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Which of these describes the lithosphere and the asthenosphere? A. The lithosphere is rigid and immovable, and the asthenosphere is hot and flowing. B. The asthenosphere is rigid and immovable, and the lithosphere is hot and flowing. C. Both are hot inner layers of earth capable of bending and moving. D. Both are hard and rigid layers of earth close to the surface. Science Starter 9.4-5.13 The Plate Tectonic Theory Tectonic Plates Plates are large sections of the earth’s crust and upper mantle that ride on top of the asthenosphere. Plate Tectonics • In 1912, German scientist Alfred Wegener proposed two ideas that are known as continental drift Theory – Earth’s continents were once joined in a single large landmass called Pangaea that broke apart millions of years ago. – Continents have drifted to their current location. Continental Drift • It has taken the continents about 225 million years since the breakup of Pangaea to move to their present locations. Pangaea What do you notice about the movement of the continent Pangaea to present day continents? The Moving Continents… Plate Tectonics Explains Continental Drift Theory of Plate Tectonics • Claims that Earth’s plates are always in motion – Plates move • slowly • at different rates • in different directions – Explains the formation, movement, and subduction of Earth’s plates Theory of Plate Tectonics • Crust rides on top of the plates • Plates ride on top of mantle which is in motion due to convection currents – Convection currents can cause plates to move away from each other or toward each other. Two Types of Lithospheric Plates • Continental crust: made of rocks that are less dense and ride higher on the mantle than oceanic crust – Made of granite rock • Oceanic crust: rocks that are more dense and ride lower on the mantle than continental crust – Made of basalt rock Plate Boundaries • Movement of Earth’s plates are responsible for most major geological events and landforms. – Volcanoes, earthquakes, mountain formation • Plate boundaries are where edges of plates interact. Earth’s Major Plates Convection Currents Convection Currents • Heat from the core causes convection currents in the mantle. • Heating and cooling of the fluid, changes in the fluid’s density, and the force of gravity combine to set convection currents in motion. Convection Currents in the Mantle animation