Volcanic bulge called key clue to volume of ash


Bulging in land that occurs before a volcano erupts points to how much ash will be spewed into the sky, providing a useful early warning for aviation, geologists in Iceland said Sunday.

The clue came from data from GPS sensors placed around the notorious Icelandic volcano Grimsvoetn, they said. Just before Grimsvoetn blew its stack in May 2011, the ground around the volcano started to bulge.

In a brief but violent eruption, it disgorged a 20-km-high plume of ash, equivalent to 0.27 sq. km of material.

By matching the ground deformation with the volume of the ash, the scientists got a snapshot of conditions that prevailed in the magma chamber below the volcano before the eruption.

The magma chamber is a vessel that progressively fills with injections of molten rock. When the pressure becomes too great, the magma is expelled through cracks, forming ash as it cools in its passage through the air.

In the study published in Nature Geoscience, the team, led by Sigrun Hreinsdottir of the Nordic Volcanological Center in Reykjavik, said the magma chamber was 3 km beneath Grimsvoetn.

The signature from the groundswell pointed to a drop in pressure 1.7 km beneath the surface an hour before the eruption as the magma headed upward.

In volcanoes that are under close surveillance, the method could help warn of imminent eruptions and forecast the possible altitude of ash clouds.

A 2011 eruption at the nearby Eyjafjoell volcano led to the world’s biggest airspace shutdown since World War II.