why do volcanoes form at convergent boundaries

A volcanic arc is formed on the continental plate, above the location of the downgoing oceanic slab. The volcanic arc is the surface expression of the magma that is generated by hydrous melting of the mantle above the downgoing slab. Hydrated minerals (e. g. , phlogopite, lawsonite, amphibole) within the oceanic lithosphere become unstable at certain depths due to increased temperature and pressure, causing the crystal structure of the hydrated minerals break down and release water. The buoyant fluids then rise into the asthenosphere, where they lower the melting temperature of the mantle and cause partial melting.

These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases. When two plates with oceanic crust converge, they typically create an
as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep is located in front of such arcs where the descending slab dips downward, such as the near the.

Hot spots, Divergent plate boundaries (such as rifts and mid-ocean ridges), and The origin of the for hot spots is not well known. We do know that the magma comes from partial melting within the upper, probably from depths not too much greater than 100 km. The actual source of the heat that causes the partial melting (the actual hotspot itself) is almost certainly much deeper than that, but we really don't know how deep or even exactly what a hotspot is! At a divergent margin, two tectonic plates are moving apart, and magma that is generated in the upper mantle flows upward to fill in the space.

This magma is probably generated at depths that are shallower than those for hotspot magmas. People argue about whether the magma forcing its way to the surface causes the plates to move apart or whether the plates move apart and the magma just reacts to that and fills in the space. Perhaps it is a combination of these two. The most extensive example of this type of volcanism is the system of mid-ocean ridges. Continental examples include the East African Rift, the West Antarctic Rift, and the Basin and Range Province in the southwestern US. The final major place where volcanism originates is at convergent margins (subduction zones)--where an oceanic plate dives under either another oceanic plate or perhaps a continental plate.

As the plate gets pushed further and further it starts to give off its volatiles (mostly water), and these migrate upwards into the mantle just under the overriding plate. The addition of these volatiles to this overriding mantle probably lowers the melting point of that mantle so that magma is generated. Part of the magma may also be generated by the downgoing plate actually starting to melt as it gets into the hotter and hotter interior.

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