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Chapter 3

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University of Toronto Scarborough
Environmental Science

What is Plate Tectonics?  Tectonics is the study of the origin and arrangement of the broad structural features of the earths surface, including not only folds and faults, but also mountain belts, continents, and earthquake belts  Plate tectonics- earths surface is divided into a few large, thick plates that move slowly and change in size  Intense geologic activity occurs at plate boundaries toward one another.  The eight large plates and few dozen smaller ones make up the outer shell of the earth (crust and upper part of mantle)  Plate tectonics can tell us about earthquake distribution, origin of mountain belts, origin of sea floor topography, distribution and composition of volcanoes..plate interactions at plate boundaries  Concept of plate tectonics was developed in 1960s by combining two pre-existing ideas- continental drift and sea-floor spreading o Continental drift- continents move freely over the earths surface, changing their positions relative to one another o Sea floor spreading- hypothesis that the sea floor forms at the crest of mid-oceanic ridges, then moves horizontally away from the ridge crest toward an oceanic trench...the two sides are moving in opposite directions like slow conveyor belts The early case for continental drift  In early 1900s Alfred Wegener, a german meteorologist made a strong case for continental drift. He noted that South America, Africa, india, Antarctica, and Australia had almost identical late paleozoic rocks and fossils  The plant Glossopteris is found in Pennsylvanian and Permian-age rock on all five continents, and fossil remains of Mesosauras, a freshwater reptile, is found in Permian-age rocks only in Brazil and South Africa. Fossil remains of lang-dwelling reptiles Lystrosaurus and Cynognathus are found in Triassic-age rocks on all five continents  Wegener reassembled the continents to form a giant supercontinent, called Pangea (Pangaea).  Pangea initially separated into two parts. Laurasia was the northern supercontinent (NA and Eurasia-excluding India). Gondwanaland was the southern supercontinent (southern hemisphere continents and India which has drifted north)  The distribution of Late Paleozoic glaciations strongly supports the idea of Pangea  The Gondwanaland continents all have glacial deposits of Late Paleozoic age  If continents are arranged according to wegeners Pangea reconstruction, then glaciations in teh southern hemisphere is confined to a smaller area, and the absence of widespread glaciations in the northern hemisphere becomes easier to explain. Also, the present arrangement of the continents would require that late Paleozoic ice sheets flowed from the oceans toward the continents, which is impossible  Paleoclimatalogy-study of ancient climates  Glacial features:indicate cold climate near north or south pole  Coral reefs: indicate warm water near equator  Crossbedded sandstones: indicate ancient deserts near 30 degrees N and 30 degrees S latitude  Wegener looked at ancient sedimentary rocks and found paleoclimatic reconstructions suggested polar positions different to those at present...he hypothesized that this indicated the continents had moved and gave strong support to continental drift Scepticism about continental drift  Wegener proposed that continents ploughed through the oceanic crust, crumpling up mountain ranges on the leading edges of the continents where they pushed against the sea floor...most geologists thought that this idea violated what was known about the strength of rocks at the time  Wegener said the driving force for continental drift was combo of centrifugal force from earths rotation and gravitational forces that cause tides...but these forces too small to move continents  Little support in NA Study of the Sea Floor  Several ways to get sample of rock and sediments from sea floor
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