Which plate is subducted

The resulting minerals are denser and they don't contain the bonded water. This metamorphic dewatering process liberates water from the descending crust. The water gradually seeps upward into the overlying wedge of hot mantle. The addition of water to the already hot mantle rocks lowers their melting temperature resulting in partial melting of ultramafic mantle rocks to yield mafic magma.

Melting aided by the addition of water or other fluid is called flux melting. It is somewhat more complicated than this, but metamorphic dewatering of suducting crust and flux melting of the mantle wedge appears to account for most of the magma at subduction zones. Magma formed above a subducting plate slowly rise into the overriding crust and finally to the surface forming a volcanic arc , a chain of active volcanoes which parallels the deep ocean trench.

Beneath the active volcanic arc lie intrusive igneous rocks formed from magma that didn't make it all the way to the surface before crystallizing. As a result, only oceanic plates are subducted.

When an oceanic plate collides with a continental plate, the denser oceanic plate is bent downward and slides under the edge of the continent. A trench forms on the seafloor above the bend. Trenches are the deepest places on the surface of the earth, dropping off as much as 10 kilometers below the rest of the seafloor.

The leading edge of the continent is buckled and compressed by the force of continuing impact. A folded mountain belt rises parallel to the plate boundary, a belt with deep roots that extend down into the upper mantle. As the mountains rise, they shed enormous amounts of rocky debris, which rolls off the continent and into the trench.

This is joined by seafloor mud and ooze scraped off the descending plate creating a thick organic-rich sediment wedge. Far beneath the mountains, the descending plate has carried seawater down with it.

As the plate heats up from friction and from exposure to the hot mantle, the water is expelled. The presence of water lowers the melting point of the surrounding rocks and magma begins to form.

The magma rises up into the plate above melting and incorporating bits of the continent and becoming more and more silica-rich. Most of the magma cools and solidifies within the continent forming huge granite masses called plutons. Some magma eventually reaches the surface and erupts. As the continental margin crumples as magma pushes into volcanoes, and as the seafloor is driven back into the mantle, both plates are shaken by earthquakes again and again. The eventual fate of the oceanic plate is unclear.

Some studies suggest it melts away into the upper mantle, while others indicate it sinks nearly whole all the way down to the core. Subduction also occurs where two oceanic plates converge, with many of the same results.

When collision begins, the denser plate is forced downward. Because oceanic crust becomes denser with age, the plate that has the oldest rock on its leading edge is the one that is subducted. Above the rim of the descending slab, a deep trench forms, and sediments begin to accumulate within it. Multimedia Gallery. Park Passes. Technical Announcements. Employees in the News. Emergency Management. Survey Manual. These plates collide, slide past, and move apart from each other.

Where they collide and one plate is thrust beneath another a subduction zone , the most powerful earthquakes, tsunamis, volcanic eruptions, and landslides occur.