Subduction

Oceanic-continental convergence zone
Oceanic-continental convergence zone
Image Credit: MBG
Oceanic-oceanic convergence zone
Oceanic-oceanic convergence zone
Image Credit: MBG
Cayambe Stratovolcano (Ecuador)
Cayambe Stratovolcano (Ecuador)
Image Credit: MBG
Illinizas Stratovolcano (Ecuador)
Illinizas Stratovolcano (Ecuador)
Image Credit: MBG

At oceanic spreading centers, new lithosphere is continually being generated. However, since the total surface area of Earth remains constant, lithosphere must also be destroyed elsewhere. This takes place at zones of plate convergence. There, usually the older, colder, and therefore denser plate sinks below another plate. Continental plates mostly consists of lighter material and can at most be partially subducted. The geodynamic process of one plate sliding beneath another occurs along deep-sea trenches and is called subduction.

The enormous forces acting during subduction are periodically expressed by great earthquakes. Volcanic and sedimentary material scraped off the descending oceanic plate accumulates along the leading edge of the overriding plate as an accretionary prism and is eventually incorporated into the adjacent plate margin as a microplate or accreted terrane.

Futhermore, when the subducting oceanic plate reaches a depth of about 100 kilometers, partial melting of the water-rich oceanic crust and some of the overlying mantle takes place. The newly formed magma created in this manner is less dense than the surrounding mantle rocks and will consequently slowly rise when sufficient quantities of molten rock  have gathered. Most of the rising magma will be emplaced in the overlying continental crust where it will cool and crystallize at a depth of several kilometers. The remaining magma will eventually migrate to the surface where it can give rise to numerous and occasionally explosive volcanic eruptions along a volcanic arc. A volcanic arc is usually formed by a chain of volcanoes, paralleling trenches approximately 100 km inland. When two oceanic plates collide, the older and therefore heavier of the two usually subducts beneath the other, initiating volcanic activity in a manner similar to that which occurs at an oceanic-continental convergent plate boundary. As two continents approach each, other the intervening sea-floor is subducted causing arc-type volcanism.