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Plate Tectonics Mrs. Griffin Drifting Continents • Average human lifetimedrift is slow • On a geologic time scaleWhoa Nelly! • South Africa is moving away from Africa- 2 to 3 cm/y • Hawaii volcanic islands are migrating NW ~8 to 9 cm/y • What’s the cause? Early Observations • Eduard Suess- single land mass know as Gondwanaland. • 1st hypothesis taken seriously was proposed by Alfred Wegener – Called his hypothesis continental drift – 200 mya supercontinent Pangaea began to break apart Evidence from Rock Formation • If mountains were once formed together then fractured…… • Wegener hypothesized similar rocks types must exist on opposite sides of the Atlantic Ocean. – Appalachian Mountains – Greenland and Europe Evidence from Fossils • Wegener also gathered evidence from fossils. • Land dwellers could not have swam the great distances that exist today. • Glossopteris fossils have been found on many continents Ancient Climatic Evidence • What types of rocks • Glacial deposits in Africa, provide clues to the past? India, Australia, and South America. – Sedimentary – Continents once near the • Wegener also found south pole? evidence of vast climatic changes on some continents. – Coal deposits in Antarctica, forms from dead swamp plants…..??? – Was Antarctica close to the equator? A Rejected Hypothesis • Wegener's theory was not accepted by scientists due to two major flaws, what were they? – What is the driving force behind the movement of the continents? – How were the continents moving without fracturing? • Wegener died in 1930, and it wasn’t until the early 1960’s that new evidence was revealed that could explain how and why. Where do you think this evidence was found? – Seafloor Alfred Wegener Review • The theory of continental drift was proposed by which individual? • This supercontinent means “all the Earth.” • What type of evidence did the above individual base his hypothesis on? • What were the two “flaws” regarding the theory of continental drift? • Where was the evidence found to explain the two missing flaws? Seafloor Spreading • Help from technology: – Development of echo-sounding methods. • Sonar, uses sound waves to measure water depth. • Magnetometer- a device that can detect small changes in magnetic fields, towed by ships, records the magnetic field strength in rocks on the ocean floor. – Used to make maps of the seafloor. Sea-floor Topography Ocean Floor Topography • What did these maps reveal? – Underwater mountain chains called ocean ridges. • Form the longest continuous mountain ranges on Earth; earthquakes and volcanoes are common among these areas. – Maps also revealed deep-sea trenches, counterparts to ocean ridges. • Mariana Trench is over 11 km deep. Ocean Rocks and Sediments • Rock samples taken from areas near ocean ridges are younger than samples taken from areas near deep-sea trenches. • Ocean floor rocks are much younger than continental rocks. Why is this so? Seafloor Spreading • Topographic, sedimentary, age, and magnetic data combined laid the foundation for Harry Hess to propose his theory: – Seafloor spreading: new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches. Seafloor Spreading • Seafloor spreading provides the answer to the how question. • Continents are merely passengers that ride with ocean crust as it slowly moves away from ocean ridges. The Theory of Plate Tectonics • Why are some regions of Earth dotted with many active volcanoes while other regions have none? Why do earthquakes occur frequently in certain areas? – The theory of plate tectonics: Earth’s crust and rigid upper mantle are broken into enormous slabs called plates. • They move in different directions and at different rates. Earth’s Tectonic Plates Plate Boundaries • Tectonic plates interact at places called plate boundaries which include divergent, convergent, and transform boundaries. Plate Boundaries • Places where two tectonic plates are moving apart are called divergent boundaries. – Found mostly on the seafloor, form ocean ridges. – Some form on continents; when crust separates, the stretched crust forms a long narrow depression called a rift valley. Convergent Boundaries • Places where two tectonic plates are moving toward each other are convergent boundaries, 3 types. – Oceanic-oceanic – Oceanic-continental – Continental-continental • In subduction zones- one of the two plates descends beneath the other ->this creates a deep-sea trench. – Recycling of the plates begins Transform Boundaries • Where two plates slide horizontally past one another. – Here crust is only deformed or fractured. – Characterized by long faults. – Rarely occurs on continents…..except for where? • San Andreas Fault Causes of Plate Motions • Convection currents are thought to be the driving mechanism of plate movements. • Convection: the transfer of thermal energy by the movement of heated matter. – Convection currents in the asthenosphere are set in motion by the transfer of energy b/w Earth’s hot interior and its cooler exterior. Mantle Convection • How is mantle convection related to movement of plates? • Hypothesized: ridge push/slab pull. – Ridge push: formation of ocean ridge causes asthenosphere to rise; the weight of uplifted trench pushes plate. – Slab Pull: sinking region of a mantle convection could be pulled down at subduction zones.