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Plate Tectonics Plate Tectonics Why is Plate Tectonics important? • Plate Tectonics explains the distribution of: – – – – – – – Mid-Ocean Ridges, Deep-Sea Trenches, Earthquakes, Volcanoes, Continents, Mountains, and Fossils around the world. • Plate Tectonics also helps to explain the formation of Earth’s atmosphere and oceans. Plate Tectonics Plate Tectonics is the scientific theory that: • The Earth is divided into lithospheric plates (7 major plates, many smaller plates). • Lithospheric plates are in motion and continually changing in shape and size. • Movement of lithospheric plates is controlled by movement within the Mantle. Plate Tectonics: The Early Years • Pre-1960’s accepted view that ocean basins & continents had fixed locations. • By the 1970’s a revolution in the form of Plate Tectonics swept the geologic community. • Where did it all start? - Continental Drift Hypothesis. Continental Drift • Alfred Wegener (German meteorologist & geophysicist) like many others since the 1600’s noticed that South America and Africa seemed to “fit together”. – In 1915 proposed a hypothesis, called continental drift, that the continents had drifted apart. – He suggested that at one time all the continents were connected together. This super continent he called Pangaea (Greek for “all land”). – Wegener hypothesized that roughly 200 MYA Pangaea broke apart and the continents “drifted” towards their present positions. Wegener’s Evidence: • Wegener’s evidence came from four sources. – Fit of continents. – Fossil distribution. – Rock types and geologic features. – Ancient climates (Paleoclimates). Fit of Continents • Coastline of South America and Africa seem to fit well. • Continental margins fit better! Fossil Evidence • Identical fossil organisms found on opposite sides of large oceans. • Mesosaurus – Fresh water reptile – Elongated head & snout – ~ 0.5 m long • Lystrosaurus – Mammal like reptile probably an herbivore kind of like a hippo – Tusks – Called “spoon lizard” • Glossopteris – Seed fern – Large seed kind of like modern day trees. • Too large to be carried long distances by wind. Rock types and Geologic Features • 2.2 billion year old rocks in Brazil closely resemble rocks of similar age in Africa. • Matching mountain ranges across the North Atlantic. • Rocks from the Appalachian Mountains matched in age and structure to rocks found in the British Isles and Scandinavia. Ancient Climates • Wegener was a meteorologist. Interested in climate changes. • Evidence came from South America, Africa, India and Australia. • Distribution of glacial deposits in southern hemisphere from 300-220 MYA. – Glacial till suggested large continental glaciers. • Direction of ice movement. – Looked like in some areas ice moved from sea onto land. Very strange! – If land masses were clumped by south pole, that would explain glaciers! The Great Debate • Hypothesis widely criticized. • One major problem was the mechanism of the movement. – Wegener suggested that possibly tidal forces from sun & moon caused the slow movement. • Harold Jeffreys (physicist) countered that the magnitude of the tidal forces would have to so great that they would stop the rotation of the earth in a matter of a few years. • Wegener had the concept right, but some of the details were wrong. Thus the hypothesis was treated with skepticism. – Some considered the idea plausible & continued the search for evidence. The Great Debate • Mid 1950’s new evidence began to emerge that supported Wegener’s Continental Drift Hypothesis. – Came from exploration of the seafloor and new field of Paleomagnetism. Paleomagnetism • Earth is like a gigantic magnet. – Spinning outer core generates a magnetic field. Paleomagnetism • As iron rich igneous rocks cool, iron in rocks will become magnetized & align w/ current magnetic field. – Once rocks cool enough (below Curie point – 585oC) magnetism becomes “frozen” in place. • Points to the location of the magnetic poles at time of rock formation. • Paleomagnetism is the term used for the preserved magnetism. Apparent Polar Wander • 1950’s – Paleomagnetic data from lava flows of different ages indicated that the magnetic poles shifted over time. – W/ respect to Europe, magnetic north moved from near Hawaii to it’s present location. • This strongly suggested that either the magnetic poles migrated, or Europe had drifted. Apparent Polar Wandering • Comparing the polar wander paths for multiple continental bodies helped determine which explanation was correct. – If continents stayed stationary, then polar wander paths would be the same for all continents. – If continents moved, polar wander paths would be different. Polar wander path in current continent configuration Polar wander path with reassembled continents. Seafloor Spreading • Submarines mapped the longest mountain chain in the world. – It wrapped around the entire planet and it was in the middle of the oceans. • Before this time, people thought the oceans were mostly flat and boring. – Found that the deepest areas were not in the middle of the oceans, but along the edges (deep-ocean trenches) – The shape of the Mid-Ocean Ridges also matched the shape of some continental margins! Seafloor Spreading • Early 1960’s – Harry Hess hypothesized that the mid ocean ridges were above areas of upwelling in the mantle. – Convection in the mantle caused the motion of the seafloor. – Crust was continuously moving away from the ridge areas, allowing new crust to be formed. – Trenches were areas where the crust dipped back down into the mantle to be “recycled”. Seafloor Spreading: Magnetic Reversals • Early 1960’s – geophysicists discovered evidence of magnetic reversals in iron rich igneous rocks. – Magnetic reversal – magnetic north & south switch places. – Normal polarity – same magnetism as present. – Reverse polarity – opposite magnetism than present. • Age dated each lava flow & recorded the direction on the magnetic field. Seafloor Spreading: Magnetic Reversals • Magnetic time scale established the general timing of reversals. – Chron – major division of the magnetic time scale where the magnetic field is dominantly normal or reverse. • Short periods of the opposite polarity can occur during chron – generally around 200,000 long. Seafloor Spreading: Evidence • Magnetic reversals & the seafloorMagnetic surveys of ocean floor revealed alternating bands of high and low intensity magnetism – often called “zebra stripes” – The bands correspond to times of normal and reverse polarity. – The bands are parallel to mid-oceans ridge & are remarkably symmetrical on either side of the ridge. Seafloor Spreading: Evidence • If oceanic crust was created all at one time, then the entire seafloor would have the same magnetic polarity. • “Zebra stripes” exist because new crust is continuously added at the ridge slowly, pushing the older crust away from the ridge making the seafloor spread. Plate Tectonics • 1968 - Seafloor spreading merged with continental drift giving rise to plate tectonics. – The Earth is divided into lithospheric plates. – Lithospheric plates are in motion and continually changing in shape and size. – Movement of lithospheric plates is controlled by movement within the Mantle. 7 major plates: North American, South American, Pacific, African, Eurasian, AustralianIndian, and Antarctic. Plate Tectonics 3 types of plate boundaries • Divergent • Convergent • Transform Plate Tectonics Divergent boundaries are where two pieces of crust move apart. • Hot mantle material wells upwards creating new seafloor. • Volcanoes and earthquakes occur at divergent boundaries. • Geologic Features: Mid-Ocean Ridges, Continental Rift Valleys. http://www.wwnorton.com/college/geo/animations/t he_process_of_rifting.htm The East African Rift Valley is an example of a divergent plate boundary. Plate Tectonics Convergent boundaries where two plates move towards each other. • Oceanic lithosphere is subducted & destroyed. – Continental lithosphere uplifted to create mountains. • Volcanoes and earthquakes occur here. • Geologic Features: Deep-Sea Trenches, Continental Volcanic Arcs, Volcanic Island Arcs. • Three types of convergence. http://www.wwnorton.com/college/geo/animations/t he_process_of_subduction.htm Continental-Ocean Collision Denser oceanic crust is being subducted beneath (carried down below) the less dense continental crust. Ocean-Ocean Collision Continental-Continental Collision Plate Tectonics Transform boundary where two plates slide past each other. • Lithosphere neither created nor destroyed. • Earthquakes. • No volcanoes (usually). • Geologic Features: Fracture Zones. http://www.wwnorton.com/college/geo/anima tions/transform_faulting.htm The San Andres Fault is a transform plate boundary created by the Pacific plate moving past the North American plate. Plate Tectonics: More Evidence • Deep Sea Drilling Project (1968-1983) – Hundreds of holes drilled into seafloor through sediments & into the crust. • Used fossils to date the sedimentary layers. – Found that youngest sediments & crust were located near the ridges & oldest parts of seafloor near the continents. • Also found that sediment layers grew in thickness with distance from the ridge. Plate Tectonics: More Evidence • Mapping projects of volcanoes & sea mounts in the Pacific Ocean revealed linear chains of volcanic structures. • Well know example is Hawaii – Radiometric dating showed that the ages of the islands increased with increasing distance from the Main island. • Main island ~ 1 MY old • Midway island ~27 MY old • Suiko seamount ~ 65 MY old Plate Tectonics: More Evidence • Hot Spots – formed by a plume of hot mantle material. – Creates an area of volcanism & high heat flow in the crust that is independent of the plate boundaries. • Mantle plumes do not move, they are in fixed locations. – As plates move over a hot spot, it leaves be hind a chain of volcanic structures (hot spot track). • Age of volcanic structure indicates how long it has been since that area of the plate was located over the hot spot. Hot spots and hot spot tracks Plate Motion • Plates move at a slow but continuous rate relative to each other. • Average rate is about 2-3 centimeters (1 inch) per year. • Can track plate motion in several ways: – Take age of seafloor and divide if by distance from spreading zone. – Hot Spots – Width of magnetic strips in seafloor. – Satellites Current rates motion of selected surface points collected from satellite data. What Drives Plate Motion? • Evidence for mantle motion seen in Seismic tomography. – Tomography measures density of materials from velocities of seismic waves. • More and less dense regions in Earth’s interior can be linked to higher and lower velocities. • Has been used to locate hot spots and mantle plumes. Seismic-velocity image of earth’s interior. Blue regions are cooler (faster moving) areas and red-brown are warmer (slower moving areas). What Drives Plate Motion? • Convection (=heat circulation) in asthenosphere is a major force that drives plate motions. • Upwelling of heat and magma occurs at divergent plate boundaries. – New oceanic crust formed. • Cooler, denser slabs of oceanic lithosphere descend into the mantle at subduction zones. • Asthenosphere that does not be come crust spreads outwards along the bottom of the lithosphere, cooling and eventually sinking. What Drives Plate Motion? • Convection not the only force. • Gravity also drives plate motion. – Slab Pull • Cold dense slabs of subducted crust sink & pull on the trailing plate. – Ridge-push • Rising magma pushes new crust created at mid ocean ridges higher that older crust. Thus pushing the whole plate down & away from the ridge. Mantle – Plate Convection • Two models for convection in the mantle – Need to account for compositional differences in rocks created from mantle material Mantle – Plate Convection • Layering at 660 km – 2 zones of convection w/ line between at 660km (bottom of the transition zone). – Hot spot source is from deeper convection cell & mid-ocean ridge from upper convection cell. – Subducting slabs incorporated into upper convection cell. • Recent data shows that some subducting oceanic lithosphere makes it down below 660km to the core-mantle boundary Mantle – Plate Convection • Whole-Mantle Convection – Subducted oceanic material sinks to coremantle boundary & helps stir the entire mantle. – Models show that whole-mantle convection would cause the entire mantle to be mixed w/in a few MY, eliminating composition differences. • Doesn’t fit with what we observe w/ hot spots.