Indications that glacier flood may have begun
The GPS equipment measures changes in land elevation. If a flood of volcanic meltwater has begun under the volcano, it usually takes quite some time for it to reach the coast. A large jökulhlaup can cause significant damage to infrastructure.
Oddur confirmed with RÚV that civil protection authorities have been in contact with the police to give advance warning of a possible flood, though no visual signs are yet visible.
Grímsvötn is Iceland’s most active volcano and has erupted close to 100 times since the age of settlement—including 13 times since 1902. It is part of a volcanic system that stretches over 100 kilometres, all the way south to the Laki Craters. Melted glacier water collects in a lake in the crater of Grímsvötn and when it reaches a critical mass, it forces the ice up and floods out over Skeiðarársandur in South Iceland.
The largest jökulhlaup events in that area cause significant damage. In 1996, for example, parts of the Route 1 highway were washed away. Most floods are small, however.
The same is true of Grímsvötn itself, which is known for its regular small eruptions, mixed in with occasional very significant events. The Skaftáreldar eruption, for example, affected weather patterns across the whole northern hemisphere in the 18th century. The most recent Grímsvötn eruption was in 2011, which brought significant ashfall to towns and villages—which is unusual for Grímsvötn eruptions.
Grímsvötn is known for explosive eruptions from its main crater under the glacier, though there are also occasional examples of lava eruptions. The most recent example of a lava eruption from the system was the Skaftáreldar in 1783 to 1784, which caused a famine. That eruption lasted eight months, gushing lava from a 27-kilometre-long fissure and covering 600 square kilometres.