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There are three kinds of plate boundaries: convergent, divergent, and transform. Volcanoes and earthquakes can happen at each type of plate boundary. This week of the course will focus on volcanoes and mountains that result from the interactions between plates and how they, in turn, affect the other spheres of Earth. The following is a brief explanation of each type of plate boundary:

Convergent
There are several kinds of convergent plate boundaries.

1. Oceanic-Continental Convergence

In an oceanic - continental boundary the oceanic plate moves under the continental plate, forming volcanic mountain chains. For example: the Nazca Plate being subducted beneath the South American Plate to form the Andes Mountains. The Cascade Mountain range in the northwestern United States was also formed in this way, by subduction of an oceanic plate beneath a continental plate.

2. Oceanic-Oceanic Convergence

At an oceanic-oceanic convergent plate boundary, one oceanic plate is subducted beneath another oceanic plate. Deep ocean trenches are associated with this type of convergent plate boundary. For example: the Mariana Plate being subducted beneath the Pacific Plate, forming the Mariana Trench in the Pacific ocean. Volcanic islands can also be result of oceanic-oceanic boundaries. An example of this is the Aleutian Islands in the northern Pacific Ocean, off the coast of Alaska.

3. Continental-Continental Convergence

A continental-continental convergent plate boundary is created when two continental plates collide. Continental-continental boundaries cause mountains to form. For example: the Indian Plate colliding with the Eurasian Plate, forming the Himalayan Mountains.

Divergent
Divergent boundaries are areas in which plates are moving away from each other. For example, divergent boundaries can be found in the mid-Atlantic ocean ridge, where magma pushes the plates apart to form new ocean crust.

Transform
Transform boundaries are areas in which two plates slide past each other; neither plate moves over or under the other. The San Andreas fault is an example of a transform boundary.




Source
http://pubs.usgs.gov/gip/dynamic/understanding.html (public domain)