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Lesson 7-3 The Theory of Plate Tectonics The Plate Tectonics Theory The theory of plate tectonics states that Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other. The word tectonic describes the forces that shape Earth’s surface and rock structures that form as a result. The Pacific Plate is the largest of the tectonic plates Earth’s Structure Earth’s crust and upper mental make up the lithosphere. These layers are cold, rigid rock. Below the lithosphere is the asthenosphere. This layer is hot and flows like plastic (meaning that it can be molded or change shape without breaking) - Plates on the lithosphere move because they rest on the flowing asthenosphere. Plate Boundaries Plate Boundary: where two plates meet. Divergent Plate Boundaries A Divergent Plate Boundary is when two plates move away from each other. In the ocean, mid-ocean ridges are located at divergent plate boundaries. On a continent a divergent plates cause rift valleys to form. Transform Plate Boundaries When two plates slide by each other, a(n) transform plate boundary forms. This type of movement cause earthquakes. Convergent Plate Boundaries When two plates collide, a convergent plate boundary forms. When the denser plate slides under the lessdense plate during the collision, subduction occurs. Convergent Plate Boundaries When oceanic and continental plates collide a deep ocean trench forms with volcanos nearby. Ex. Mount Rainier When two oceanic plates collide, a trench forms. When two continental plates collide, mountains form Ex: The Himalayas Evidence of Tectonic Plates Today, scientists use satellites to measure how fast the continents move. By using the theory of plate tectonics, scientists can explain why earthquakes and volcanoes occur in certain locations. Plate Motion The asthenosphere moving underneath the lithosphere causes the continents to move. Convection currents cause tectonic plates to move. Convection is the circulation of material caused by differences in temperature and density. Convection currents form in the mantle when thermal energy transfers from the core to the mantle. Forces Causing Plate Motion Three forces interact to cause tectonic plate motion. Convection currents in the mantle produce a force that cause motion called basal drag. The force of ridge push causes the plates to be pushed away from each other at mid ocean ridges. When a plate sinks below another plate, it pulls on the rest of the plate, exerting a force called slab pull. A Theory in Progress Plate tectonics is the unifying theory of geology. It explains the connections between continental drift and the formation and destructions of Earth’s crust along plate boundaries It also explains the occurrence of earthquakes, volcanos, and mountains. Scientists still have several unanswered questions.