A cross-section through an erupting volcano A volcano—often, but not always, a cone-shaped mountain—is an opening in the Earth’s crust through which molten rock, called magma erupts. When a volcano erupts and hurls out its red-hot rock, this is one of the most awesome events of nature. It happens at a hole, crack or weak point in the solid rocks of the Earth’s crust. Melted rock called magma from deep below forces its way up under incredible temperature and pressure. As it emerges it is called lava. When it cools and hardens, it forms a type of rock known as igneous rock.
In a volcanic eruption, magma is blasted out of the volcano’s crater. If the magma is thick and pasty, the gas trapped inside it cannot escape, so it builds up and up until it explodes. The eruption will be a very violent one. The erupted magma, called lava, is shattered into pumice, fragments of rock once full of gas bubbles, and ash, lava blown to powder by the force of the explosion. The pumice and ash form a huge cloud. In explosive eruptions, the thick lava moves slowly and hardens close to the volcano’s vent or crater. As this type of volcano erupts time after time, the lava builds up in layers to form a cone-shaped, steep-sided mountain known as a stratovolcano.
In many volcanoes, eruptions are less violent than those of explosive volcanoes. This is because the lava is thin and runny: the gas inside it can escape more easily. Lava oozes like boiling syrup from the volcano, flows down the gentle slopes and spreads over a wide area. As it cools, it turns into a “shield” of solid rock known as basalt. Each time the volcano erupts, it adds to the shield in layers of lava up to 10 metres (more than 30 feet) thick. These are known as shield volcanoes.
The largest shield volcano chain in the world is the Hawaiian Islands, a chain of hot-spot volcanoes in the Pacific Ocean. There are also shield volcanoes in Iceland and in the Great Rift Valley of East Africa.
A diagram of a shield volcano
Volcanic bombs are blasted out during an eruption
Ash and bombs
The temperature of lava erupting from a volcano can be more than 1000°C (1800°F). Volcanoes eject other substances, too: gases and fumes rich in sulphur. Some give out clouds of ash that fly high in the air. The ash may fall near the volcano and build up in thick layers around it. Some volcanoes have such explosive power that they blast out huge lumps of molten rock as big as houses. These volcanic bombs crash to the ground nearby. The ash is often blown away by the wind and may fall over a very wide area.
Capulin Volcano cinder cone, New Mexico
Cinder cones are steep, conical hills made of loose fragments of volcanic rock. They are formed by explosive eruptions or lava fountains from a single vent. As the lava is blasted violently into the air, it breaks into small fragments that fall around the vent to form a cone that is often symmetrical in shape—slopes of between 30 and 40° and with a nearly circular base. Most cones have a single bowl-shaped crater at the summit.
Cinder cones are often found on the flanks of other larger, volcanoes. There are, for example, about 100 cinder cones on the slopes of Mauna Kea, a shield volcano on the island of Hawaii.
The most famous cinder cone, Paricutin, grew out of a cornfield in Mexico in 1943 in a matter of hours. Eruptions continued for nine years, with the cone eventually building to a height of 424 metres (1391 feet).
Interior of a cinder cone
The Earth's volcanoes
Most volcanoes are situated along the edges of the giant, jigsaw-like tectonic plates that make up the Earth’s surface. The boundaries between plates have many weak points. In particular, volcanoes form along subduction zones where one plate slides down beneath another. As the lower plate melts back into the mantle, its gases and lighter molten rock “boil” and force their way up through cracks with enormous pressure, causing eruptions.
More than half of the world's active volcanoes above sea level encircle the Pacific Ocean, which is nearly surrounded by subduction zones, to form the so-called "Ring of Fire".
A map of volcanoes (red triangles)
The typical cone-shaped mountains on land are what we think of as volcanoes. But they make up less than one-hundredth of all the volcanic activity on Earth. Most magma oozes to the surface deep under water, along the crack-like fissures of the Mid-Oceanic Ridge or through smaller weak “holes” known as hot spots. If underwater volcanoes build cones tall enough they emerge at the surface as islands, such as the Hawaiian Islands in the Pacific and the Canary Islands in the Atlantic. New islands are emerging all the time: one has been recently forming in the Red Sea off the coast of Yemen.
When a volcano has been known to erupt in the last few hundred years, it is said to be active. An active volcano regularly erupts lava, ash, fumes and other materials. In very active volcanoes, this happens almost continuously. In others, there are weeks or months between eruptions. There are at least 1500 active volcanoes above sea level around the world, with possibly more than 10,000 active volcanoes under the oceans.
A view of Mount St Helens two years after it eruptedWhen a volcano has not erupted for many years or centuries, but still might in the future, it is dormant (sleeping). Mount St Helens in the USA, for example, was a dormant volcano that came back to life spectacularly in 1980. It is often difficult to distinguish an extinct volcano from a dormant one. Fourpeaked Mountain in Alaska, for example, had not erupted since before 8000 BC and had long been thought to be extinct before it burst into life in September 2006.
Arthur's Seat, EdinburghWhen there have been no eruptions for tens of thousands of years the volcano is described as extinct. Arthur's Seat in Edinburgh, Scotland, is an example of an extinct volcano. It is part of an extinct volcano system from the Carboniferous Period (approximately 350 million years old).
Volcanoes: 10 deadly dangers
- 1 Blasted by the explosion
- 2 Being buried by mudslides
- 3 Being struck by lava bombs, chunks of molten rock ejected during an eruption and solidifying before they reach the ground
- 4 Falling ash making it hard to breathe
- 5 Inhaling poisonous gases
- 6 Buildings collapsing under the weight of falling ash
- 7 Ash making crops inedible, leading to famine
- 8 Dust causing pneumonia and illnesses
- 9 Burned by fires caused by lava flows
- 10 Being engulfed by a pyroclastic flow, the extremely hot mixture of volcanic fragments and gases, sweeping down the volcano's slopes at speeds of more than 300 km/h and destroying everything in their path (the unfortunate residents of Pompeii perished in this way when Vesuvius erupted in AD 79)
An explosive eruption also produces tonnes of hot gas, dust, ash and steam in a glowing cloud called a pyroclastic flow (the word “pyroclastic” means “shattered by fire”). They happen when the great cloud of ash and pumice that blasts into the sky after a volcano erupts collapses back down to Earth. The cloud surges down the side of the volcano at high speed as an avalanche of red-hot rock particles and gas, destroying everything in its path.
Scientists studying a recent volcanic eruption (Mount St Helens, United States, in 1980) discovered that a pyroclastic flow could travel at speeds of up to 300 km/h (about 200 mph). There is strong evidence that pyroclastic flows surged down the slopes of Vesuvius and destroyed the city of Pompeii in AD 79.
Vines growing in fertile soilsVolcanic ash and lava are full of the minerals and chemicals needed by plants to thrive. Although newly erupted lava and ash is a hostile environment for plants, over time the rocks break down, releasing the nutrients within them. Eventually, highly fertile soils can develop on old lava and ash flows.
Some of the world’s finest wines and coffee are made from crops grown on volcanic soils. But growers should beware. There is always the danger that the very same volcano that provided their fertile soil may erupt again and turn fields back into barren wastelands.
There are warning signs to look out for before a major eruption. Smoke and steam being ejected from the main vent, or even small lava eruptions, can indicate that a much bigger eruption is on the way. Earth tremors are also a sign of impending trouble. Sometimes the sides of a volcano bulge before an eruption as pressure builds up inside.
Consultant: Ian Fairchild