Page last updated at 17:39 GMT, Thursday, 15 April 2010 18:39 UK

A guide to Iceland's volcanoes

Iceland and tectonic plates

Iceland is a volcanic hot spot on the Mid-Atlantic Ridge - the dividing line between the Eurasian and North American continental plates.

The country's three major volcanoes - Hekla, Katla and Grimsvotn - lie along this boundary.

Altogether there are 35 active volcanoes on and around the island.

Eyjafjallajokull means "island mountain glacier"
Eyja means "islands"
Fjall means "mountain"
Jokull means "glacier"
It's the mountain with the glacier seen from, or around, the Westman islands, off Iceland's south-western coast

Offshore volcanic activity has resulted in the formation of separate volcanic islands, one of which, Surtsey, appeared as recently as 1963.

Icelandic volcanoes have long drawn tourists from across the globe, eager to catch a glimpse of a pillar of smoke or a spectacular lava flow. But there have also been cataclysmic events.

Eruptions at the eastern volcanoes of Grimsvotn and Laki from 1783 to 1785 produced a lava flow which consumed vast swathes of land, blotted out the sun and killed a quarter of the population through poisoning or famine.

Angry sisters

Eyjafjallajokull, which is currently filling the sky above northern Europe with ash, is a minor player in Icelandic terms - though its last eruption lasted for more than a year, from December 1821 to January 1823.

The most ominous thing about Eyjafjallajokull is that its eruptions have historically preceded eruptions by one of Iceland's most feared volcanoes, Katla.

Fortunately, Katla, one of the two so-called Angry Sisters, has shown no sign of unusual activity in 2010.

1783-85: Grimsvotn-Laki - lava inundated 300 sq km, sulphur dioxide and hydrogen fluoride emitted killed 9,000 people
1918: Katla eruption
1947-48: Hekla - 11-month eruption, major lava flow, 24,000 tonnes of carbon dioxide emitted
1973: Eldfell - seven-month eruption on island of Heimaey
1996: Grimsvotn erupts, causing a glacial burst (where a build-up of water under the ice of a glacier is suddenly released)
2004: Grimsvotn - eruption sends ash up to 12km into the air, some of it landing in Norway, Sweden and Finland

Katla is located under the vast Myrdalsjokull glacier. One of the highest volcanoes in Europe at 1,512m (4,961 ft), its crater has a diameter of 10km. Any eruption would raise fears of major flooding due to the melting of the glacier.

Its last major eruption occurred in 1918, with a smaller eruption in the 1950s. Historically, it tends to erupt every 40-80 years.

The second of the Angry Sisters is 1,490m Mount Hekla, Iceland's most active volcano.

In the Middle Ages it was believed to be one of the gateways to hell, or Judas's prison. An eruption in 2000 spewed columns of ash several kilometres into the sky, and caused a lava flow 7km long.

In 1973, there was an eruption near the nation's premier fishing port, Vestmannaeyjar, on the southern island of Heimaey. A mile-long fissure rapidly opened, bisecting the island. Spectacular lava fountains followed.

The lava flow continued for five months and around 400 homes close to the rift were destroyed by ash-fall, or consumed by lava flow.

But within hours of the eruption, nearly all of Heimaey's 5,300 residents were evacuated to the mainland by the island's fishing fleet.

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