Online Encyclopedia
Subduction
Subduction zones exist where an oceanic plate collides with a continental plate and sinks below the latter plate into the asthenosphere. Oceanic lithospheric plates are subducted because of their higher density. The oceanic plate is melted down into magma and once again becomes part of the earth's mantle.
Subduction results in creation of oceanic trenches, such as the Mariana trench, and mountain building. Volcanoes that occur along these boundaries, such as Mt. Saint Helens and Krakatau, are called subduction zone volcanoes. There are more volcanoes associated with subduction zones than any other type of tectonic boundary.
When many volcanoes form along a subduction fault, they are called volcanic arcs.
When two oceanic plates collide, the younger of the two plates, being less dense, will ride over the edge of the older plate. Oceanic plates grow more dense as they cool and move further away from the Mid-oceanic ridge. When oceanic plate collides with continental plate, the oceanic plate is subducted, creating regions such as the Cascadia subduction zone of northwestern North America.
Subduction zones are also notorious for producing earthquakes because of the intense geological activity. In some cases these earthquakes are of such a high magnitude that they produce tsunamis such as the ones that devistated the areas around the Indian Ocean on December 26, 2004. Small tremors that create tiny, unnoticable tsunamis happen all the time because of the dynamics of the earth.