Plate Tectonics Exercise (20 points)
PLATE BOUNDARIES There are three different types of lithosphere plate boundaries associated with different types of topographic features and tectonic activity. These are: Divergent boundaries (also called “spreading centers). At divergent boundaries, plates are moving apart. The most common kind of spreading center is the midocean ridge where new ocean floor is created. Spreading may also take place within a continent. In this case, blocks of crust may drop down as the land is pulled apart, creating a rift valley. Convergent boundaries These boundaries involve collision of lithosphere plates, which may occur in three different circumstances. 1. Oceanic – continental plate convergence. If the edge of an oceanic plate collides with the edge of a continental plate a subduction zone is formed. The denser oceanic plate subducts (plunges) beneath the continent, producing a deep oceanic trench. As the oceanic lithosphere descends, partial melting of the rock along the contact zone occurs. The magma that is produced rises toward the surface and produces a chain of volcanic mountains on the land mass parallel to the collision boundary, such as the Andes in South America. 2. Oceanic – oceanic plate convergence. In this case, subduction also takes place. A deep ocean trench forms along with a chain of volcanic islands. An example of such an island arc is the Aleutian Islands in Alaska. 3. Continental – continental plate convergence. If the edge of a continent collides with the edge of another large land mass, subduction does not take place because of the low density of the continental crust. Instead, a non-volcanic mountain range is uplifted. An example of such a mountain range is the Himalayas in Asia. Transform boundaries Plates slide past one another at
