The theory of plate tectonics was developed in the 1960s and 1970s, and transformed our understanding of geology. According to the theory, Earth's crust is broken into pieces, or plates, that are constantly in motion. Three primary types of plate tectonic boundaries exist: transform boundaries, divergent boundaries and convergent boundaries. The nature of convergent boundaries varies depending on whether they occur in the ocean floor or continental floor.
Transform Boundaries
Transform boundaries, known more commonly as faults, occur where two plates slide horizontally past each other. Most faults occur along the ocean floor and produce shallow earthquakes. However, some faults--such as the San Andreas fault in California--occur on land. Earthquakes along the San Andreas fault result from the Pacific Plate sliding past the North American Plate, which it has been doing for 10 million years at a rate of several inches per year.
Divergent Boundaries
At divergent boundaries, neighboring plates pull away from each other. This produces new crust, which expands the ocean floors or creates a rift zone on land. Diverging landmasses will eventually become distinct, as the divide between them fills with water. For this reason, Iceland will someday be split down the middle into two islands. The Mid-Atlantic Ridge, between South America and Africa, is another example of diverging plates.
Convergent Boundaries: Oceanic Convergence
Oceanic convergence can occur where two oceanic plates meet, or when an oceanic plate collides with a continental plate. A deep oceanic trench can develop when two oceanic plates collide, as one plate tends to get subducted under the other. Ocean volcanoes also form at oceanic-oceanic convergent boundaries, collecting lava and debris until they grow out of the water and become island volcanoes. This process often forms island arcs that are subject to frequent, strong earthquakes. When an oceanic plate converges on a continental plate, the oceanic plate generally gets pushed under the continental plate, creating mountain ranges, generating powerful earthquakes and breaking the subducted oceanic plate into pieces.
Convergent Boundaries: Continental Convergence
Continental rocks are a relatively light part of the Earth's crust. Thus, when two continental plates meet, the surface gets pushed upward. Unlike convergence with oceanic plates, neither plate will get pushed under the other; instead, the convergence is like icebergs colliding. For this reason, earthquakes are common, but volcanism is comparatively rare. The Himalayas resulted from the convergence of the Eurasian Plate and the Indian Plate.
Tags: oceanic plate, continental plate, oceanic plates, Andreas fault, Convergent Boundaries, convergent boundaries
