Plate Tectonics A Powerful Unifying Theory Plate 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 17 Plate Tectonics Introduced Earth 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 21 Plates Interact They converge diverge or slide horizontally past one another Refer to pages 17 21 Continental Drift Alfred 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 17 Continental Drift The position of the continents today The continents are still slowly moving at about the speed your fingernails grow Wegener s Evidence for Continental Drift Continents fit together like a puzzle e g the Atlantic coastlines of Africa and South America Refer to pages 13 17 Wegener s Evidence for Continental Drift Fossils of plants and animals of the same species found on different continents Refer to pages 13 17 Wegener s Evidence for Continental Drift Mesosaurus 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 Drift The distribution of climate sensitive sedimentary rocks on the different continents Refer to pages 13 17 Coal deposits are found abundant in Pennsylvania and Siberia Why is this unusual Wegener 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 17 Wegener s Evidence for Continental Drift Although 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 17 The Revival of Continental Drift 1940 s 1950 s Work 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 trenches Harry Hess and Sea Floor Spreading 1960 s Hess suggested that the continents may be moving along with the sea floor not plowing through it as Wegener suggested Refer to pages 13 17 Harry Hess and Sea Floor Spreading 1960 s Harry 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 17 Evidence of Sea Floor Spreading Paleomagnetism Throughout earth s time the magnetic north and south have switched RANDOMLY and sporadically every 100010 000 years When magnetism switches its called a REVERSAL Refer to pages 13 17 Evidence of Sea Floor Spreading Paleomagnetism Rocks 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 17 Evidence of Sea Floor Spreading Paleomagnetism Frederick 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 17 Evidence of Sea Floor Spreading Paleomagnetism At 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 17 Evidence of Sea Floor Spreading Ocean Floor Ages The 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 million years Anything older has been recycled during subduction so there is no sea floor older than the last Pangaea 200250 mya Refer to pages 13 17 Evidence of Sea Floor Spreading Hot Spot Volcanic Islands A hot spot is a persistent volcanic center located directly above a rising plume of hot mantle rock Hot spot mantle plumes remain stationary while the lithosphere moves over it This process forms a chain of volcanic islands Refer to pages 27 29 The chain of islands formed indicates the direction of plate movement over the hot spot Hot Spot Volcanic Islands Evidence of Sea Floor Spreading Interactions at Plate Boundaries The majority of earthquakes and volcanic eruptions are concentrated in belts or linear chains at the boundaries of the lithospheric plates Refer to pages 21 Plate Boundaries Notice the three different types of plate boundaries All plate boundaries are associated with either volcanism earthquakes or both Plate Boundaries Plate Boundaries Divergent Plate Boundaries Also called spreading centers and rifts occurs where two plates move apart horizontally and new oceanic lithosphere is created Refer to pages 21 23 Rates of sea floor spreading vary globally 1 17 cm year Continent Continent Divergent Plate Boundaries Continental rifting
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