April 15, 2010 / 1:50 PM / 8 years ago

Factbox: Volcanoes and ash clouds

(Reuters) - A huge cloud of ash from a volcano in Iceland turned the skies of northern Europe into a no-fly zone on Thursday, leaving hundreds of thousands of passengers stranded.

The eruption under the Eyjafjallajokull glacier -- 10 times more powerful than another one nearby last month -- showed no sign of abating after more than 24 hours of activity.

Here are a few facts on volcanic eruptions:

THE DANGERS:

* VOLCANIC GASES - The most dangerous gases released during an eruption are sulphur dioxide, carbon dioxide and hydrogen fluoride. High concentrations of sulphur dioxide injected into the atmosphere by large explosions can result in lung ailments, acid rain, lower surface temperatures and depletion of the earth’s protective ozone layer. When carbon dioxide, which is heavier than air, settles in low-lying areas or collects in the soil, it can be lethal to people and animals.

* ERUPTION COLUMNS - Billowing clouds of gas and debris can reach more than 12 miles above a volcano, posing a serious threat to aviation. Some commercial jets have nearly crashed after flying into clouds of ash. Large rock fragments falling to the ground can kill people and destroy property.

* ASH FALL - Large volumes of ash can settle on buildings, resulting in their collapse. High levels of ash particles can cause increased coughing and irritate the eyes and skin and sometimes result in serious lung conditions. When the acid coating on ash is removed by rain, it can pollute local water supplies and damage vegetation. On the other hand, ash deposits can be beneficial by improving the fertility of soil.

* PYROCLASTIC FLOWS - An avalanche of hot ash, rock fragments and gas can flow down the side of a volcano at speeds of up to 150 miles an hour during explosive eruptions, burning everything in its path. People on the margins of the flows can suffer serious injury or even death from burns and inhalation of hot ash and gases.

THE RING OF FIRE:

* Of the world’s 1,500 active volcanoes, almost 90 percent are in the Ring of Fire, a horseshoe-shaped band of volcanoes and fault lines circling the edges of the Pacific Ocean. It runs from Chile, northwards along the South American coast through Central America, Mexico, the west coast of the U.S. and the southern part of Alaska, through the Aleutian Islands to Japan, the Philippines and Indonesia before curving back to New Guinea, the southwest Pacific islands and New Zealand.

VOLCANIC ERUPTIONS IN HISTORY:

* Pompeii was buried under a deep layer of ash in an eruption of the Vesuvius volcano, which towers over the Bay of Naples in southern Italy, in AD 79.

* In 1815 the Tambora volcano eruption in Indonesia killed an estimated 92,000 people. It has been claimed that volcanic ashes swept all the way to Europe blanketing the continent in a veil and turning 1816 into a “year without summer.”

* In 1883 Krakatoa erupted in Indonesia killing 36,000. The impact of the eruption spewed molten rock and sulphate aerosols into the atmosphere. The debris in the atmosphere created vivid red twilights in Europe from November 1883 through February 1884 - a fact reflected by painters of the period.

* An earthquake measuring 9.5 struck the coast of central Chile in May 1960, triggering tidal waves and volcanic eruptions. Some 5,000 people were killed and 2 million made homeless.

* Mount Pinatubo, 80 km (50 miles) north of Manila, has one of the 20th century’s three largest eruptions. The June 1991 eruption killed more than 700 and 200,000 buildings are destroyed.

* The Mount St. Helens volcano in the U.S. state of Washington erupted in May 1980, creating a cloud of ash 2,500 miles long and 1,000 miles wide. The eruption was triggered by a 5.1 magnitude earthquake. 57 died.

* In June, 2006, eruptions at Mount Sakurajima, on the southern Japanese island of Kyushu, spews volcanic gases 2,000 meters (6,600 ft) into the air.

* An explosion from the 1,500-meter (5,000-ft) summit of Bulusan volcano in June 2006 in the Philippines sends ash and steam 1,000 meters (3,300 ft) into the air and showers ash on surrounding villages.

* In April 2010 an eruption from below the Eyjafjallajokull glacier in Iceland hurled a plume of ash six to 11 kilometers (3.8 to 7 miles) into the atmosphere.

Writing by David Cutler, London Editorial Reference Unit; Editing by Dominic Evans

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