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17.3 Theory of Plate Tectonics Plate Boundaries • Tectonic plates interact at places called plate boundaries. • At some plate boundaries: 1. Plates come together, or converge 2. Plates move away from one another, or diverge 3. Plates move horizontally past one another Divergent Boundaries – Divergent boundaries are places where two tectonic plates are moving apart. – Most divergent boundaries are found in rifts. – A rift valley, which is a narrow depression, is created when a divergent boundary forms on a continent. Convergent Boundaries – Convergent boundaries are places where two tectonic plates are moving toward each other. – There are three types of convergent boundaries: 1. Oceanic crust converging with oceanic crust 2. Oceanic crust converging with continental crust 3. Continental crust converging and colliding with continental crust. Convergent Boundaries Convergent Boundaries – Subduction occurs when one of the two converging plates descends beneath the other. – A subduction zone forms when one oceanic plate, which has become denser as a result of cooling, descends below another plate creating a deep-sea trench. – Some of the magma forms new oceanic crust at the ridge or is forced back to the surface, forming an arc of volcanic islands that parallel the trench. Convergent Boundaries – When an oceanic plate converges with a continental plate, the denser oceanic plate is subducted. – Oceanic-continental convergence produces a trench and a series of volcanoes along the edge of the continental plate. – Two continental plates collide when an ocean basin between converging oceanic and continental plates is entirely subducted. – Because continental rocks are too buoyant to be forced into the mantle, the colliding edges of the continents are crumpled and uplifted to form a mountain range. Transform Boundaries – A transform boundary is a place where two plates slide horizontally past each other, deforming or fracturing the crust. – Transform boundaries are characterized by long faults. – The San Andreas Fault is an exception to the fact that transform boundaries rarely occur on continents. Section Assessment 1. Match the following terms with their definitions. B divergent ___ boundary E rift valley ___ A convergent ___ boundary D ___ transform boundary C ___subduction zone A. place where two tectonic plates are moving toward each other B. place where two tectonic plates are moving apart C. when one of two converging plates descends beneath the other D. place where two plates slide horizontally past each other E. result of a divergent boundary forming on a continent Section Assessment 2. What happens to an oceanic plate once it is subducted? What is created with the material? The subducted plate melts in the mantle. Some of the resulting magma is forced to the surface creating a series of volcanoes that are parallel to the subduction zone. Some of the magma is recycled into new oceanic crust at the ridge. Section Assessment 3. Why does uplift occur when two continental plates converge? Give an example of this process. The rocks that make up a continental plate are too buoyant to be forced into the mantle. As a result, the colliding edges of the continents are crumpled and uplifted to form a mountain range such as the Himalayas.