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Transcript
Plate Tectonics Earth Science Continental Drift scientific theory proposing the slow, steady movement of Earth’s continents Early Observers In the late 1500s, A Dutch mapmaker noticed that the continents fit on either side of the Atlantic Ocean. Abraham Ortelius proposed that North and South America had been separated from Europe and Africa by earthquakes and floods. Early Observers Then, a man named Eduard Suess hypothesized that the present southern continents had once been joined to form a landmass called Gondwanaland. Alfred Wegener In 1912, created his hypothesis called “continental drift.” Continental drift = Alfred Wegener’s hypothesis which proposes that the Earth’s continents had once been joined as a single landmass. Wegener’s Continental Drift Hypothesis Supercontinent called “PANGAEA” began breaking apart about 200 million years ago – Continents “drifted” to present positions – Continents “broke” through the ocean crust Evidence Used by Wegener Fit of South America and Africa Matching Fossils Matching Rock Structures Ancient Climates Main Objection to Continental Drift Wegener’s theory did not provide a mechanism for the movement of continents Plate Tectonics Theory proposed in the 1960’s that replaced the continental drift theory Could provide an explanation for the movement of continents Plate Tectonics Involved Earth’s Rigid Outer Shell, the Lithosphere – Consists of about 20 slabs (plates) Plates are moving slowly Largest Plate is the Pacific plate Plates are mostly beneath the ocean Asthenosphere The partially molten, hot, flowing layer of the mantle Found beneath the lithosphere Allows for motion of lithosphere Lithosphere Asthenosphere Plate Boundaries Where tectonic plates meet and interact Some boundaries converge, diverge, or move horizontally Associated with: – Seismic activity (earthquakes) – Volcanism – Mountain building Divergent (Spreading) Boundaries Plate tectonics that MOVE AWAY from each other Most exist along oceanic ridge crests, in a rift or valley Process initiates Seafloor Spreading – Forms fractures (openings) on the ridge crests – Fracture fills with magma—creates new seafloor Divergent (Spreading) Boundary When Divergent Boundaries are on continents, Rift Valleys form Rift Valley: long narrow depression Convergent (Together) Boundary 3 types of Convergent Boundary – Oceanic-Continental Boundary – Oceanic-Oceanic Boundary – Continental-Continental Boundary Oceanic-Continental Boundary Forms a subduction zone with a deepocean trench Dense ocean plate subducts beneath continent Results in volcanic mountains Examples: – Andes South America – Sierra Nevadas California Oceanic-Oceanic Boundary 1 ocean plate subducts beneath another Creates deep sea trenches Often forms volcanoes on the ocean floor Island arc forms as volcanoes emerge Examples: – Aleutian Trench & Islands – Mariana Trench and Islands Continental-Continental Boundary 1 continental plate pulled into subduction zone, but not forced into mantle Can produce mountains Examples: – Himalayas (India and Asia) – Other possibilities Alps Appalachians Transform Plate Boundary 2 Plates slide past one another – No new crust is created – No crust is destroyed, but deformed or fractured – Associated with long faults Transform faults – Most often in the ocean: 1 land example San Andreas Fault in California Transform Plate Boundaries Evidence to Support Plate Tectonics 1. Paleomagnetism: study of Earth’s magnetism – Ancient Magnetism preserved in rocks – Paleomagnetic records show Earth’s Magnetic Field Reversals – Recorded in the Seafloor Rocks – Confirms seafloor spreading Evidence to Support Plate Tectonics 2. Earthquake Patterns – Associated with Plate Boundaries – Deep-focused Earthquakes along trenches provide a method for tracking the plate’s descent Evidence to Support Plate Tectonics 3. Ocean Rock Age – Youngest are near ocean ridges – Oldest at a distance from ocean ridges – Ocean basins are geologically young 4. Ocean Sediment – Thicker further from o. ridge Driving Mechanisms of Plate Tectonics No one model explains all plate motions Earth’s heat is the driving force Several models have been proposed – Convection currents in mantle – Slab-pull and slab-push model Slab pull-push Slab pull and slab push – Descending oceanic crust pulls the plate Convergent boundary – Elevated ridge system pushes the plate Divergent boundary Volcanism at convergent boundaries Most, 80% volcanoes occur at convergent plate boundaries Oceanic-oceanic volcanic islands OceanicContinental volcanic mountains The island of Bora Bora Mount Shasta in the Sierra Nevada’s of Oregon Volcanism at Convergent boundaries Plates come together at convergent boundariescreates a subduction zone plate descends into the mantle and melts into magmacomes back up through the boundaryforms volcanoes when it reaches the surface. Volcanism at divergent boundaries 15% of volcanoes are found at divergent boundaries Magma is forced upward into the fractures and faults that form as the plate separates Iceland is an example of this volcanism, its part of the midAtlantic ridge Hot Spots 10. 5% of volcanoes are hot spots. 11. These volcanoes are found far away from any plate boundary. 12. Unusually hot regions of the earths mantle where high temperature plumes of mantle material rise toward the surface Hot Spots Plume melts rock melt straight through the mantle and crust become volcanoes on earth’s surface A Hot Spot!!! http://link.brightcove.com/services/li nk/bcpid1329217914/bctid18774390 6
