REYKJAVIK – A volcano erupted under Iceland’s biggest glacier on Saturday, with a white plume shooting 18,000 feet into the air, scientists said. However, geologists played down the likelihood it would cause the same disruption to air traffic chaos like that caused by ash from the Eyjafjallajokul volcano in April 2010.
The eruption began at approximately 17:30 UTC, May 21st 2011 and was followed by around 50 small earthquakes, the largest of which measured 3.7 on the Richter Scale, according to Iceland’s meteorological office.
Eruptions in Grímsvötn start as subglacial eruptions, which quickly break the ice cover. At 21:00 UTC, the eruption plume had risen to an altitude of over 65,000 ft (~20 km). Initially, the plume is expected to drift to the east and subsequently to the north-east.
Grimsvötn volcano is situated near the center of the Vatnajökull ice cap in central Iceland and is one of Iceland’s most active volcanoes. It has a complex of calderas and a subglacial caldera lake sustained by geothermal heat. Small eruptions have occurred at the volcano in 1983 and 1998. In 1996, the Gjálp subglacial eruption occurred north of the volcano and yet again in the fall of 2004. As Bardarbunga, the Grimsvötn centre is a part of a a fissure system which includes the Laki fissure, which in 1783 produced about 12-14 km3 of basaltic lava. Within the ice filled Grimsvötn caldera intense geothermal activity continuously melts the ice to form a subglacial lake, which at intervals of 5 to 10 years is emptied along subglacial channels to create large floods (jökulhlaup) on the sandur plain, Skeidararsandur, on the Icelandic south coast.
The eruption was preceded by both long-term and short-term precursors, and finally triggered by release of overburden pressure associated with a glacial outburst flood (jökulhlaup), originating from the Grimsvötn subglacial caldera lake. Accumulation of magma in a shallow magma chamber under the Grimsvötn caldera has been ongoing since its last eruption in 1998. GPS measurements show uplift of 5-10 cm/year in the caldera center, and horizontal displacements away from the caldera. Earthquake activity increased in middle of 2003, at about the same time uplift exceeded its 1998 maximum. Pressure in the Grimsvötn magma chamber is likely to have exceeded its pre-eruption level from 1998 at this time. Additional uplift and expansion of the volcano since then suggested approaching failure of the volcano. Earthquake activity increased further in late October, 2004. Geothermal heat sustains a lake in the caldera that intermittently causes glacial outburst floods. On October 26 high frequency seismic tremor indicated increased water flow from the caldera lake and suggested that a glacial outburst flood was about to begin. On October 29 discharge increased in river Skeidara. The outburst flood was caused by high water level in the Grimsvötn caldera lake from ice melting by geothermal activity. The release in overburden pressure associated with the outburst flood triggered the eruption. The drop in water level in the Grimsvötn caldera at the onset of the eruption is uncertain, but is probably on the order of 10-20 meters, corresponding to a pressure change of 0.1-0.2 MPa on the volcano surface. This modest pressure change triggered the eruption because internal pressure in the Grimsvötn shallow magma chamber was high after continuous inflow of magma to the volcano since 1998.
In November, melted glacial ice began pouring from Grimsvotn, signaling a possible eruption. That was a false alarm but scientists have been monitoring the volcano closely ever since.
Sparsely populated Iceland is one of the world’s most volcanically active countries and eruptions are frequent.
Eruptions often cause local flooding from melting glacier ice, but rarely cause deaths.
Last year’s Eyjafjallajokul eruption left some 10 millions of air travelers stranded worldwide after winds pushed the ash cloud toward some of the world’s busiest airspace and led most northern European countries to ground all planes for five days.
Whether widespread disruption occurs again will depend on how long the eruption lasts, how high the ash plume rises and which way the wind blows.
No people were threatened by the volcano, which was in an uninhabited area where the nearest farm was 100 kilometres away.
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