Volcanoes Volcanoes arise as a result of buoyancy, coupled with fluid mechanics as a result of the effects of pressure deep underneath the earth. Volcanoes can largely be separated into two groups, mafic and felsic volcanoes. The causes of these are very different. Felsic volcanoes typically arise near subduction zones. This is due to the action of oceanic plates sinking into the crust. Oceanic plates at subduction zones are usually old, and have a buildup of felsic sediments on top of them that were deposited from the ocean. When the plate subducts, some of the felsic sediments are dragged down along with the mafic oceanic plate, even though they are as dense as the rock they are sinking into. Eventually, the felsic rocks melt and rise up through the crust, because the liquid rock is less dense than solid rock. As the magma rises through the crust, its pressure decreases, which lowers the melting point of the magma further, helping it to remain a liquid. This is referred to as decompression melting. When it reaches the surface, it forms a volcano. Typically these volcanoes are found in chains along coastlines, such as the Aleutian islands. Graph depicting the melting point of ultramafic material. As pressure decreases, the metling point also decreases, helping magma to maintain buoyancy. Since the felsic sediments in subduction come from the ocean, they are very high in water content, and the resulting magma is as well. When this magma reaches the surface, the magma is depressurized to atmospheric conditions, and the water rapidly boils off into steam. The water releases a huge amount of work through this phase change, leading to the catastrophically explosive nature of many felsic volcanoes such as Mt St Helens.      The cause of mafic volcanoes is much more poorly understood than felsic volcanoes, and there are more types. One type is the shield volcano, such as those found in Hawaii. Shield volcanoes are thought to arise as a result of a “hot spot” in the mantle, where magma is much hotter than normal. Due to the decrease in density caused by heating, the mafic material of the mantle rises up and eventually reaches the surface. An even more poorly understood form of mafic volcanoes are flood basalt. These involve large, rapid eruptions of huge volumes of basalt that cover a large area, such as in the Columbia River Basalt Group in Washington. (West)