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Stratovolcano

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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

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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 one layer 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. or basalt). </s> 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.

Stratovolcanoes are a by-product of the subduction of tectonic plates. The source melts of these volcanoes arise as a result of the dewatering of oceanic crust at specific pressure/temperature conditions as the plate subducts to lower depths. The water freed from the subducting slab allows partial melting of the overlying mantle rock, which rises due to its lower density relative to the surrounding mantle rock, and pools at the base of the crust. The melt is initially basic in composition. As it rises through the crust, it incorporates more acidic adjacent crustal rock, leading to a final intermediate composition. The water remains in the melt and upon eruption contributes to the explosive power of these volcanoes as superheated steam.

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.