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Plate Tectonics The theory --the surface of the Earth is broken into large plates. The size and position of these plates change over time. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. Plate tectonics is a combination of two earlier ideas, continental drift and seafloor spreading. Continental drift is the movement of continents over the Earth's surface and in their change in position relative to each other. Seafloor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges. http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/introduction.html Earth Structure Mexico quake-- magnitude at 7.0, and epicenter was in the western Pacific state of Michoacan. Its depth was about 40 miles Indonesia-- earthquake off coast of Indonesia Wednesday, April 11 with magnitude of 8.6, sparking tsunami warnings Lithosphere Consists of continental, oceanic and upper part of mantle Continents composed of granite-type rock, quartz and feldspar minerals, density+2.8g/cm3 Oceanic crust formed of basalt; basalt rich in iron/magnesium minerals, density+3.0 g/cm3 Lithosphere is rigid layer of crust and mantle overlying partially-molten asthenosphere Continental Drift Evidence Researchers noted geographic fit of continents – e.g. Africa and S. America – Atlantic formed by separation of Africa from S. America Seuss, 1885, proposed super continent by studying fossils, rocks, mountains Wegener and Taylor, early 1900’s, proposed continental drift and Pangaea Evidence supporting the idea that the continents had drifted. – – – – Geographic fit of continents Fossils Mountains Glaciation Continental Drift Geographic Fit Continents seem to fit together like pieces of a puzzle Continental Drift Fossils Similar distribution of fossils such as the Mesosaurus Continental Drift Mountains Mountain ranges match across oceans Continental Drift Glaciation Glacial ages and climate evidence Objections to the Continental Drift Model Wegener did not provide a plausible mechanism to explain how the continents could have drifted. Seafloor Spreading Continental drift reexamined in 1960’s with new information New theory developed – Seafloor spreading Supporting evidence for seafloor spreading – World seismicity – Volcanism – Age of seafloor – Paleomagnetism – Heat flow Theory combining continental drift and seafloor spreading termed “Plate Tectonics” Seafloor Spreading New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenches Evidence for Seafloor Spreading World Seismicity Earthquake distribution matches plate boundaries Evidence for Seafloor Spreading Volcanism Volcanoes match some plate boundaries; some are hot spots Evidence for Seafloor Spreading Age of Seafloor Youngest sea floor is at mid-ocean ridge Oldest sea floor away from mid-ocean ridge Evidence for Seafloor Spreading Paleomagnetism Earth has a magnetic field - Probably caused by rotation of solid inner core in liquid outer core (both mostly Fe) When rocks cool at the Earth’s surface, they record Earth’s magnetic field (normal or reverse polarity) Evidence for Seafloor Spreading Paleomagnetism Paleomagnetic studies indicate alternating stripes of normal and reverse polarity at the mid-ocean ridge. Seafloor Spreading Heat Flow In 1960, convection currents were proposed as driving force to move continents Theory of Plate Tectonics John Tuzo Wilson combined ideas of continental drift and seafloor spreading into “Plate Tectonics” Principles of Plate Tectonics Earth’s outermost layer composed of thin rigid plates moving horizontally Plates interact with each other along their edges (plate boundaries) Plate boundaries have a high degree of tectonic activity – mountain building – earthquakes – volcanoes Plate Boundaries Three types Plate Boundaries Divergent Plates move away from each other New crust is being formed Divergent Plate Boundaries Examples East African Rift Mid-Atlantic Ocean Ridge Plate Boundaries Convergent Three Types: Ocean-continent Ocean-ocean Continent-continent Plates are moving toward each other Crust is being destroyed Convergent Plate Boundaries Mount Fuji, Japan Examples Mount Lassen, California Andes, South America Plate Boundaries Transform xxxxxxxxxxxxxxxxxxx Plates slide past one another Crust is neither created nor destroyed Transform Plate Boundaries Examples Calexico, California San Andreas Fault Carrizo Plains, Central California Stop Here Origin of the Earth Meteors and Asteroids bombarded the Earth Gravitational compression Density Stratified planet Earth’s Interior Core – dense – Nickel and Iron – Inner core – solid – Outer core – liquid Mantle – Less dense than core – Iron and Magnesium silicates – Partially molten Crust Outermost layer Continental – granite-type, density=2.8 g/cm Oceanic – basalt, density=3.0 g/cm Rigid Evidence of Internal Structure Density – calculate density of Earth – Speculate on probable compositions Meteorites – Use composition and age to determine composition and age of Earth Seismic waves – Travel times and direction give indication of internal structure of Earth Granite Basalt Types of Seismic Waves P waves – Primary, pressure, push and pull – Travel fastest of the seismic waves (average 6 km/sec) – Travels through solids and liquids S waves – Secondary, shaking, side-to-side – Slower (average 4 km/sec) – Travel through solids only Seismic Waves Through Earth