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9/8/2015 Structure of Earth Notes Part 2 https://www.khanacademy.org/science/cosmolog y-and-astronomy/earth-history-topic/platetechtonics/v/plate-tectonics-difference-betweencrust-and-lithosphere 1 9/8/2015 Plate tectonics 2 9/8/2015 Lithosphere and Lithospheric plates, pt. 1 • Lithosphere – rigid outer shell of the Earth – Composed of crust and some of the upper mantle – Composed of a series of moving plates – Mature oceanic plate is approx. 80-100 km thick • The upper 5-7 km is oceanic crust – Continental plate is somewhat thicker, 150-200 km thicker • Includes continental crust, 5-70 km thick 3 9/8/2015 Lithosphere and Lithospheric plates, pt. 2 • A single plate may be part oceanic and part continental. In this type of case, the boundary between ocean crust and continental crust is NOT the plate boundary. • The mobile part of the mantle under the lithosphere is called the asthenosphere. 4 9/8/2015 Boundaries between two Lithospheric plates, pt 2 • Plate boundaries are most readily identified by seismicity, the location of significant earthquake activity. • About 95% of the world’s Earthquakes occur on plate boundaries and ALL earthquakes occur in the Lithosphere. 5 9/8/2015 USGS.gov diagram • The diagram on the next page shows 23 plate boundaries and their direction of motion. • Use this labeled diagram to help you label your diagram. • LEGEND • Orange = divergent boundary • Green = convergent boundary • Blue = transform boundary 6 9/8/2015 Plate boundaries and direction of movement 7 9/8/2015 Boundaries between two Lithospheric plates, pt 1 • Can be –Divergent –Convergent –Or –Transform 8 9/8/2015 Divergent Plate Boundaries • At divergent plate boundaries, two lithospheric plates move apart and new plate (and crust) are created at the boundary. • The new crust is created through volvanic activity. • This process ALWAYS PRODUCES OCEAN CRUST, and thus oceanic plate. • Thus divergent boundaries ultimately become mid-ocean ridges. 9 9/8/2015 Divergent plate boundaries, pt 2 • As the lithosphere moves away from the ridge, it thickens by a process of underplating, in which asthenosphere solidifies onto the underside of the plate as the plate cools. 10 9/8/2015 • Thickening lithosphere subsides as it ages. Ridges are a result of this. Young lithosphere near the ridge sits up high, while older lithosphere sits lower. • Ultimately, ocean lithosphere becomes more dense than the asthenosphere, beneath it. Thus old oceanic lithosphere will sink in to the asthenosphere if it gets the chance. 11 9/8/2015 Convergent boundaries, pt 1 • Two lithospheric plates move together. The denser plate sinks in a process called subduction. • The subduction process produces magmas through other processes (that we will discuss as needed later.) • Only oceanic plate can subduct. Continental crust is too thick and low density to sink. (acts like a float). 12 9/8/2015 Convergent, pt 2 • Subducting plates (sinking into the mantle) remain more rigid than the surrounding mantle to depths of about 700km. This is the maximum depth of earthquakes (which only occur in rigid lithosphere). 13 9/8/2015 Convergent, pt 3 • At some convergent plates, oceanic plate meets continental plate. • An example: west coast of southern Mexico, Central America and South America. – An oceanic trench forms offshore. – Chain of volcanic mountains forms just inland of the coast. – Called an active continental margin. 14 9/8/2015 Convergent, pt 4 • Other convergent plates: oceanic meets oceanic. • Results in oceanic trench and a chain of volcanic islands known as an island arc. • Japan, the Phillipines, and the Aleutions are examples. 15 9/8/2015 Convergent plates, pt 5 • At other convergent plate boundaries, two continents collide. Neither can subduct. • Result: like a car crash. • Both are crumpled. Large mountain belts are formed, their insurance rates go up. 16 9/8/2015 Transform Boundaries, pt 1 • Occur when two plates move parallel to one another. • Neither divergence nor convergence • Rare: occurs for short intervals along mid ocean ridges. • Along and perpendicular to mid ocean ridges. • The San Andreas Fault in California is a transform boundary. Another cuts across southern New Zealand. • No volcanism at transform boundaries 17 9/8/2015 What causes plate tectonics?, pt 1 • The ultimate cause of plate tectonics is the cooling of the earth. The core and inner Earth have retained enormous amounts of heat from the initial formation of Earth. Since the mantle is made of rock, it is a very good insulator. 18 9/8/2015 What causes plate tectonics, pt 2 • The material of the mantle convects, enabling the release of heat. Like a hot cup of coffee, the quickest way to cool the Earth is to stir it. The movement of plates is the surface manifestation of that mantle convection. 19 9/8/2015 What causes plate tectonics, pt 3 • Oceanic plates are driven by their own sinking. Thick, cold, dense oceanic lithosphere sinks at subduction zones, pulling the rest of the ocean plate. Since plates have limited tensional strength, this "subduction pull" cannot be the only force acting on an ocean plate. 20 9/8/2015 What causes plate tectonics, pt 4 • "Ridge push" is the sliding of the young thin part of the plate downhill away from the ridge. A combination of "subduction pull" and "ridge push" move the ocean plates along. These forces are not separate from mantle convection, but are both caused by and help cause that convection. Continental plates are pushed around by oceanic plates. 21