A volcano is an opening in the Earth's surface that allows hot, molten rock, ash and gases to escape from deep below the surface.

Volcanoes come in many different shapes and sizes, but all have a central vent that is connected to a magma chamber where lava builds up. Most of these chambers are several kilometres wide.

The flanks of a volcano are unstable and often fracture, allowing lava and volcanic gases to escape out of the sides.

There are more than 550 active volcanoes that have erupted since historical records began. About another 1,000 dormant volcanoes still have the potential to erupt again.

Some are sleeping giants, like the supervolcano beneath Yellowstone National Park in the United States that has not erupted for 640,000 years but is still considered active because of its functioning geothermal system and frequent earthquakes.

Yellowstone's magma chamber is 10 km (6 miles) high and 40 km (25 miles) wide. If this erupts again, the consequences would be unimaginably catastophic.

Many volcanoes in densely populated areas are constantly monitored by scientists, but hundreds in remote areas are not.

The signs of a volcano about to blow include an increased number of tremors near the volcano, new gas emissions and shifts on its surface.

An eruption happens when magma rises to the earth's surface, fracturing the surface rock. The volcano produces...

...lava (called magma when it is beneath the surface)

...hot gases

...ash and tephra (larger fragments, including lava bombs)

...pyroclastic flows (a mixture of hot gas and tephra that flow at high speed)

...lahars (mudflows of ash and tephra mixed with water)

Lava flows destroy everything in their path but usually move too slowly to kill people.

More dangerous are superheated clouds of ash and hot gas that can move at hurricane speeds, hugging the ground and destroying everything in their path. These pyroclastic flows killed all but two of the 29,000 inhabitants of St Pierre, capital of Martinique in the Caribbean, in 1902. They died within seconds of the eruption.

Volcanic ash is made up of tiny jagged pieces of volcanic rock and glass formed from magma during an eruption. Heavy ash falls can cause roofs to collapse, darken skies and make it difficult to breathe.

When ash and lava erupt into the air, they often cause intense thunder and lightning storms and heavy rainfall. This can trigger floods, landslides and rivers of mud that wash downhill covering towns and villages.

About 23,000 people died when Nevado del Ruiz in Colombia erupted in 1985 - not because of the eruption itself but because of mudflows caused by the sudden melting of the volcano's ice cap.

Another danger during an eruption is earthquakes caused when the lava breaks the surface rock. These tremors in turn can trigger dangerous landslides.

Most eruptions end within three months. Some have lasted 30 years, including volcanoes in Ethiopia, Papua New Guinea, Vanuatu, Indonesia, Guatemala, Costa Rica and Ecuador. Stromboli volcano in Italy has been erupting for 2,400 years.

WHERE DO VOLCANOES FORM?

The earth's surface is broken up into slow-moving tectonic plates, each about 100 km (62 miles) thick. Most volcanoes form on the edges of these plates where they are either pushing towards or pulling away from each other, as you can see from this map. A few exceptions are some highly active volcanoes that have formed in the middle of a plate, in areas called hotspots.

Volcanoes can form on the bottom of oceans as well as on land.

Many explosive volcanoes make up the Ring of Fire, a chain of volcanoes that encircles the Pacific Ocean from South America up to Alaska, across to Russia and down through Japan and Indonesia.

But scientists say they may not yet have found all the world's volcanoes - even the most dangerous ones. Some volcanoes are hard to detect if they are covered by dense vegetation, glaciers or water, according to Global Volcanism Program.

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