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Commotion Beneath the Ocean Introduction to Plate Tectonics Plate Tectonics • Originally proposed by Wegener - but ridiculed !! • 1960’s new evidence found • Earth’s Surface Continually Shifting • Plate - Large, Rigid Slab of Solid Rock • Tectonics- to build Theory of Continental Drift • Pangaea – 225 mya • Earth/Solar System - 4.6 bya • Life – 3.4 bya Evidence of Continental Drift • Coastline shapes • Rocks Minerals • Fossils Inside the Earth • Crust • Mantle • Core Convection • Heated material expands - becomes less dense - more buoyant • It rises and cold material takes it’s place Heat • Decay of Radioactive Elements inside Earth • Trying to Escape • Convection transport of mass by heat (heat changes the density) Mid- Ocean Ridges • Sonar • Mapping Began during WWII • By 1960’s clear maps of mid ocean ridges Ocean Maps • All the major oceans have long ridges Underwater Volcanoes • Opening of the earth where hot molten rock, ash, and gases to escape from deep below the surface • Underwater volcanoes are called Hydrothermal vents So what do the ridges have to do with plate movement? • Undersea volcanoes • Deposit molten rock from mantle convection • Seafloor spreading Supported Theory of Plate Tectonics 1962 - Harry Hess Seafloor Spreading as proposed by Harry Hess • New Ocean Floor deposited at the ridges • Moves laterally away from the ridges • Moves back down at the continental margin • Continents are not “islands” they are parts of larger plates that move on the soft asthenosphere Earth’s Magnetic Field • Liquid Iron in the outer core • Moving Electrical charges (current) INDUCE a magnetic field • Aligns approximately with the spin axis but periodically flips N<-->S Evidence for Seafloor Spreading • Magnetic Reversals • Rocks have magnetic fields • When rocks cool magnetic minerals align to Earth’s Mag Field • 1960’s drilling found Mirror images on each side of mid Atlantic ridge How old is the lithosphere? • Dates of rock increased on both sides of mid-ocean ridges The Plates • Earth - egg • Shell - cracked, but stronger than the interior • Tectonic plates in the lithosphere Plates • Tectonic plates are in the lithosphere • The lithosphere floats on the Asthenosphere • Asthenosphere is undergoing convection • Plates are moving and interact with other plates Plate Boundaries Transform Divergent Convergent Divergent Plates • Lithosphere is being produced • Happens at mid ocean ridges • Plate motion is away from the boundary • Sea-floor spreading • Well if two plates are diverging….. Convergent Boundaries • Lithosphere sinks back into the asthenosphere (its recycled) • One plate moves under another plate - SUBDUCTION • The ocean plate is denser !! Continent - Continent When two continental crusts converge, the result is a huge mountain range. Both pieces of crust are buoyant. Example - Himalayas Indian plate -->> Asian plate. Numerous earthquakes but very little volcanism. Transform Plate Boundaries • One plate slides parallel to another plate • San Andreas Fault • • Plate boundary details for the West Coast of North America Divergent Plate Boundaries Hot Spots and Hawaiian Islands • 1963 Tuzo Wilson (also discovered transform faults) • Noticed Volcanism in some areas active for a really long time • Heat produced magma, melted through the plates. • Magma lighter than the rock rises. Hawaiian Chain Hot Spots Measuring Hot Spots • Yellowstone • Plate moves • Hotspot does not • Determine and compare the age of the rocks • New evidence shows that hot spot may also move somewhat Hot Spots • Yellowstone: several craters or calderas Energy fuels hot springs and geysers - Old Faithful May be a Super Volcano !! Review