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Katla: Iceland's most notorious volcano is kept under close surveillance

By Haraldur Sigurðsson

  • The fire within Iceland's most active and potent volcanos are in the south part of the country. This photo was taken in 2010 when a small volcano at Fimmvörðuháls ridge erupted. Since 2010 two other volcanos in south Iceland have erupted. Photo/Vilhelm Gunnarsson

  • From the 2010 eruption in Eyjafjallajökull in 2010. Photo/Vilhelm Gunnarsson

  • Grímsvötn volcano in Vatnajökull glacier south Iceland is Iceland's most active volcano. In the spring of 2011 an eruption in Grímsvötn caused disruption to air travel in North-Western Europe from 22–25 May 2011. Photo/Vilhelm Gunnarsson

A new report by British scientists highlights the immediate risk for a large portion of northern Europe from a volcanic eruption in Iceland. The ash cloud from the 2010 eruption in Eyjafjallajökull shut down all aviation across the North Atlantic for one week. Katla volcano, it's next-door neighbor, has the potential of generating an eruption that is at least ten times larger. Haraldur Sigurðsson, the world-renowned volcanologist and the owner of the Volcano Museum, profiles the volcano that is keeping the nation on edge.


When Eyjafjallajökull volcano erupted explosively in 2010, it shut down all aviation across the North Atlantic for one week. The dispersal of the ash cloud led to the closure of 313 airports, and cancellation of 104,000 international flights between Europe and North America.

But there was immediately some concern that this event would be followed by an even larger eruption of Katla volcano, which is located only 20 km to the east.

History tells us that Katla has generally erupted shortly after the three previous eruptions of Eyjafjallajökull, in 960 AD, 1612 and 1823. No one knows if there is a connection between these volcanoes, or if this is just a coincidence. After all, Katla has erupted at least 22 times since Iceland was settled 1,100 years ago and another eruption in the not-too-distant future is therefore inevitable.

Steam Explosions

The major problem with Katla and many of Iceland’s volcanoes is that they are generally capped by a thick ice sheet. All the highest mountains in Iceland are volcanoes, and they have accumulated layers of ice on top that may be 400 to 700 meters thick. When 1,200°C (2,192° F) hot magma rises up in the volcano beneath the glacier it causes massive melting of the ice and violent steam explosions. Normally, this magma would create relatively harmless lava flows on land, but the steam explosions in the glacier change the nature of the eruption to explosive, with production of huge volumes of ash that are dispersed widely in the atmosphere.

The glacier on top of the erupting Icelandic volcanoes causes another and even larger problem, which is called jökulhlaup or glacial outburst flood. When the hot magma rises up beneath the glacier it leads to enormous melting of the ice sheet. This causes a flash flood within hours of the beginning of an eruption, often with devastating results. The rate of flow may be from 50-300,000 cubic meters per second. The largest glacial floods will briefly reach a flow rate equal to that of the Amazon River and about fifteen times that of the Mississippi

Terrifying Flood

The largest glacial flood in Iceland’s history occurred in the  beginning of the Katla eruption in 1918. Tales of this flood  are terrifying. Although it was a thick mixture of meltwater, volcanic ash, and ice, it advanced so fast that one could  only escape its path on horseback. It covered hundreds of square kilometers; today such a flood would destroy the Iceland ring road and many important facilities in the south of the country.

And then there is the question of the impact of a future eruption of Katla on aviation. Because of these serious concerns, there is a lot of effort being put into monitoring the Katla volcano. It is literally “wired” with ten seismometers that detect the heart-beat in the magma chamber below, with GPS stations that measure inflation or stretching of the crust, and with scientists who study the chemistry of the rivers that flow from the volcano.

Their work is analogous to the medical doctor, who studies the chemistry of urine, blood and other body fluids of his patient in order to see what may be happening inside the body. 

Hekla volcano's latest eruption was in 2000. Geophysicists have been anticipating an eruption in Hekla for a few years. Hikers on the mountain are advised to be careful. Photo/Vilhelm Gunnarsson


The hope is that this monitoring may detect changes that give a warning or perhaps a basis for a prediction of the next eruption. Katla has a large magma reservoir which is located at only 2 km below the glacier. It therefore has the potential of generating an eruption that is at least ten times larger than the Eyjafjallajökull eruption that occurred in 2010.

Icequakes or Earthquakes

All was quiet in Katla after the Eyjafjallajökull eruption in 2010, but seventeen months later, in May 2011, the number of earthquakes began to increase,  eaching over 400 events per month in September and October. Was the earlier prediction that Katla would follow Eyjafjallajökull perhaps correct? Icelanders braced themselves for another eruption and the tension increased. Then the earthquakes gradually dropped off earlier this year, but increased again this June and have remained high since, as the graph shows.

The roller-coaster ride continues as uncertainty about future developments in the volcano prevails. A major source of uncertainty is the nature or origin of these quakes. Are they icequakes or earthquakes? They are all small events, too small to be felt by us, only detected by geophysical instruments. When glaciers move, and when they melt, the thick slab of ice fractures and produces earthquake-like shocks that are picked up by the seismometers.

Increased melting below the glacier may be occurring because of increased hot spring activity in the Katla volcano. Melting may also be increasing because of the exceptionally warm summer that Iceland is experiencing, as a result of climate change. Thus, Icelanders remain in limbo with respect to interpretation of the signs from Katla volcano. Is the increasing frequency of earthquakes a signal that magma or molten rock may be moving within the crust, in advance of an eruption, or are they mostly icequakes, caused by climate change and hydrothermal melting?

Stay tuned to get the answer to this important question.

The Volcano Museum in Stykkishólmur town west Iceland is open from May 1 to September 30 between 11 and 7 pm. Talks are given daily in English and Icelandic . 

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