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)
Mount St Helens helped scientists understand these powerful events.
The 1980 eruption of Mount St Helens and El Chichon two years later were the first times that pyroclastic flows were studied using modern scientific methods, as volcanologist Haraldur Sigurdsson explains in this clip. These devastating flows of hot ash and rock speed down the slopes of volcanoes destroying everything in their path.
Find out how the people of Herculaneum and Pompeii died.
The people of the ancient Roman towns of Herculaneum and Pompeii died when Mount Vesuvius erupted in AD79 and pyroclastic flows of hot gas, ash and rock burnt and suffocated them. In modern times, archaeologists have found cavities in the ash left by their bodies and filled them with plaster to show how they fell.
Iain Stewart tells the story of Mount St Helens' 1980 eruption.
Professor Iain Stewart tells the story of Mount St Helens' 1980 eruption. An earthquake-triggered landslide unleashed a sideways blast of hot gas and ash from this famous stratovolcano volcano (also known as a composite volcano) in the state of Washington, USA.
In 1815 an Indonesian volcano killed an estimated 200,000 people worldwide.
Dr Iain Stewart visits Mount Tambora, site of the largest known volcanic eruption of the last millennium, on the island of Sumbawa, Indonesia. In 1815 Tambora blasted 50 cubic km of rock high into the atmosphere, leaving an 8km-wide caldera. Falling as ash, the ejected rock stopped crops from photosynthesising. Volcanic debris in the upper atmosphere reflected sunlight back into space and caused global cooling. An estimated 200,000 people worldwide were killed by the eruption itself and famines caused by widespread crop failures.
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 AD. Both eruptions claimed thousands of lives.