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. See UK monitoring and deposition of tephra from the May 2011 eruption of Grímsvötn, Iceland.
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 Grímsvötn 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 a devastating fissure eruption in 1783-1784.
Grímsvötn is covered by an ice cap, similar to the Eyjafjallajökull volcano which erupted in April 2010.
The explosive nature of the 2011 Grímsvötn 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'.
Contact Dr Sue Loughlin for further information.