Grimsvötn volcano (Iceland): accelerating rates of subsidence increase likelihood of glacial flood and perhaps an eruption

 

The ice sheet on top of the volcano continues to subside and does so at accelerating rate: Compared to a week ago, the surface has sunk by 4 meters, but more than half of that occurred during the past 48 hours, and it is not clear how much is yet to come.

In any case, it suggests that more and more melt water normally trapped above the summit area is now flowing away to different locations further at the side of the ice shield. This makes a glacial flood very likely in the next days - once enough water has accumulated beneath the rims of the glacier, it can lift it and surge out, and/or melt and erode itself a channel through its bottom, to exit in a massive, violent glacial flood known as jökulhlaup (literally "glacial run").

According to geologist Björn Oddsson from Civil Defense in an article on grapevine.is, "melt water is accumulating under an ice cave that lies at the top. There's a GPS measuring device over the ice cave which indicates the situation with the water. When the water accumulates, the ice cave rises." He estimated that the total volume of water involved was about one cubic kilometer.

There are concerns that the pressure changes on the volcano involved by sudden changes in overlying weight also increase the likelihood of the volcano, by many thought to be "overdue".

Update(12/2):

As we reported in the latest report, the ice sheet on top of the volcano continues to subside.

Today, on 1 December, the Icelandic Meteorological Office (IMO) reported that GPS measurements confirmed the surface subsidence by 10 meters since it was measured highest. A gradually increasing water level is now in progress that has been occurred in Gígjukvísl yesterday and last night.

Hydrologists from the IMO continue to monitor meltwater accelerating rate in the river and expect a glacial flood very likely in the next few days Gígjukvísl, which may turn to develop into dangerous jökulhlaup (literally "glacial run").

Glaciologists at the Institute of Earth Sciences at the University of Iceland have published a forecast model that assumes that the maximum flow in this event will be reached around next weekend or the beginning of next week. Based on the latest data, the maximum discharge rate is expected to be about 4000 m3/s.

Source: Icelandic Meteorological Office volcano activity update 1 December 2021

3/12 Update:

Grimsvötn volcano (Iceland): subglacial meltwater flood started while top of glacier subsides 15 m

The glacial flood (jökulhlaup) from Grímsvötn volcano has started. Meltwater has made its way under the glacier to its exit at the Gígjukvísl outlet river, whose water levels are now quickly increasing and were at approx. 1200 cubic meters per second earlier today. Scientists expect the flood to go on for days and reach its maximum in 4-6 days at values of around 4,000 cubic meters per second.
The modeled water discharge rate to release a total of about 1 cubic kilometer can be seen in this embedded graph:

Worries the flood might trigger volcanic eruption
On top of the volcano, the ice sheet has continued to drop and reached about 15 m in total subsidence. A crack has formed in this area, releasing fumarolic gasses and steam from the volcano underneath. Scientists are worried that the massive drop in pressure caused by the release of melt water might be able to trigger a volcanic eruption.
Einar Hjörleifsson from the Icelandic Meteorological Office explained in an interview with mbl.is that he does not rule out a volcanic eruption in the wake of the flood: "When all this water is pouring out, there is an immense pressure drop," he said, "and that is a scenario that we keep an eye on and monitor with all of our instruments."
The second most recent eruption of the volcano in 2004 did follow a glacial flood and is thought to have been triggered by it.

https://m.youtube.com/watch?v=uygaiN9zHmA

Update(12/4):

The meltwater rate has been increasing since the last update. The IMO's hydrologists measured 1600 cubic meters per second at 10:15 and 12:20 local time today in Gígjukvísl.
The latest measurements confirm estimated forecasts and if continue to be in line with them, the glacial flood will likely reach its peak on Sunday. Based on the latest data, the maximum discharge rate is expected to be about 4000 m3/s.
The ice sheet has continued to drop and reached already about 27 m in total subsidence.
Higher-than-usual gas emissions levels have been detected at the glacier rim of the volcano.
Recent eruptions at the volcano occurred every 5-10 years and based on current measurements and observations, volcanologists agree that an impending eruption is highly expected.
Source: Icelandic Meteorological Office volcano activity update 3 December 2021

https://m.youtube.com/watch?v=x0cDsTdhJis

Update(12/7):

The Icelandic Meteorological Office (IMO) raised the alert level of the volcano orange, as chances that it might erupt have increased.

This morning, a magnitude 3.6 earthquake occurred at shallow depth at the eastern summit caldera rim of the glacier-covered volcano. The recent glacial flood has more or less ended and resulted in a subsidence of 77 meters of the ice sheet above the summit. Whether or not the resulting significant pressure drop on the volcano's system will trigger an eruption or not is likely something that will be seen in the next few days.

 

Update Dec 9:

The Icelandic Meteorological Office (IMO) lowered the alert level of the volcano back to yellow, as relatively few quakes were detected in the monitoring area and decreased again to normal values. It happened after almost 2 days when the observatory raised the alert level to orange due to elevated seismic activity.
The IMO's earthquake hazards coordinator Kristín Jónsdóttir explained: "When the seismicity increased we needed to react as it could have been a typical precursor of a volcanic eruption. We need to react to any relevant changes in the volcano activity. In this case it was likely that an eruption would have been triggered by the flood, and sometimes the time between the two events is not that long so we needed to be ready. This scenario, that a flood will trigger an eruption is becoming less likely as time passes."
The ice sheet has continued to drop and reached already about 80 m in total subsidence.
The detected precursor is very short which is why an explosion may come as a surprise and need to be monitored closely.
Source: Icelandic Meteorological Office volcano activity update 8 December 2021

12/13 Update:

Kristín Jónsdóttir from the Icelandic Meteorological Office (IMO) notified on her Twitter account that the ice sheet has continued to drop and reached about 60 cm in total subsidence.
The glacial flood (jökulhlaup) from Grímsvötn volcano has likely started. New measurements reveal an estimated glacial flood peak in the next 5-10 days. The origin peak on 5 December didn't happen.