London, Jan 13 (IANS): As they prepare to erupt, volcanoes shift their base, says an interesting study that paves the way for volcanologists to forecast future eruptions.

A Global Positioning System (GPS) instrument fixed on an Icelandic volcano known as Grímsvötn volcano showed the ground shifting noticeably just an hour before the eruption.

This Icelandic volcano spurted out a 20-km high ash plume in May 2011, temporarily grounding flights in parts of Britain.

The GPS data, streamed in real time back to volcanologists, revealed not only that the eruption was imminent but also its likely size, said the study published in the journal Nature Geoscience.

"A GPS site can tell you not only that there’s unrest at a volcano, but that it’s about to erupt and then how high its plume will be,” said Sigrún Hreinsdóttir, a geophysicist at the University of Iceland.

Knowing that an eruption is about to occur helps emergency officials to prepare for a disaster by closing roads or evacuating nearby residents.

Knowing how high a volcano’s ash plume may reach helps airlines to plan for whether they need to re-route flights, or even close airports, said the study.

The scientists translated the GPS-measured ground movement into changes in pressure inside the chamber.

That, in turn, was linked strongly with the height of the ash plume that came from the eruption.

“Seismic instruments can detect an impending eruption because earthquakes usually come fast and furious right before such an event. But the GPS data can only hint at the actual size of the eruption to come,” added Hreinsdóttir.

The work could prove useful at remote volcanoes, such as those in Alaska's Aleutian Islands, where there are few webcams watching to see whether an eruption is occurring.

Pilots flying across the North Pacific often have to guess whether there is an ash plume they need to avoid.