Composite volcanoes

Popocatepetl volcano

Composite volcanoes (also called stratovolcanoes) are much more explosive than shield volcanoes, the other important type of volcano.

The large and generally cone-shaped volcanoes form along plate boundaries called subduction zones where one of the Earth's plates moves below another. The sinking plate melts as it falls back into the Earth's mantle. Trapped seawater and ocean sediment is added to this molten mix, which in turn melts the overlying continental crust. These magma plumes rise to the surface and form chains of volcanoes.

Composite volcano magma contains more silica, SiO2, than that of a shield volcano and is therefore stickier (more viscous). This stickiness "plugs up" the volcano, causing pressure to build-up. The result is an explosive, dangerous eruption.

Image: Popocatepetl, an active composite volcano in Mexico (credit: Peter Menzel/SPL)

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Composite volcanoes

A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic explosive eruptions and effusive eruptions, although some have collapsed craters called calderas. The lava flowing from stratovolcanoes typically cools and hardens before spreading far due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica (as in rhyolite, dacite, or andesite), with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).

Stratovolcanoes are sometimes called "composite volcanoes" because of their composite layered structure built up from sequential outpourings of eruptive materials. They are among the most common types of volcanoes, in contrast to the less common shield volcanoes. Two famous stratovolcanoes are Krakatoa, best known for its catastrophic eruption in 1883 and Vesuvius, famous for its destruction of the towns Pompeii and Herculaneum in 79 CE. Both eruptions claimed thousands of lives. In modern times, Mount Saint Helens and Mount Pinatubo have erupted catastrophically.

Existence of stratovolcanoes has not been proved on other terrestrial bodies of the solar system with one exception. Their existence was suggested for some isolated massifs on Mars, e.g., Zephyria Tholus.

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