Unit VI Solid Earth Circulation A Structure of the Solid Earth 1 Crust Thin brittle variable composition least dense a Continental Crust older thicker more variable lots of water volatiles b Oceanic Crust thinner younger more uniform composition Unit VI Solid Earth Circulation A Structure of the Solid Earth 1 Crust Thin brittle variable composition least dense 2 Mantle Uniform composition upper Mantle behaves like a soft plastic rock that can slowly deform 3 Core Very hot very dense Outer core liquid inner core solid Mostly iron 85 and nickle 6 Motion of core creates magnetic field Unit VI Solid Earth Circulation A Structure of the Solid Earth How do we look into the center of the Earth By tracking seismic waves produced by Earthquakes Volcanic explosions Large human noises atomic bombs P compression waves can pass through solid and liquid media S shear waves can only pass through solid media P compression waves S shear waves Unit VI Solid Earth Circulation A Structure of the Solid Earth B Earth s Magnetic Field 1 Core is a magnetic dynamo Transforms fluid motion convection into electrical currents that create magnetic field Unit VI Solid Earth Circulation A Structure of the Solid Earth B Earth s Magnetic Field 1 Core is a magnetic dynamo Magnetic mineral grains in lava or sediment align themselves with the Earth s magnetic field preserving a record of the field direction when they were deposited Throughout recorded history the compass has always pointed North But Earth s magnetic field occasionally collapses and reforms in the opposite direction These are called magnetic reversals Unit VI Solid Earth Circulation B Earth s Magnetic Field 1 Core is a magnetic dynamo Magnetic field occasionally collapses and may reform in the opposite direction Magnetic reversals occur every 0 5 to 1 million years Most recently 0 7 Myr ago Normal and Reversed Polarity Unit VI Solid Earth Circulation B Earth s Magnetic Field 1 Core is a magnetic dynamo Transforms fluid motion convection into electrical currents that create magnetic field 2 Why does the core convect Density differences Radioactivity Radioactive decay releases heat energy Hot core heats the overlying mantle so the outer edge core is coolest Causes a fundamental instability Unit VI Solid Earth Circulation A Structure of the Solid Earth B Earth s Magnetic Field C Plate Tectonics In the 1920s Alfred Wegener proposed that the continents used to be together based on similar continental outlines distribution of fossils and mountain chains Unit VI Solid Earth Circulation C Plate Tectonics But there was no physical basis for drift and his ideas were dismissed for half a century In the early years after WWII ocean exploration revealed mountain ranges in the middle of the oceans mid ocean ridges and ships measured the orientation of magnetic minerals on the sea floor They found magnetic stripes Unit VI Solid Earth Circulation C Plate Tectonics 1 Sea floor spreading Magnetic stripes get older away from mid ocean ridges proving that oceanic crust is constantly being formed at mid ocean ridges Where does all that ocean crust go Oceanic crust is destroyed along subduction zones where it is recycled back into the mantle Unit VI Solid Earth Circulation C Plate Tectonics 1 Sea floor spreading 2 Earth s Tectonic Plates 3 Plate margins determine earthquakes and volcanism a Divergent Margins where Earth s lithosphere is being pulled apart Examples Mid Ocean Ridges African Rift Valleys Unit VI Solid Earth Circulation C Plate Tectonics 3 Plate margins a Divergent Margins where Earth s lithosphere is being pulled apart Commonly associated with mild volcanism shallow earthquakes Fluid volcanism Why Unit VI Solid Earth Circulation C Plate Tectonics 3 Plate margins a Divergent Margins where Earth s lithosphere is being pulled apart b Convergent Margins where two plates are forced together subduction collision Examples Pacific Northwest Aleutian Island Chain off Alaska Japan Italy India West coast of South America b Convergent Margins where two plates are forced together subduction collision Subduction of oceanic plate is commonly associated with volcanism and deep earthquakes What kind of volcanism Explosive volcanism because heated rocks have lots of volatile materials in them water water tied to mineral hydrated When do we get collision and when subduction Ocean Crust Continental Crust subduction because oceanic crust is more dense Continental Crust Continetal Crust collision equal densities neither can easily sink Still get subduction but not of continental crust Example India Himalaya Unit VI Solid Earth Circulation C Plate Tectonics 3 Plate margins a Divergent Margins where Earth s lithosphere is being pulled apart b Convergent Margins where two plates are forced together subduction collision c Transform Margins Where two plates slide past each other Examples Southern California and the San Andreas fault 4 What Drives Plate Tectonics In order to get rock to move we need Density differences Created by an inherent instability Earth is hottest in its interior due to radioactive decay Liquid core circulates relatively quickly heats the lower mantle which becomes less dense than the overlying mantle and SLOWLY starts to rise Buoyant Rigid lithosphere just along for the ride Plasticity heat pressure make rock deformable Unit VI Solid Earth Circulation C Plate Tectonics D Earthquakes sudden release of stored energy as a result of rapid movement between two blocks of rock Can only happen where rocks are brittle in the lithosphere Unit VI Solid Earth Circulation C Plate Tectonics D Earthquakes E Volcanos Explosive vs Fluid Where they occur and why it matters Most occur at Plate Boundaries Most explosive volcanism occurs along subduction zones in continental crust Unit VI Solid Earth Circulation C Plate Tectonics D Earthquakes E Volcanos Explosive vs Fluid Hot Spots Intra Plate volcanism Hawaii Mid ocean hot spot Iceland Hot spot on mid ocean ridge Yellowstone Mid continent Hot Spot Iceland a Hot Spot on a Mid Ocean Ridge Yellowstone The last three Yellowstone eruptions Caldera forming Age ash flow tuff millions of years Lava Creek Tuff 0 6 Mesa Falls Tuff 1 3 Huckleberry Ridge Tuff 2 1 Difference Volume erupted km3 1 000 280 2 450 0 6 Myr 0 7 Myr 0 8 Myr For our purposes volcanic eruptions are most important if they are explosive and place lots of material in the stratosphere But they also pose a very real threat to nearby and sometimes very distant communities