Iceland: The ice sheet at Grimsvötn is subsiding, which could lead to a volcanic eruption

The Scientific Advisory Board for Civil Defense met Nov. 24, 2021, to discuss changes recorded at Grimsvötn. GPS measurements show that the ice sheet has begun to settle, indicating that an eruption of the Grimsvöttn volcano is likely to begin. There have been examples of Grimsvöttn eruptions in the past that began after such events.

Over the past few days, the ice sheet had subsided about 60 cm (23.6 inches) by November 24, and the rate of subsidence had increased since November 23. These measurements indicate that it is likely that water has begun to escape from Lake Grimvötn and that Gigükvösl will be flooded, the Icelandic Meteorological Office (IMO)1 reported.

Throughout the night and into the morning of November 25, the ice cover continued to descend evenly. IMO’s GPS instruments show that it has dropped about 25 cm (9.8 inches) since 10:00 UTC on November 24. At this time, there are no signs of rising water levels, electrical conductivity, or gas in Hygiuquistle.

Based on observations of past floods, floodwaters are expected to exit the edge of the glacier in the next 48 hours, with peak flooding occurring in 4 to 8 days thereafter.

So far, no increase in electrical conductivity has been detected in the Giguquistle, which is the clearest indication that the floodwaters of the Grimsvötn have exited the glacier. IMO has also installed gas monitors along the Gigiukwisle that will show whether floodwaters have reached these points in the river.

The maximum flow expected from this flood is estimated to be about 5,000 m3/s (176,000 ft3/s). This size of flooding is not likely to affect infrastructure in the area, such as roads or bridges. At this early stage, however, these projections are uncertain, the IMO noted.

There have been instances in the past where a Grimsvöttn volcano eruption has started after a flood.

The loss of water from Lake Grimsvöttn reduces pressure on the top of the volcano, which can lead to the start of an eruption.

It happened in 2004 and before that in 1934 and 1922, IMO reports.

In 2004, the eruption began three days after the first flood observations were made. A series of earthquakes occurred in the days leading up to the eruption. No such earthquakes have been recorded to date.

According to Benedikt Gunnar Ofeigsson, a deformation scientist at IMO, there is every indication that the Grimsvöttn volcano is ready to erupt2.

The last eruption of this volcano occurred in 2011, which was a large and powerful VEI 4 eruption. The average frequency of eruptions over the last 1100 years is 1 eruption per 10 years.

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