Ice sheet in Grímsvötn subsided 17 m (55.8 feet) in 9 days, Iceland


The ice sheet in Grímsvötn has subsided about 17 m (55.8 feet) from November 24 to December 2, 2021. This is about 9 days since it started subsiding and running water began to break its way under the glacier.

The Meteorological Office's hydrologists measured the flow in Gígjukvísl at about 11:00 UTC on December 2 which was then almost 930 m3/s (32 850 ft3/s) and the flow has almost tripled in about three days, the Icelandic Met Office said in an update released at 16:00 UTC on December 2, 2021.1

This flow is 10 times the flow of the river compared to the season. 

Electrical conductivity, which indicates the amount of running water in the river, has also increased in recent days and was measured at 272 uS/cm at 13:00 UTC on December 2 and is rising. Gas is measured in small quantities at the glacier tail and is well within the danger limits.

"The latest measurements fit fairly well with the flow forecasts that have been made and assume that the flood will probably reach its peak next Sunday. If there are major changes in the development of the flow from Grímsvötn, it will affect when the flood peak is reached," IMO said.

There are examples of eruptions in Grímsvötn after water flows from there.

"The last time this happened was in 2004 and before that in 1934 and 1922. In 2004, traces of the start of the eruption were seen on October 28 and the eruption began about three days later, or at the end of the day on November 1. 

In the days before, the earthquake in Grímsvötn was a sign that an eruption was imminent. However, no such earthquakes have been detected so far, IMO said.

The last eruption in Grímsvötn took place in 2011, but this time it had run out of Grímsvötn just over six months before. Since 2011 it has run a total of 6 times from Grímsvötn without an eruption.

The eruption has taken place every five to ten years from Grímsvötn, and scientists agree that measurements show that conditions are such that Grímsvötn is ready to erupt. 

However, it is not possible to state that this situation will trigger an eruption, and seismic activity in Grímsvötn needs to be closely monitored, which could give indications that an eruption is imminent.

Geological summary 

Grímsvötn, Iceland's most frequently active volcano in historical time, lies largely beneath the vast Vatnajökull icecap. The caldera lake is covered by a 200 m (650 feet) ice shelf, and only the southern rim of the 6 x 8 km (3.7 x 5 miles) caldera is exposed.

The geothermal area in the caldera causes frequent jökulhlaups (glacier outburst floods) when melting raises the water level high enough to lift its ice dam.

Long NE-SW-trending fissure systems extend from the central volcano. The most prominent of these is the noted Laki (Skaftar) fissure, which extends to the SW and produced the world's largest known historical lava flow during an eruption in 1783.

The 15 km3 (3.6 mi3) basaltic Laki lavas erupted over a 7-month period from a 27 km (16.7 miles) long fissure system. Extensive crop damage and livestock losses caused a severe famine that resulted in the loss of one-fifth of the population of Iceland.2


1 Icesheet in Grímsvötn subsiding – IMO

2 Grímsvötn – Geological summary – GVP

Featured image credit: IMO

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