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Continental Drift & Plate Tectonics Chapter 3 Continental drift - observations 1620, Francis Bacon noticed that S America and Africa appear to fit together and proposed that they had once been joined (as were Europe and Africa) 1885, Edward Suess, S American and Africa fit and similarities in fossils found on these continents Fit got even better if continental shelves were included. No mechanisms Evidence that continents were together Fit of continents Ecosystems – Rock weathering (e.g., glacial weathering in current tropics) – Fossils (similar across continents, coal in Antarctica) Continental drift proposed In 1910, Alfred Wegner proposed the idea of continental drift. Proposed a super-continent, Pangaea, existed 225 MY ago (about the time of dinosaurs in the Cretaceous period), and that continental movements caused friction resulting in volcanic activity. – N part was Laurasia, S part was Gondwanaland, partially separated by the Tethys Sea Proposed mechanism – centrifugal effect from earth’s spinning and tidal drag from combined effects of sun and moon. Hang-up was the mechanism. Idea largely discounted. Lack of understanding of the mantle and isostatic support of crustal material. Post-WWII New instruments make new observations possible. Lines of evidence – Seismology – Volcanoes, hot spots – Sediment distribution – Heat flow patterns from earth – Magnetism Additional observations Better understanding of earth’s structure – seismology Charts of earthquake and volcanic activity (Pacific Ring of Fire) 1925, mid-Atlantic ridge was mapped Seismological evidence of deformable, non-rigid upper mantle (asthenosphere) Post WWII, Radiometric dating of ocean crust (max. age of 200 MY old! Young relative to continental crust more than 4 BY old) Seismic events worldwide between 1977-1986. Seismic activity Earthquakes and volcanoes near trenches Earthquakes at mid-ocean ridges – Plus transform faulting because of rotational stress Focus of earthquakes is below the surface in the crust or mantle; epicenter is the site on the earth surface above the focus – Shallow focus - < 75 km (e.g., transform quakes) – Deep focus - > 300 km (e.g., trenches) Plate formation and destruction explains worldwide seismic distribution More observations Detailed mapping of ocean floor crust and sediments – post WWII, echo sounding – Ridge conformed to coasts – Sediments thicker near continents and thicker near mid-ocean ridges. Mantle studies – International Geophysical Year in 1957. Lithosphere isostatically balanced on partially melted upper layer Age of rocks and sediments Rocks increase in age with increasing distance from the mid-ocean ridge Sediment cover increases with distance from the ridge (older rocks exposed to sediment rain for longer periods) – this isn’t foolproof because of sedimentation from land and movement of sediments. Hotspots Sources of magma other than those that drive the main convection cells Sources static while lithosphere moves over them Erupt as volcanoes for awhile and form chains (e.g., Hawaiian Islands) Youngest mountain in the chain is closest to the hot spot (big island, Hawaii, most active) Emperor Seamount chain formed from same hot spot long ago Seafloor spreading 1960 Harry Hess and Robert Dietz proposed seafloor spreading Mechanism – conveyor belts of oceanic crust moving up at center and down at edges; analogous to a convection cell in water New crust formed at Mid-Atlantic Ridge (hot and less dense) and spreads outward (cools, shrinks and collects sediments) Continents carried along Mid-Atlantic Ridge Mass balance Earth either expanding or there is Consumption of crust Wadati-Benioff zones – subduction zones where crust is being consumed – Oceanic trenches – continental crust less dense and floats on oceanic crust which dives down – Violent so, very seismically active – No direct exchange of mantle material but a window into mantle composition Magnetism Earth behaves like there’s a giant magnet inside. – Magnetic N pole in Hudson Bay – Magnetic S pole opposite in the Pacific Outer liquid core (Ni, Fe) rotating around inner solid core. Rotation calculated at 1 mm/sec (90 m/d) When magnetic materials melt and then solidify in the presence of a magnetic field, solid material lines up with prevailing field. Dating of magnetized rocks (radiometric dating) reveals that rocks of different ages sometimes have opposite magnetic orientation Magnetism (continued) Earth’s magnetic field must have reversed in the past Causes are unknown History of magnetic polarity and reversals recorded in volcanic (basaltic) rocks Ocean crust is high in basalt Magnetometers towed across ocean floor revealed large scale pattern of alternating polarity parallel to mid-ocean ridge (stripes!) If pole moved and not the continents (would have to have been in different places at the same time) If continents moved Magnetic stripes Not evenly spaced Stripes used to calculate rates of seafloor spreading – E.g., rocks 1000 km from ridge are 50 MY old 1000km/50,000,000 = 2 cm/year Magnetism aside Reversals take many years as field collapse and reorient May allow cosmic radiation to penetrate normal magnetic field and cause changes in surface living organisms during these periods (fossil records?) Magnetosphere protects us from solar wind (radiation) Cause of reversals is unknown Current polarity has lasted 730,000 yr 170 reversals in last 76 MY Decrease in field of 7% in the last 160 yr which would predict a reversal in 1500-2000 years! Seafloor spreading 1963 – Vine and Matthews proposed that magnetic patterns were created by seafloor spreading – New ocean floor formed by vulcanism at mid-ocean ridge – Rock solidifies and takes on polarity of prevailing field – Matching patterns on each side of spreading center – Stripes of alternating polarity with increasing age and distance from the ridge axis – Reversal stripe in sea floor correlated with age-dated reversals on land – Youngest rocks at mid-ocean ridges and oldest at margins and on continents Plate tectonics 1965 – John Wilson. Lithospheric plates floating on asthenosphere – Plates diverge when heated mantle (asthenosphere) becomes less dense & rises up – Plates converge where cool, dense crustal rocks collide and are pulled down 1966-1967 Debate 1968 Glomar Challenger drilled deep-crustal cores Reexamination of scientific disciplines – e.g., similar fossils on different continents, coal in Antarctica Plate movement Plate movement is slow about 5 cm/yr (2 inches) & powered by mantle’s heat Crust is formed and consumed Plate boundaries – Convergence – Divergence – Transform Seven major lithospheric plates Each have continental or continental plus ocean crust Ridges, faults and trenches form boundaries Some highlights Lithospheric plates float on asthenosphere Asthenosphere is where convection cells occur Lithospheric plates move past each other but are not created and destroyed as is oceanic crust Wegner’s date for Pangaea not so bad! The Earth: # of Millions of Years Ago 550 136 220 65 190 Grand Canyon 0 Plate boundaries Major features of plates Why no trench on the west coast of N America? Probably used to be and then Pacific plate changed direction Current spreading has lasted about 200 MY but, earth is older so previous history depends on fossils & rocks on land Take home points Lithosphere versus asthenosphere also the Moho Continental drift, seafloor spreading and plate tectonics Continental drift – fit of continents & fossil evidence Seafloor spreading – theory supported by several lines of evidence made possible by post WWII technology (magnetism, seismology, sedimentology, heat flow, distribution of volcanoes and hot spots) Plate tectonics – provided a mechanism Types and features of plate boundaries