The icescheet above the volcano has been subsiding in the past days, likely because a pool of glacial melt water has moved away and might soon break out as a glacial flood.
According to information from Iceland’s Met Office (IMO), the subsidence in total is more than 85 cm, with its speed accelerating. So far, however, there are no signs of increased water level, electrical conductivity or gas in the Gígjukvísl outlet river at this time.
A glacial flood can be disastrous for areas in the path of the flow, but it can also trigger eruptions of the volcano as it decreases load on the magma chamber, something which can lead up to the formation of gas bubbles and create conditions allowing it to erupt to the surface.
The Scientific Advisory Board for the Civil Defense met yesterday to discuss changes measured at Grímsvötn and published the following report:
„GPS measurements are showing that the icesheet has started subsiding which indicates that a flood from Grímsvötn is likely starting.
The icesheet has now subsided about 60 cm in the last few days and the speed of subsidence has increased in the past 24 hours. These measurements indicate that it is most likely that water has started to leave from Grímsvötn lake and that Gígjukvísl will flood.
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 flood waters 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 5000 m3/s. This size of flood will most likley not affect the infastructure in the area such as roads or bridges. These forecasts are uncertain at this early stage.
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. 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 preceeding the eruption. No such earthquakes have been measured at this point in time.
IMO continues to monitor Grímsvötn closely and will keep updating this news as things develop.„