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Plate Tectonics
In the early 1960s, the theory of plate tectonics took shape. According to this theory, the outermost layer of the Earth's crust is comprised of large, rigid slabs of rock. These slabs are fragmented into approximately a dozen plates. Furthermore, these plates move relative to one another, a process known as continental drift. This motion creates four types of plate boundaries: divergent boundaries, convergent boundaries, transformational boundaries and plate boundary zones. Convergent boundaries occur where two or more plates are colliding together. There are three types of convergent zones: oceanic plate verses oceanic plate, oceanic plate verses continental plate and continental plate verses continental plate.
Ocean Plate Verses Continental Plate
The first type of convergent zone occurs where an oceanic plate is colliding with a continental plate. Oceanic plates are very dense and force their way underneath the buoyant, continental plates. A deep trench forms where the two plates converge. As the oceanic plate plunges downward, it enters the asthenosphere. Here, huge forces cause this subducting plate to release trapped water and gases. As these work their way upward, they cause a chain of chemical reactions, melting the mantle above. The melting mantle pools into magma chambers and, when it reaches the surface, creates volcanic mountain ranges. This type of convergent boundary is known for producing extremely powerful earthquakes. An example of this type of zone is the Northwest coast of the United States, where the Juan de Fuca plate is forcing its way under the North American plate. The San Andreas Fault and Mount Saint Helens are a product of this convergence.
Ocean Plate Verses Ocean Plate
Oceanic plates are born in midocean ridges, where molten rock rises, cools and solidifies. This causes oceanic plates to slowly expand, cooling as they move away from the ridgeline. The oldest plate will be the coolest and densest and will subduct under the newer oceanic plate. The outcome of this collision is the same, with the volcanic mountain range becoming a chain of islands. The Aleutian Peninsula of Alaska is an example of this type of convergent boundary.
Continental Plate Verses Continental Plate
When equally buoyant continental plates collide, neither is able to force its way under the other. Instead of a subduction zone, this type of convergent zone crushes the two plates together under tremendous pressure. The result is the formation of very high mountains. The tallest mountains in the world, the Himalayas, are an example of this type of convergent boundary. In fact, the pressure in this convergent boundary is so great, it is forcing a large piece of the Asian plate to shear sideways.