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Volcanoes: Magma
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Volcanoes are often formed where two plates of crust decide that they don't love each other anymore, and move apart. They can ... be formed when the two plates get into a fight, and charge at each other with blind ferocity (much like angry sumo wrestlers), often as quickly as two inches-per-year. If the new bulge of ground that they form is unstable, perhaps due to low self-esteem or bad grades in school, it can erupt its vast reservoir of emotions, or "magma." When this happens, a volcano has formed. Some will continue to erupt sporadically for the duration of their lifetimes. Others, meanwhile, will simply wonder what the hell they were thinking and move on.
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Not all volcanoes show a graceful, symmetrical cone shape, such as that exemplified by Mount Fuji, Japan. Most volcanoes, especially those near tectonic plate boundaries, are more irregular, though of grossly conical shape. Such volcanoes, called stratovolcanoes or composite volcanoes, typically erupt explosively and are composed dominantly of andesitic, relatively viscous and short lava flows, interlayered with beds of ash and cinder that thin away from the principal vents. Volcanoes constructed primarily of fluid basaltic lava flows, which may spread great distances from the vents, typically are gentle-sloped, broadly upward convex structures. Such shield volcanoes, classic examples of which are Mauna Loa volcano, Hawaii, tend to form in oceanic intraplate regions and are associated with hot-spot volcanism. The shape and size of a volcano can vary widely between the simple forms of composite and shield volcanoes, depending on magma viscosity, eruptive style (explosive versus nonexplosive), migration of vent locations, duration and complexity of eruptive history, and posteruption modifications.
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Composite volcanoes are made from alternate layers of lava flows and exploded rock. Their height ranges from 100 to 3,500 meters tall. The chemistry of the magma of these volcanoes is quite variable ranging from basalt to granite. Magmas that are more granitic tend to be very explosive because of their relatively higher water content. Water at high temperatures and pressures is extremely volatile. Examples of composite volcanoes include Italy's Vesuvius, Japan's Mount Fuji, and Washington State's Mount Rainier and Mount St. Helens (Figures 5).
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Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes, except when a mud volcano is actually a vent of an igneous volcano.
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