Volcanoes
Volcanoes are dramatic evidence of the powerful forces at work inside the Earth. Eruptions of ash, gas and lava destroy entire cities and kill large numbers of people.
Volcanoes also add nutrients to soils, creating perfect conditions for many crops. Some types of volcano make new sections of the tectonic plates that make up the surface of the Earth. Without volcanoes and our planet's plates, the dry land we live on would not be renewed, and weathering and erosion by water, wind and ice would eventually carry it all into the oceans leaving Earth a water world.
There are three common types of volcano: composite volcanoes, often the most deadly; shield volcanoes, which are large but generally less violent; and cinder cones.
Image: The Indonesian volcano Anak Krakatau erupts at night (credit: Getty Images/Tom Pfeiffer/VolcanoDiscovery)
Introduction
Volcanoes
TV clips (13)
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Mount Tambora
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.
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Mount Kilauea
Lava flows from this spectacular volcano have built the island of Hawaii.
Professor Iain Stewart explains how Mount Kilauea's eruptions of lava have built up the island of Hawaii over millions of years as a magma plume known as a hotspot rises up through the Earth's crust.
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Iceland's volcanoes
Iceland's volcanism is linked to its position on the Mid-Atlantic Ridge.
Professor Iain Stewart discusses Iceland's volcanoes, including the massive eruption that created the island of Surtsey in 1963. Iceland's position on the Mid-Atlantic Ridge makes it a volcanically active place.
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Silent killer
In 1986 mysterious deaths in Cameroon were evenutally linked to a volcanic crater lake.
Professor Iain Stewart recounts the story of hundreds of mysterious deaths that occurred on 21 August 1986 in villages near Lake Nyos, Cameroon. Lake Nyos sits in the crater of an inactive volcano. It was later found that a landslide disturbed volcanic gas - carbon dioxide - that had dissolved in water at the bottom of the lake and sent it down the slopes of the volcano. Approximately 1,700 people in nearby villages were asphyxiated in and around their homes. The event at Lake Nyos is a rare type of natural disaster called a limnic eruption or lake overturn.
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Giant volcanoes on Mars
Why does Mars have volcanoes that are so much bigger than Earth's?
Professor Iain Stewart looks at some of the reasons why Martian volcanoes such as Olympus Mons are so much bigger than Earth's.
Volcanoes
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface.
Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust in the interiors of plates, e.g., in the East African Rift, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "Plate hypothesis" volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core-mantle boundary, 3,000 km deep in the Earth.
Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. Volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Historically, so-called volcanic winters have caused catastrophic famines.
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