Plate Tectonics – A Powerful Unifying TheoryPlate Tectonics IntroducedPlates Interact: They converge, diverge or slide horizontally past one another. Refer to pages 17 - 21Continental DriftSlide 5Wegener’s Evidence for Continental DriftSlide 7Slide 8Slide 9Slide 10Slide 11The Revival of Continental Drift 1940’s & 1950’sHarry Hess and Sea Floor Spreading 1960’sSlide 14Evidence of Sea Floor Spreading PaleomagnetismSlide 16Slide 17Slide 18Evidence of Sea Floor Spreading Ocean Floor AgesEvidence of Sea Floor Spreading Hot Spot Volcanic IslandsHot Spot Volcanic IslandsEvidence of Sea Floor Spreading Interactions at Plate BoundariesPlate BoundariesSlide 24Slide 25Divergent Plate BoundariesContinent-Continent Divergent Plate BoundariesSlide 28Ocean-Ocean Divergent Plate BoundariesConvergent Plate BoundariesSlide 31Continent-Ocean Convergent Plate BoundariesSlide 33Continent-Continent Convergent Plate BoundariesSlide 35Ocean-Ocean Convergent Plate BoundariesSlide 37Transform Plate BoundariesTypes of Plate Boundaries: Dynamics, Results, and ExamplesDriving force is not just convection? Ridge Push and Slab PullRidge Push and Slab PullPlate Tectonics – A Powerful Unifying TheoryPlate tectonics is a relatively new scientific concept, introduced some 30 years ago, but it has revolutionized our understanding of the dynamic planet upon which we live. Refer to pages 13 - 17Earth’s lithosphere, which consists of the earth’s crust and upper mantle, is cut up into roughly 20 plates that move relative to one another atop of the asthenosphere. Refer to pages 17 - 21Plate Tectonics IntroducedPlates Interact: They converge, diverge or slide horizontally past one another.Refer to pages 17 - 21Continental DriftAlfred Wegener in the early 1900’s proposed that the continents were once joined together in a single large land mass he called Pangea (meaning “all land” in Greek). He proposed that Pangea had split apart around 200-250 million years ago moving gradually to their present positions - a process that became known as continental drift.Refer to pages 13 - 17Continental DriftThe position of the continents today. The continents are still slowly moving, at about the speed your fingernails grow.Wegener’s Evidence for Continental DriftContinents fit together like a puzzle…e.g. the Atlantic coastlines of Africa and South America.Refer to pages 13 - 17Wegener’s Evidence for Continental DriftFossils of plants and animals of the same species found on different continents. Refer to pages 13 - 17Wegener’s Evidence for Continental DriftMesosaurus a freshwater reptile fossil found in Africa and South America.Glossopteris; a fern that requires warm climates was found on Antarctica, Southern South America, Australia, Southern Africa and India.Wegener’s Evidence for Continental DriftThe distribution of climate sensitive sedimentary rocks on the different continents.Coal deposits are found abundant in Pennsylvania and Siberia. Why is this unusual?Refer to pages 13 - 17Wegener’s Evidence for Continental Drift•Glacial sediment deposits found in places where glaciers do not exist today.•Glacial Scratches (scratches on rock caused by glacial movement) line up like a jigsaw puzzle when continents are reassembled.Both show that the land masses were all joined and partially covered by a single large ice cap over the ancient south pole! Refer to pages 13 - 17Wegener’s Evidence for Continental DriftAlthough today we know that Alfred Wegener was correct about continental drift, at the time his theory was not widely accepted because he never supplied a viable mechanism to explain how continental movement could be accomplished.Confirmation of continental drift would have to wait until the 1960’s, when a better understanding of the ocean floor lead to the concept of sea floor spreading. Sea floor spreading would eventually vindicate Wegener and lead to the most important unifying concept in geology: the theory of plate tectonics. Refer to page 17The Revival of Continental Drift 1940’s & 1950’sWork in the 1940’s and 50’s set the stage for the revival of Wegner’s work. During that time intense oceanographic research and technological advancements were taking place that provided maps of the sea floor showing mid-ocean ridges and deep sea trenchesHarry Hess and Sea Floor Spreading 1960’sHess suggested that the continents may be moving along with the sea floor, not plowing through it as Wegener suggested.Refer to pages 13 - 17Harry Hess and Sea Floor Spreading 1960’sHarry Hess concluded that new sea floor was being created at mid-ocean ridges (MOR) by volcanic activity. But the earth is not getting larger. Therefore he concluded that an equal amount of oceanic crust is probably being lost at trenches.The driving force is convection. Locations of spreading ridges (upwelling) and trenches (downwelling) are determined by the convection cells.Refer to pages 13 - 17Evidence of Sea Floor SpreadingPaleomagnetismThroughout earth’s time the magnetic north and south have switched RANDOMLY and sporadically every 1000-10,000 years. When magnetism switches, its called a REVERSAL.Refer to pages 13 - 17Evidence of Sea Floor SpreadingPaleomagnetismRocks record the direction of the earth’s magnetic field at the time the rocks form. Small magnetite (Fe) crystals in cooling magma act like compass needles that record the direction of the earth’s magnetic field when the magma solidifies. Refer to pages 13 - 17Evidence of Sea Floor SpreadingPaleomagnetismFrederick Vine and Drummond Matthews found that alternating normal and reversely polarized rock form a symmetrical stripe-like pattern parallel to the ridge crest.Refer to pages 13 - 17Evidence of Sea Floor SpreadingPaleomagnetismAt MOR new sea floor is added and spreads laterally from the axis. As the magma cools and the iron bearing minerals crystallize they align themselves parallel to the lines of force of the earth’s magnetic field. Therefore, the sea floor is a ticker tape recording of the earth’s magnetism through geologic time. (Only for about ~200million years….why?)Refer to pages 13 - 17Evidence of Sea Floor SpreadingOcean Floor AgesThe rocks of the sea floor, are youngest close to the MOR and become progressively older the farther away they are from the ridges on either side. The age pattern is symmetrical across the ridge. Sediments deposited on the sea floor and radiometric dating of basalt have ages no older than ~200
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