Ice sheet in Grímsvötn subsiding, could lead to an eruption, Iceland

The Scientific Advisory Board for the Civil Defense met on November 24, 2021, to discuss changes measured at Grímsvötn. GPS measurements are showing that the ice sheet has started subsiding which indicates that a flood from Grímsvötn is likely starting. There are past examples of Grímsvötn eruptions starting following such floods.

The ice sheet has subsided about 60 cm (23.6 inches) by November 24 in the last few days and the speed of subsidence has increased since November 23. These measurements indicate that it is most likely that water has started to leave from Grímsvötn lake and that Gígjukvísl will flood, the Icelandic Met Office (IMO) said.¹

The ice sheet has continued to subside evenly throughout the night and into the morning of November 25. IMO's GPS instruments show that it has sunk about 25 cm (9.8 inches) since 10:00 UTC on November 24. There are no signs of increased water level, electrical conductivity or gas in Gígjukvísl at this time.

Based on observations of past floods, it is anticipated that flood water will be exiting the glacier edge in the next 48 hours and the flood will peak 4 – 8 days after that.

At this moment no increase in electrical conductivity has been measured in Gígjukvísl which is the clearest sign that Grímsvötn floodwaters have exited from under the glacier. IMO also has gas monitors along Gígjukvísl which will indicate if flood water has reached those points in the river.

The maximum discharge anticipated from this flood is calculated to be around 5 000 m³/s (176 000 ft³/s). This size of flood will most likely not affect the infrastructure in the area such as roads or bridges. However, these forecasts are uncertain at this early stage, IMO said.

There are past examples of Grímsvötn eruptions starting following a flood.

The loss of the water from Grímsvötn lake reduces the pressure on top of the volcano and this can allow an eruption to begin.

This happened in 2004, and before that in 1934 and in 1922, IMO said.

In 2004 the eruption started three days after the first observations were made of flood onset. There were a series of earthquakes in the days preceding the eruption. No such earthquakes have been measured at this point in time.

According to Benedikt Gunnar Ófeigsson, deformation scientist at the IMO, there is every indication that the Grímsvötn volcano is ready to erupt.²

The last eruption of this volcano took place in 2011 — it was a large and powerful VEI 4 eruption. The average eruption frequency during the last 1 100 years is 1 eruption per 10 years.

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.³

References:

¹ Icesheet in Grímsvötn subsiding – IMO

² Grímsvötn volcano ready to erupt, Iceland – The Watchers

³ Grímsvötn – Geological summary – GVP

Featured image credit: IMO