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GEO 200 ICA 2: Plate Tectonics

The model of plate tectonics is the starting point for understanding the distribution and formation of many collections of landforms around the world. The lithospheric plates are 65 to 100 kilometers (40 to 60 miles) thick and consist of the crust and upper mantle. The plates move over the layer of the mantle known as the asthenosphere at speeds averaging from 2.4 to 10 centimeters (1 to 4 inches) per year.

Major Lithospheric Plates.

The three different kinds of plate boundaries are associated with different kinds of topographic features and tectonic activity.

Divergent Boundaries: At divergent boundaries (also called “spreading centers”) plates are moving apart. The most common kind of spreading center is the midocean ridge where new basaltic 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, producing a continental rift valley (such as the Great East African Rift Valley).

Convergent Boundaries: At convergent boundaries, where plates collide, three circumstances are possible:

1. If the edge of an oceanic plate collides with the edge of a continental plate a subduction zone is formed. The denser oceanic plate is subducted below the continent, producing an oceanic trench. As the oceanic lithosphere descends, water and other volatile materials are driven out of the ocean rocks, leading to a partial melting of the mantle. The magma that is generated rises, producing intrusions of plutonic rock such as granite and a chain of andesitic volcanoes, such as the Andes in South America or the Cascades in North America.

2. If the edge of an oceanic plate collides with the edge of another oceanic plate, subduction also takes place. An oceanic trench forms, along with a chain of andesitic volcanic islands known as an island arc, such as the Aleutian Islands in Alaska and the Mariana Islands of the western Pacific Ocean.

3. If the edge of a continent collides with the edge of another continent, the relatively buoyant continental material is not subducted. Instead, a mountain range is uplifted. The Himalayas are a dramatic example of this kind of plate boundary interaction.

Transform Boundaries: Plates slide past each other at transform boundaries, such as along the San Andreas fault system in California.

Evidence supporting the theory of plate tectonics comes from global patterns of landforms and tectonic activity. In addition to the matching shape of the continental margins on both sides of the Atlantic Ocean (which spread apart from the Mid-Atlantic Ridge), the age of the ocean floor provides evidence of movement. The ocean floors are youngest at midocean ridges, where new lithosphere is being formed, and become progressively older away from a ridge in both directions. This was verified through ocean core samples, as well as paleomagnetic evidence (changes in Earth’s magnetic field that have been recorded in the volcanic rocks of the ocean floor).

Plate boundaries are often the sites of significant volcanic activity. At spreading centers, magma is moving up to the surface, creating new lithosphere as the plates spread apart. Magma generated in subduction zones can produce a chain of continental volcanoes or a volcanic island arc.

The distribution of earthquakes also provides clues to plate activity. Most earthquakes around the world occur in association with plate boundaries. Shallow-focus earthquakes, within about 70 kilometers (45 miles) of the surface, occur at all plate boundaries. However, in subduction zones, bands of progressively deeper earthquakes are observed, produced when an oceanic plate is thrust deep down into the asthenosphere.