Grímsvötn, near the western end of the Vatnajökull ice cap, is Iceland's most active volcano. At least 60 eruptions have been documented in the Grímsvötn volcanic system during the past 800 years, most of them occurring in the Grímsvötn caldera. It is one of the most active geothermal areas in the world, with a natural thermal loss of 2000-4000 MW.
Glacial bursts (jökulhlaups) originate in Grímsvötn and charge down Skeiðará river. They were the main reason that the Ring Road around Iceland was not completed until 1974.
Start of the eruption in Grímsvötn, 21 May 2011.
Volcano and lake under the ice
Most of the Grímsvötn area lies beneath several hundred metres of ice. Measurements made during geophysical investigations in the 1990s and confirmed using radio echo technology give a picture of the sub-glacial landscape of the area. They show that the Grímsvötn volcano is approximately 15 km in diameter and rises up to 900 m from a hilly base to a height of over 1000 m above sea level. A composite caldera sits in the middle of Grímsvötn. It consists of three parts: the north caldera (12 km2), the south or main caldera (20 km2) and the east caldera (16-18 km2).
Grímsfjall abuts the principle caldera on its south side. Two nearby peaks stand prominently above the surface of the ice. The Icelandic Glaciological Society has built three cabins close to the eastern peak, 1722 m above sea level. They serve as a base camp for research expeditions and other visitors, and provide facilities for seismographic and other equipment that monitors Grímsvötn.
Volcanic activity and glacial bursts
Geothermal activity is greatest in the main caldera, which has a floor at an altitude of 1060-1100 m north of Grímsfjall. The caldera contains a lake which is covered by an ice slab 200-300 m thick. Creeping ice and continuous melting caused by geothermal heat cause water to accumulate and the water level of Grímsvötn Lake rises. The ice slab also rises. When the water reaches a critical level it seeks an outlet north-east of Grímsfjall. The water begins to gush from the foot of the glacier into the bed of Skeiðará river. A glacial burst begins, slowly at first, increasing in volume until peak flow is reached. The flow then suddenly decreases and the exit channel closes.
Until 1934, intervals between glacial bursts were commonly around 10 years. The volume of each outburst was anything up to 4 or 5 km3 and maximum flow was generally several tens of thousands of cubic meters per second. Between 1940 and 1996, the interval between bursts was nearer 5 years and they were smaller (1¬-3 km3 with a maximum flow of 2000-10,000 m3/s). Glacial bursts also frequently occur either before or after an eruption in Grímsvötn. A glacial burst results in reduced weight lying on the central volcano's magma chamber, which can allow magma to break out. This scenario was quite common until 1934, but did not occur again until November 2004.
Grímsvötn changed considerably during the large glacial burst that followed the eruption in 1996. Increased geothermal activity weakened the ice dam, and since 1996 there have been many small outbursts at irregular intervals and continuous leakage into the river for longer periods. No further large glacial bursts are expected as long as these conditions prevail.
No eruptions were recorded in Grímsvötn between 1938 and 1996 with the exception of a small event at the end of May 1983. After outbursts in 1945 and 1954, geothermal activity increased in the calderas in Grímsvötn. One theory suggests that small eruptions did occur, although no indications of any eruptions were noticed on flights over Grímsvötn, and no volcanic material has been found that can be traced to eruptions from this time.
On the other hand, changes to geothermal caldera similar to those in 1945 and 1954 have been seen in recent years that are unconnected to volcanic eruptions. A new period of activity has begun in Vatnajökull after three quiet decades. Four volcanic eruptions have occurred since 1996: the Gjálp eruption in 1996 (north of Grímsvötn) and Grímsvötn eruptions in 1998, November 2004 and again in May 2011.
The water level in Grímsvötn is measured by GPS units located on the ice cap. Seismometers housed in the facilities of the Icelandic Glaciological Society and GPS units monitor the rise of magma and any uplift or subsidence. Changes to the glacier due to geothermal activity are recorded and a range of glaciological measurements are taken in and around Grímsvötn.