GPS data from the eruption of Grímsvötn opens a new avenue in the forecast area.

The Grimsvotn , an Icelandic volcano under the Vatnajokull glacier, propelled in May 2011 a plume 20 km . height , following a basaltic magma eruption, equivalent to 0.27 km³ of dense rocks ... and just give to the volcanologists a possible tool for predicting future eruptions .

                 The eruptive plume of Grímsvötn in May 2011 - photo Björn Oddsson
 

One hour prior to the eruption , GPS installed on his sidewall measured changes of the ground level. This tool is well known to be a means of monitoring the impending eruption ... A geophysicist of the Icelandic Reykjavik University , Sigrún Hreinsdóttir , established a correlation between the measurements of ground deformation , pressure changes within the magma chamber and the height of the eruption plume emitted .

The Grímsvötn is Iceland 's most active volcano, with an average of an eruption every 10 years in the 20th century , but it remains difficult to monitor because of its location under an icecap . The team of Sigrún Hreinsdóttir has positioned its GPS station on a rocky outcrop, kept free of ice by a pipeline channeling geothermal heat.

                 Location of Grímsvötn under the Vatnajökull in Iceland and seismicity in 2011

                  Grímsvötn - 21.05.2011 - Photo Halldora Kristin Unnarsdottir / AP

Grimsvotn - May 22, 2011 - the ash cloud and its dispersion - doc.NASA GSFC Jeff Schmaltz MODIS Land Rapid - Response Team.

Geodetic measurements before and during the eruption, allow an interpretation of the deformation of the volcano as a drop in pressure in the magma chamber located at about 1700 m. depth. The estimate of the magma discharge and the evolution of the plume associated is made by differentiation of the fall of co- eruptive pressure over time . The time between the start of the pressure drop and the start of the eruption is about 60 minutes with a 25% change in the total pressure before the eruption . As Hreinsdóttir said, " If you have a ball full of water and you highly compress, the height reached by the water coming out of the ball is in correlation with the compression force ... it seems simple , but we don't have not been able to show before . Seismometers can detect an eruption in preparation, but only the GPS data are able to give us a reference to its size. "

The biggest challenge remains to install the GPS equipment correctly, and  Hreinsdóttir wants more validate its discovery by observing another Icelandic eruption. To be continued !

Estimate position of the ash cloud from the eruption of Grímsvötn for 24-25.05.2011 - Red Aviation Code. - doc.VAAC London / IMO .

Geodetic observations in real time can be useful as a timely warning of an eruption, and to master the evolution of the volcanic plume and its impact on air traffic ... remember the disruption caused by the eruption of Eyjafjallajökull .  

This type of installation would be wise to remote volcanoes , as in the Aleutian arc on the road of the far north , but the decrease of budgets of many laboratories is not likely to afford these costly investments .

Sources :

- Institute of Earth Science - “Volcanic plume height correlated with magma pressure change at Grímsvötn Volcano, Iceland” by Sigrún Hreinsdóttir et al. / Nature Geoscience 12.01.2014

- Nature - Volcanoes shift before they sprew - GPS data from 2011 Icelandic eruption hint at new ways to forecast hazards. - link

- CIMSS blog - Eruption of the Grimsvötn volcano in Iceland ( 2011) - link