Grímsvötn volcano in Iceland began to erupt during the evening of 21 May 2011.
The London Volcanic Ash Advisory Centre issued regular forecasts of the ash plume location.
A small amount of fine ash fell over parts of the UK on 23 to 24 May, but shifting winds then tended to carry the ash away from the British Isles from the 25 May.
The Icelandic Meteorological Office and the Institute of Earth Sciences declared that the eruption ended on 28 May.
Many schools and individuals across the UK put out samplers during May 2011 and sent the results to the BGS for analysis.
Many of these samples included basaltic ash from the Grimsvotn volcano.
The image (right) shows a grain of ash from a sample sent in by Kirkwall Grammar School in Orkney, which was collected on 24 May.
The grain with concave surfaces is a fragment of basaltic glass, about the size of a pollen grain, and the concave surfaces are bubble walls.
View a map of volcanic ash observations across the UK.
The resulting ash plume rose to heights of over 17 km at the start of the eruption, but reduced to around 10 km during 22 May, and to less than 7 km by 24 May.
By the afternoon of 25 May, the Icelandic Meteorological Office (IMO) were unable to detect an ash plume above the volcano, although continuing seismic tremor showed that the eruption had not ended completely.
The eruption ended on 28 May.
Grímsvötn is the most active volcano in Iceland and last erupted in 2004. Grímsvötn typically erupts basalt lava and was responsible for the world's largest known lava flow in 1783.
Grimsvötn is covered by an ice cap, similar to the Eyjafjallajökull volcano which erupted in April 2010.
The explosive nature of the 2011 Grimsvötn eruption was due to the interaction of hot magma with ice and water, producing ash particles.
Basaltic eruptions tend to produce a relatively large proportion of coarse ash particles that fall out closer to the volcano.
However, some fine ash is also produced, and may be transported over long distances.
Basaltic eruptions are associated with volcanic gases such as sulphur dioxide, carbon dioxide and hydrogen sulphide which will be dispersed in the atmosphere in a similar manner to the ash. The gases partially transform in the atmosphere to acid aerosols.
Some of the ash fall from Grímsvötn in southern Iceland is visible from a web cam at Jökulsárlón.
The Iceland Meteorological Office reported that meltwater began flooding from the Grímsvötn glacial lake in Iceland on 29 October 2010 and discharge progressively increased through to 1 November.
This activity may have been an early signal that magma was rising beneath the volcano, although no volcanic eruption occurred at the time.
Grímsvötn volcano is situated near the centre of the Vatnajökull ice cap in central Iceland and is Iceland's most active volcano. It has a subglacial caldera lake (a lake beneath the glacier) warmed by geothermal heat.
The caldera at Grímsvötn is like a bowl which holds a meltwater lake beneath the 200 m thick ice cap. Geothermal activity at the base of the caldera can raise temperatures and cause melting of the base of the ice cap thus raising the level of the subglacial lake.
If enough meltwater is generated the water lifts the ice cap and escapes over the caldera rim to cause a 'glacier outburst flood' — or jökulhlaup. This meltwater flooding is the trigger for the current alert.
As magma rises in the crust it increases the geothermal gradient thus a glacier outburst flood can be a sign that magma is rising towards the surface, the magma may not reach the surface and remain in the crust as an 'intrusion'.
The last eruption of Grímsvötn was in 2004 when the plume height reached 12–14 km; the high plume phase lasted only a few days.
A brief eruption of the Grímsvötn volcano produced an ash cloud which was carried eastwards over Scandinavia, leading to diversion and grounding of some flights. At the time, newspapers carried quotes saying that the disruption would have been much worse had the winds been north-westerly.
The previous eruption was in 1998 and prior to that 1996. These were all explosive eruptions. There was a large explosive eruption in 1902.
There have been numerous smaller non-explosive eruptions at Grímsvötn during the 20th century — mainly defined by 'glacier bursts'.
Iceland is a unique volcanic island. It lies on the Mid-Atlantic Ridge, the great chain of underwater volcanoes that runs through the North Atlantic Ocean, along which the Eurasian and North American plates are moving apart at about 2.5 cm per year — or roughly the rate your fingernails grow. But Iceland also sits atop a mantle plume, where hot magma upwells from deep in the Earth’s mantle. The interaction of these two types of volcanism, over the last 15 million years or more, has created the island of Iceland.
'BGS have been working in collaboration with the Iceland Met Office to install seismic stations in the vicinity of Eyjafjallajokull and Katla. These stations provide real-time data for monitoring any future eruptions. They also recorded some of the recent earthquake activity around Grímsvötn volcano.'
The majority of volcanism in Iceland occurs along volcanic rift zones that cut through the centre of the island. The single North Volcanic Zone, running broadly southwards from the north coast, gives way to two north-east–south-west rift zones (the East and West Volcanic Zones) in southern Iceland. Within these volcanic rift zones are numerous individual volcanoes, which tend to have the classic volcano profile of a broad cone with a summit crater or craters. In Iceland, though, many of these volcanoes are capped by glaciers.
Modern techniques for dating rocks tell us that the oldest lavas on Iceland are some 15 million years old, and that volcanic eruptions in Iceland have been going on ever since. However, by far the best record of Iceland’s volcanic past comes from the last 1100 years, since the first settlers arrived in Iceland from Europe. Written records have been studied in conjunction with careful fieldwork that has identified the products of individual eruptions, preserved in Iceland and further afield. Over this time period, more than 200 volcanic events have been identified, some of them lasting for months or even years (Thordarson and Larsen, 2007). Thus, on average, there is roughly one volcanic eruption in Iceland every five years.
Contact Dr Sue Loughlin for further information.