Volcanoes are places where magma (red-hot liquid rock from the Earth’s interior) emerges through the crust and onto the surface.
The word ‘volcano’ comes from Vulcano Island in the Mediterranean. Here Vulcan, the ancient Roman god of fire and blacksmith to the gods, was supposed to have forged his weapons in the fire beneath the mountain.
There are many types of volcano (see kinds of volcano). The most distinctive are the cone-shaped composite volcanoes, which build up from alternating layers of ash and lava in successive eruptions.
Beneath a composite volcano there is typically a large reservoir of magma called a magma chamber. Magma collects in the chamber before an eruption.
From the magma chamber a narrow chimney, or vent, leads up to the surface. It passes through the cone of debris from previous eruptions.
When a volcano erupts, the magma is driven up the vent by the gases within it. As the magma nears the surface, the pressure drops, allowing the gases dissolved in the magma to boil out. The expanding gases — mostly carbon dioxide and steam — push the molten rock upwards and out of the vent.
If the level of magma in the magma chamber drops, the top of the volcano’s cone may collapse into it, forming a giant crater called a caldera. Caldera is Spanish for `boiling pot. The world’s largest caldera is Toba on Sumatra, Indonesia, which is 1775 sq km.
When a volcano with a caldera subsides, the whole cone may collapse into the old magma chamber. The caldera may fill with water to form a crater lake, such as Crater Lake in Oregon, USA.
All the magma does not gush up the central vent. Some exits through branching side vents, often forming their own small ‘parasitic’ cones on the side of the main one.
At Urgiip, Turkey, volcanic ash has been blown into tall cones by gas fumes bubbling up. The cones have hardened like huge salt cellars. People have dug them out to make homes.
Volcanic eruptions are produced by magma, the hot liquid rock under the Earth’s surface. Magma is less dense than the rock above, and so it tries to bubble to the surface.
When magma is runny, eruptions are ‘effusive,’ which means they ooze lava gently all the time.
When magma is sticky, eruptions are explosive. The magma clogs the volcano’s vent until so much pressure builds up that the magma bursts out, like a popping champagne cork.
The explosion shatters the plug of hard magma that blocks the volcano’s vent, reducing it to ash and cinder.
Explosive eruptions are driven by expanding bubbles of carbon dioxide gas and steam inside the magma.
An explosive eruption blasts globs of hot magma, ash, cinder, gas and steam high up into the air.
Volcanoes usually erupt again and again. The interval between eruptions, called the repose time, varies from a few minutes to thousands of years.
Magma near subduction zones contains 10 times more gas, so the volcanic eruptions here are violent.
The gas inside magma can expand hundreds of times in just a few seconds.
One of the biggest- ever eruptions occurred 2.2 million years ago in Yellowstone, USA. It poured out enough magma to build half a dozen Mt Fujiyamas.
In 1645 BC the Greek island of Thera erupted, destroying the Minoan city of Akoteri. It may be the origin of the Atlantis myth.
On August 24, AD79 the volcano Mt Vesuvius in Italy erupted. It buried the Roman town of Pompeii in ash.
The remains of Pompeii were discovered in the 18th century, preserved under meters of ash. They provide a remarkable snapshot of ancient Roman life.
The eruption of the volcanic island of Krakatoa near Java in 1883 was heard a quarter of the way round the world.
In 1815 the eruption of Tambora in Indonesia was 60-80 times bigger than the 1980 eruption of Mt St Helens.
Ash from Tambora filled the sky, making the summer of 1816 cool all around the world.
J. M. W. Turner’s paintings may have been inspired by fiery sunsets caused by dust from Tambora.
During the eruption of Mt Pelee on Martinique on May 8, 1902, all but two of the 29,000 townspeople of nearby St Pierre were killed in a few minutes by a scorching flow of gas, ash and cinders.
The biggest ancient Roman town eruption in the Pompeii in the past 50 years was found to be the eruption of My Pinatubo in the Philippines in April 1991.
In Worldwide there are over 1500 volcanoes; 500 of these are active. A volcano can have a lifespan of a million years and not erupt for several centuries.
Volcanoes are said to be active if they have erupted recently. The official Smithsonian Institute list of active volcanoes includes any that have erupted in the past 10,000 years. Extinct volcanoes will never erupt again.
Volcanoes occur either along the margins of tectonic plates, or over hot spots in the Earth’s interior.
Most volcanoes are found around the Pacific Ocean. They also occur in Iceland, Hawaii and southern Europe. 254
Some volcanoes erupt where the plates are pulling apart, such as under the sea along mid-ocean ridges.
Some volcanoes lie near subduction zones, forming either an arc of volcanic islands or a line of volcanoes on land, called a volcanic arc.
Subduction zone volcanoes are explosive, because the magma gets contaminated and acidic as it burns up through the overlying plate. Acidic magma is sticky and gassy. It clogs up volcanic vents then blasts its way out.
Around the Pacific there is a ring of explosive volcanoes called the Ring of Fire. It includes Mt Pinatubo in the Philippines, and Mt St Helens in Washington State, USA.
Away from subduction zones magma is basaltic. It is runny and low in gas, so the volcanoes here gush lava.
Effusive volcanoes pour out lava frequently but gently.
3D radar interferometry from satellites may pick up the minutest swelling on every active volcano in the world. In this way it helps to predict when eruptions may occur. 255
One of many volcanoes in the Ring of Fire is Mt Rainier, in Washington State, USA
About five percent of volcanoes are not near the margins of tectonic plates. They are over especially hot places in the Earth’s interior called hot spots.
Hot spots are created by mantle plumes – hot currents that rise all the way from the core through the mantle.
When mantle plumes come up under the crust, they burn their way through to become hot-spot volcanoes.
Famous hot-spot volcanoes include the Hawaiian island volcanoes and Reunion Island in the Indian Ocean.
Hot spots pump out huge amounts of lava.
Hot-spot volcanoes ooze runny lava that spreads out to create shield volcanoes.
Lava from hot-spot volcanoes also creates plateau, such as the Massif Central in France.
The geysers, hot springs and bubbling mud pots of Yellowstone National Park, USA, indicate a hot spot below.
Yellowstone has had three huge eruptions in the past 2 million years. The first produced over 2000 times as much lava as the 1980 eruption of Mt St Helens.
Hot spots stay in the same place while tectonic plates slide over the top of them. Each time the plate moves, the hot spot creates a new volcano.
The movement of the Pacific plate over the Hawaiian hot spot has created a chain of old volcanoes 6000 km long. It starts with the Meiji seamount under the sea north of Japan, and ends with the Hawaiian Islands.
Each volcano and each eruption are slightly different.
Shield volcanoes are shaped like upturned shields. They form where lava is runny and spreads over a wide area.
Fissure volcanoes are found where floods of lava pour out of a long crack in the ground.
Composite volcanoes are cone shaped. They build up in layers from a succession of explosive eruptions.
Cinder cones are built up from ash, with little lava.
Strombolian eruptions are eruptions from sticky magma. They spit out sizzling clots of red-hot lava.
Vulcanian eruptions are explosive eruptions from sticky magma. The magma clogs the volcano’s vent between cannon-like blasts of ash clouds and thick lava flows.
Peleean eruptions eject glowing clouds of ash and gas called nuee ardente.
Plinian eruptions are the most explosive kind of eruption. They are named after Pliny who witnessed the eruption of Vesuvius in AD79.
In Plinian eruptions boiling gases blast clouds of ash and volcanic fragments up into the stratosphere.