why do volcanoes form at subduction zones

The Earth recycles itself! Some of the most spectacular volcanoes on Earth are associated with subduction zones! Right: The upper picture was taken at Crater Lake in 1941. Left: Mt. Ranier in 1914. Mt. Mazama, the volcano that erupted to form Crater used to look a lot like Mt. Ranier, however when it erupted, the top collapsed in on itself and filled with water over time to produce the lake we know today. The small cone at Crater Lake is a
called Wizard Island. (National Park Service) A subduction zone forms when and collide. The continental crust is thicker and more buoyant than the oceanic crust so the oceanic crust subducts beneath the continental crust. As the plate sinks deeper, it can reach depths of 50 to 100 miles (80-160 kilometers) were it is so hot that the crust releases fluids trapped inside. The fluid melts some of the -rich minerals in the overlying material producing dark, silica-poor basaltic. The basaltic melt migrates upwards and becomes more silica-rich it melts its way toward the surface.


Sticky, silica-rich magma erupts at the surface forming steep-sided volcanoes. Where plates converge, the thin, dense oceanic crust sinks beneath the thick, buoyant continental crust. Volcanoes form where the subducting oceanic plate gets hot enough to БsweatБ fluids and initiate melting. (Modified from Lillie, 2005. ) Subduction zones produce volcanic arcs, curving chains of steep-sided volcanoes, for example the Aleutian Islands in Alaska. Volcanoes associated with subduction zones generally have steep sides and erupt explosively. Why are the subduction zone volcanoes so steep? The lavas erupted there are rich in silica. Silica acts as a thickening agent (like flour in gravy) so silica-rich magmas are thick and pasty. When the magma erupts, the lava is so thick that it canБt flow very far. Instead, it sticks to the sides of the volcano forming a tall, steep-sided cone.


Sometimes these volcanoes explode (for example, Crater Lake, Mt. St. Helens, Lassen, and Mt. Ranier). Thick silica rich magma does not release the gasses very easily so they build up inside the. As the gasses collect in the magma chamber, the pressure rises. It is possible for the pressure to rise so much that the chamber cannot contain the magma and it erupts explosively. Crater Lake National Park, OR Lassen Volcanic National Park, CA Mount Ranier National Park, WA Mount St. Helens National Volcanic Monument, WA Anickchak National Monument, AK Katamai National Park, AK Lake Clark National Park, AK Other national parks lie along ancient subduction zones. They used to contain volcanoes long ago, but over time the tall volcanoes were eroded away. Parks that show remnants of ancient volcanism are (these will be links to individual park pages): Kings Canyon National Park, CA Yosemite National Park, CA Sequoia National Park, CA 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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