why do volcanoes form at plate boundaries and hotspots

What is a volcano? A volcano is an opening in Earth s crust through which magma rises. Magma is called lava once it reaches the Earth s surface. Volcanoes form on land, in the ocean, and at the center of islands, but are most commonly formed in the ocean. A number of factors contribute to the formation of a volcano. How do volcanoes form? All volcanoes begin as magma. The magma forms when rock material melts, or becomes molten, close to where the mantle and crust meet. Molten material contains gases. Under intense pressure, the gases remain dissolved. As the molten material rises to Earth s surface, the pressure decreases.

When this happens, the gases separate from the liquid magma. This process is similar to opening a can of soda. When the cap is removed, gases escape the can of soda. Gases in the magma include water vapor, carbon dioxide, sulfur, chlorine, and fluorine. As these gases expand, they produce an explosion eruption of lava at Earth s surface. Volcanoes at Divergent Boundaries Most volcanic activity occurs in the ocean along divergent plate boundaries. Magma rises from the Earth s mantle, moving oceanic plates apart. When the magma cools, it turns into new crust. This process creates mid-ocean ridges, which are under-water mountain ranges.
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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