the continents today. The continents are still Wegener named the southern portion of Pangaea Gondwana, and breakup of Pangea formed the Atlantic Ocean. Indias eventual slowly moving, at about the speed your fingernails grow. Satellite the northern portion Laurasia. collision with Eurasia would form the Himalayan Mountains. measurements have confirmed that every year the Atlantic Ocean gets a few inches wider! Wegeners Evidence for Wegeners Evidence for Continental Drift Continental Drift Continents fit together At your table, come up with like a puzzle….e.g. the evidence or ideas of what your think Atlantic coastlines of supports the Theory of Continental Africa and South Drift America.! ! The Best fit includes the continental shelves (the continental edges under water.)! Fossils of plants and animals of the same species found on different continents. Wegeners Evidence for Wegeners Evidence for Seafloor Spreading Continental Drift Continental Drift • Everyone agreed that Wegeners evidence • Rock sequences (meaning • The same three layers are in he looked at the order of the same order in areas now was compelling. But wouldn t we feel the rock layers) in South separated by oceans. movement? America, Africa, India, • Wegener proposed that the • Also, wouldnt there be evidence to show Antarctica, and Australia rock layers were made when show remarkable all the continents were part that the continents were still moving today? similarities. of Pangaea. • Wegener was a meteorologist and his • Wegener showed that the • He proposed that they theory was not well accepted. (He died on same three layers occur formed in a smaller small at each of these places. joined land mass that was an expedition in Greenland collecting ice • later broken and drifted samples) apart. Picture from http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/ part4.html Seafloor Spreading Seafloor Spreading Seafloor Spreading • One reason scientists had a hard • Hess proposed that hot, time with Wegeners theory is • In the 1960 s, a scientist named Henry Hess made a less dense material below that there was no mechanism for discovery that would the continents motion. Earth s crust rises vindicate Wegner. toward the surface at the • Using new technology, radar, mid-ocean ridges. he discovered that the seafloor has both trenches • Then, it flows sideways, and mid-ocean ridges. carrying the seafloor Picture from USGS • Henry Hess proposed the http://pubs.usgs.gov/gip/dynamic/HHH.html away from the ridge in sea-floor spreading theory. both directions. Picture from http://library.thinkquest.org/17457/platetectonics/4.php Mechanism for Seafloor Spreading Evidence)for)Spreading) Plate Tectonics • In 1968, scientists aboard the research ship Glomar • As the seafloor spreads apart at a mid- Challenger began gathering information about the • Seafloor Spreading provided ocean ridge, new seafloor is created. rocks on the seafloor. insight to the mechanism for • Scientists found that the youngest rocks are located how the continents moved. • The older seafloor moves away from the at the mid-ocean ridges. • The magma which pushes up ridge in opposite directions. at the mid-ocean ridge provides the new land • This helped explain how the crust could pushing the plates, and the Picture from move—something that the continental subduction zones gobble up http://library.thinkquest.org/17457/platetectonics/2.php drift hypothesis could not do. the land on the the other side of the plates. The mechanism was convection currents! Picture from http://www.pbs.org/wgbh/aso/tryit/tectonics/divergent.html Plate Boundaries Plate Tectonic Theory Forces between plates (divergent boundary) ! Tension – two plates pulling apart TENSION: Divergent boundary • Both Hesss discovery and Wegners continental drift theory combined into what scientists now ! Compression – two plates pressing into each other Two plates moving away from each other creating new call the Plate Tectonic Theory. crust • Theory of plate tectonics: ! Shearing - two plates sliding/grinding past each • The Earths crust and part of the other. ! within oceanic area – sea floor upper mantle are broken into sections, called plates which move spreading on a plastic-like layer of the mantle ! within continental area – rift valley Plate Boundaries Plate Boundaries Plate Boundaries TENSION: Divergent boundaries( (convergent boundary) ! Ocean – Continent convergent boundary – subduction zone COMPRESSION: (destruction of crust), volcanic mountain chain Convergent Boundary Two plates moving toward each other; the behavior/ results depend on the type of crust… Within an ocean – Within a continent – Mid-Ocean Ridge African Rift, aka Red Sea Plate Boundaries Plate Boundaries Plate Boundaries (convergent boundary) (convergent boundary) (Transform Boundary) ! Continent – Continent convergent ! Ocean – Ocean convergent boundary – subduction zone, Shearing - Transform boundary - construction (yes (destruction of crust), volcanic island arc boundary construction) of crust, mountains ! Two plates sliding / grinding past each other Most transform boundaries are associated with Plate Boundaries fracture zones along the mid-ocean ridge (Transform Boundary) Plate Tectonic Theory • Plate Tectonics explains • Earthquakes • Mountains • Volcanoes .
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