Stratovolcano

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Stratovolcano

Mount St. Helens — a stratovolcano in the U.S. state of Washington — the day before the May 18, 1980 eruption that removed much of the top of the mountain

Mt. Rainier, a stratovolcano in Washington.

Mt. Kazbek in Georgia, a dormant stratovolcano in the Caucasus

Popocatépetl, an active stratovolcano in Mexico

A stratovolcano is a tall, conical volcano composed of many layers of hardened lava, tephra, and volcanic ash. These volcanoes are characterised by a steep profile and periodic, explosive eruptions. The lava that flows from them is highly viscous, and cools and hardens before spreading very far. The source magma of this rock is classified as acidic, or high in silica to intermediate (rhyolite, dacite, or andesite). This is in contrast to less viscous basic magma that forms shield volcanoes (such as Mauna Loa in Hawaii), which have a wide base and more gently sloping profile. Many stratovolcanoes exceed a height of 2500 m.

Although stratovolcanoes are sometimes called composite volcanoes, volcanologists prefer to use the term stratovolcano to distinguish among volcanoes because all volcanoes of any size have a composite (layered) structure — that is, are built up from sequential outpourings of eruptive materials.

[edit] Formation

Stratovolcanoes are a common feature of subduction zones, tectonic plate boundaries where oceanic crust is subsumed under continental crust. The magma that forms stratovolcanoes arises when water, which is trapped both in hydrated minerals and in the porous basalt rock of the upper ocenic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering," and occurs at specific pressure/temperature conditions for specific minerals as the plate subducts to lower depths. The water freed from the subducting slab lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its lower density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. The magma then rises through the crust, incorporating silica rich crustal rock, leading to a final intermediate composition. When the magma nears the surface it pools in a magma chamber under the volocano. The relatively low pressure of the magma allows water and other volatiles (CO2, S, Cl) dissolved in the magma to begin to come out of solution, much like when a bottle of soda is opened. Once a critical volume of magma and gas accumulates, the obstacle provided by the volcanic cone is overcome, leading to a sudden explosive eruption.