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CONTINENTAL DRIFT, SEA-FLOOR SPREADING, AND PLATE TECTONICS An ancient, but amazingly accurate world map! Early map of trade routes More modern map of sea bottom features. Look at the jigsaw puzzle outlines of the continents! Alfred Wegener • Most people thought the complementary shapes were coincidence. Alfred Wegener was an exception who sought more evidence that the continents had been joined. He found it in the rocks & fossils and proposed the hypothesis of Continental Drift. Continental Drift - A Hypothesis Three important ideas changed our understanding of the earth: • Continental drift - a hypothesis • Sea-floor spreading- a hypothesis • Plate Tectonics - a theory What Is This Supercontinent? Pangaea Who proposed the hypothesis of continental drift? Alfred Wegener in 1912 When did Pangaea break up? About 180-200 million years ago The most current theory shows the development of modern continental shape and location What Kind of Evidence Supported Continental Drift? Matching Mountain Belts • Appalachians and Caledonian Mountains • Cape Mountains, S. Africa and Sierra of Argentina • Fossil Evidence How could fossils of land animals appear on widely separated continents? (Evolved independently, On rafts, Swam there, Land bridges) • Wegener’s explanation: Continents were all one Past Climates • Now-tropical Africa, India, etc., had ice sheets 300 million years ago • Tillite • Now-midlatitude North America and Europe, once had tropical swamps • Coal Why Was Continental Drift Not Accepted? How could vast continents plow through the ocean floor? What was the driving mechanism? Rotation of earth Pull of the moon Inadequate explanation - forces too weak Wegner’s Problem • The problem with the Continental Drift Hypothesis was that Wegener had no mechanism for moving the continents through the ocean floor. • Powerful physical arguments prevented continental drift from acceptance by the majority of scientists, particularly those in the northern hemisphere. • Wegener died in 1930 with his hypothesis largely ignored. But despite the problems and an overwhelming rejection of his ideas, not everyone forgot about his work. • In the 1940’s and 1950’s geophysicists studying the record of Earth’s magnetic field began to revive some of the ideas to explain observed variations in their observations. During WWII and with the initiation of the Cold War, much research was focused attention on the ocean floor. – Researchers were surprised to find long, large mountain chains and deep trenches. – They also found that the rocks which compose the ocean floor were young, ranging in age from recent to 200 Ma. Seafloor Spreading • Sea Floor Spreading is the hypothesis in which new ocean floor is created at the mid-ocean ridges and that plates move apart at the ridges and the continents move with them. Hypothesis of Sea-Floor Spreading Who proposed this hypothesis? Harry Hess What Goes On At Mid-Ocean Ridges? Sea-floor spreading Shallow earthquakes High heat flow and volcanism Birthplace of new oceanic crust Youngest rocks of the sea floor Magnetic stripes Thinnest sediment What Was The Proof Of Sea-Floor Spreading? Paleomagnetism of the sea floor Ancient magnetism records flip-flops of earth’s magnetic field North orientation - normal South orientation - reverse How Does the Age of the Sea Floor Support Sea-Floor Spreading? Oceans are youngest at the Mid-Ocean ridges Oldest near the continents Summary of Sea-Floor Spreading New sea floor is created at mid-ocean ridges. Sea floor is destroyed in subduction zones. Seafloor Recycling • Since new material is created at the midocean ridges and Earth is not expanding, somewhere material must be removed from the surface. • It turns out that old ocean floor is “subducted” into the mantle at subduction zones. Plate Tectonics - What Is This Grand Unifying Theory? The lithosphere is cracked into seven or more large plates with many smaller ones. The lithosphere moves over a semi-molten asthenosphere. Most tectonic plates include both continental crust and oceanic crust. Map of Tectonic Plates What global features can now be related to the story of plate tectonics? Deep Oceanic Trenches Mid-Ocean Ridges Volcanic Activity Earthquake Epicenters • By plate we really mean a “shell” on Earth’s surface that is about 100200 km thick and includes all of the crust, and the uppermost part of the mantle. • Tectonics is the term we use in geoscience to represent the formation of folds, fractures, faults, etc. in rocks. Major Plates Newest Design - 2002 Global Seismicity New Volcanism Earthquakes • Most earthquakes occur along plate margins because plate margins are relatively weak, but about 10% of earthquakes occur within the interior of plates. • We classify earthquakes depending on where they are located • Interplate - between plates • Intraplate - within plates Earthquakes Coincide With Plate Boundaries Most geological activity occurs along plate boundaries Divergent Plate Boundaries Convergent Plate Boundaries Transform Plate Boundaries Divergent Plate Boundaries Where are most divergent plate boundaries? In the middle of oceans Fault Offset the Mid-Ocean Ridges Shallow earthquakes occur as plates move past Volcanoes erupt along the rifts What Kind of Plate Boundary is Iceland? A divergent plate boundary that forms an island Do Divergent Plate Boundaries Occur on Land? East African Rift Valleys Convergent Plate Boundaries How many ways can plates converge? Ocean-Continent Ocean-Ocean Continent-Continent What Goes On At Ocean-Continent Convergent Plate Boundaries? Earthquakes occur as the ocean plate subducts An ocean trench forms as the sea floor bends Volcanoes form as magma erupts from the subduction zone Mountains form as sediments are folded and faulted What Kind Of Convergent Plate Boundary? Ocean-Ocean Plate Boundary Earthquakes occur as one ocean plate subducts An ocean trench forms as the sea floor bends A volcanic island arc forms as magma rises above the subduction zone Continent-Continent Convergent Plate Boundary Mountain building occurs as the continental crust is folded and faulted Earthquakes occur as two continents collide NO volcanic activity India crashes into Asia Himalayan Mountains and Tibetan Plateau What Kind Of Plate Boundary? San Andreas Transform Fault Plate Boundary Earthquakes only •What Moves the Plates? Convection currents in the earth's mantle Slab-pull in the trenches Ridge-push at the ridges