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Types of Plate Boundaries

 
Convergent Boundaries
Because the amount of crust on the surface of the earth remains constant, when a divergence (or separation) occurs in one area, a convergence (or collision) must occur in another area. There are 3 types of converging plate boundaries.
When an oceanic-continental convergence occurs the denser oceanic plate will most commonly subduct beneath the less dense continental plate creating a trench. Once the subducting oceanic crust reaches a depth of about 100 kilometers, melting of the crust and the overlying mantle begins forming a magma. Some of this magma is pushed to the surface resulting in volcanic eruptions.

Earthquakes can also be caused by the collision of oceanic and continental plates. In the Philippines, the Java trench is associated with volcanic islands as well as earthquakes. This type of earthquake can be shallow, intermediate, or deep, according to its location on the downgoing lithospheric slab. Further, the movement of magma in subduction zones can also trigger deep earthquakes.
An oceanic-oceanic convergence often results in the formation of an island arc system. As the subducting oceanic crust melts within the asthenosphere the newly-created magma rises to the surface and forms volcanoes. If the activity continues, the volcano may grow tall enough to create an island.
A continental-continental convergence generally does not involve subduction.  Instead, the two plates squeeze and deform each other, resulting in a new mountain range, such as the band crossing the Himalayas. In this zone, shallow earthquakes are associated with high mountain ranges where intense compression is taking place. Intermediate- and deep-focus earthquakes also occur.

 
Divergent Boundaries
When a divergence occurs within a continent it is called rifting, when a divergence occurs in an ocean plate it is called seafloor spreading. Molten rock from the asthenosphere rises to the surface, forcing the plate to break and separate into two plates. Midocean ridges occur at divergent boundaires. Seimic activity is low, and it occurs at very shallow depths. The lithosphere is very thin andweak at these boundaries, so the strain cannot build up enough to cause large earthquakes.

 
Transform Boundaries
A transform boundary, or transform fault, involves plates sliding past one another without construction or destruction of crust. The direction of these transform faults can change. However, they generally move parallel to the general direction of the plates.
A good example of a transform boundary is the San Andreas fault in California. The type of earthquake associated with this kind of boundary is the shallow-focus event unaccompanied by volcanic activity. In these faults, two mature plates are scraping by one another. The friction between the plates can be so great that very large strains can build up before they are periodically relieved by large earthquakes.

URL for CVO HomePage is: <http://vulcan.wr.usgs.gov/home.html>                                                                                                  12/21/01, Lyn Topinka