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Transcript
A. Three stages to get there 1. Continental Drift 2. Seafloor Spreading 3. Plate Tectonic Theory B. What did people believe about the earth before Plate Tectonics? 1. The continents were in fixed positions 2. The map of the earth was unchanging a) Same past, present, and future locations of continents II. Continental Drift A. Alfred Wegener (1880-1930) German Astonomer/Meteorologist/clim atologist/geologist First to use weather balloons Wrote a textbook on Geology that Germans LOVED Just wanted to be a professor somewhere! Proposed that continents have slowly drifted to current locations All land masses were once joined together in a single large mass called “Pangea”-Greek for “All Land” Pangea split apart and the continents gradually moved to their current positions= Continental Drift 5. Most of the evidence was already known. Wegener just assembled it and proposed the theory EPIC. "Scientists still do not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter can only be reached by combing all this evidence. . . It is only by combing the information furnished by all the earth sciences that we can hope to determine 'truth' here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability. Further, we have to be prepared always for the possibility that each new discovery, no matter what science furnishes it, may modify the conclusions we draw." The Origins of Continents and Oceans (4th edition) Evidence for Continental Drift Continents fit together like a puzzle Atlantic Coastlines of South America and Africa Evidence for Continental Drift The same fossils are found on separate continents Plants (Glossopteris) and Animals (Mesosaurus, Cynognathus, Lystrosaurus) These plants and animals couldn’t fly or swim that far Evidence for Continental Drift Similar rock sequences in separate areas In South America, Africa, India, Antarctica, Australia Basalt lava on top, then sandstone/shale, then glacial till Evidence for Continental Drift Evidence of different climates in the fossil record Antarctica has fossilized plants…somewhere warmer Africa has glacial till….it was somewhere colder REJECTED The continents didn’t have a perfect fit, and most didn’t fit at all Land bridges…big ones… could have allowed crossings Coincidence or widespread events Polar wandering- the earth’s polar locations may have changed Most importantly…wouldn’t we feel them move? Couldn’t measure it Couldn’t explain the cause of movement. Alfred was ridiculed, but finally got a professorship Soon after, he reluctantly went on his 4th trip to Greenland Died collecting Ice samples Seafloor Spreading Harry Hess Naval Rear Admiral. Served in WWII Post-WWII, helped map the seafloor Worried about the Soviets and subs Discoveries by others of the undersea ridges and mountains He reviewed the data again and made a hypothesis Seafloor Spreading Sonar allowed mapping of sea floor “Scar” encircles the whole globe. Mid-ocean ridge A mirrored profile emerged Undersea mountains with a rift in the middle Mountains led to low abyssal plains on both sides Ended with continental slope and shelf on each side Not perfectly symmetrical though Proposed that hot, less dense mantle material rises toward the mid-ocean ridges Some material solidifies. Some flows sideways and drags the seafloor along with it in both directions As the seafloor spreads apart, new seafloor is created Older seafloor moves away in opposite directions This could explain how the crust moved! Supports Continental Drift Theory. Evidence for Seafloor Spreading Mid-Ocean Ridge System encircles globe Less sediments and fossils around the midocean ridges (Younger) Evidence for Seafloor Spreading 1968- Glomar Challenger Expedition Collected rock samples to see ages Younger rocks near mid-ocean ridges. Older near continents. Ocean floor age mirrored on each side of mid-ocean ridge Evidence for Seafloor Spreading Paleomagnetism When rocks form, they take on the magnetic profile of the earth at that time Magnetic field switches every so often though. Pattern mirrors on ocean floor as well Seafloor spreading explains how continental drift can occur. IV. Plate Tectonic Theory It’s a combination of continental drift and seafloor spreading The lithosphere is broken into tectonic plates. They move across the upper mantle (Asthenosphere) They collide and divide to create the major landforms on Earth today Plate Tectonic Theory Caused by convection currents Heated Asthenosphere flows upwards (less dense) When it hits the crust it spreads outwards Flowing Asthenosphere drags lithosphere with it Asthenosphere material cools, sinks. 5. Cycle repeats C. Tectonic Plates 1. Explains a) Mountains b)Earthquakes c) Volcanoes Other geologic patterns and features Can use these features to find the plates