There may be no such thing as a dormant volcano, according to scientists, who say that many could in fact be reawakened in a period of months.
It has long been thought that once a volcano's magma chamber has cooled down, it stays dormant for centuries before it can be remobilized by fresh magma.
But Alain Burgisser of the Orléans Institute of Earth Sciences, together with a US researcher, has tested a theoretical model on two major eruptions and found that this process can take place in just a few months. The findings should lead to a reassessment of the dangerousness of some dormant volcanoes, he says.
According to the team's mathematical model, reheating takes place in three stages. When fresh hot magma rises from below and arrives beneath the chamber, it melts the viscous magma at the base of the reservoir. This freshly molten magma therefore becomes less dense and starts to rise through the chamber, forcing the rest of the viscous mush to mix.
This mixing process, they say, enables the heat to spread through the chamber a hundred times faster than volcanologists had predicted.
The two researchers tested their model against the eruption of Mount Pinatubo in the Philippines in March 1991 and the ongoing eruption of the Soufriere Hills volcano in Montserrat, in the Caribbean.
In both cases, seismic shocks preceding the eruption indicated the arrival of fresh magma beneath the cooling reservoir. By taking account of various known physical parameters of the two volcanoes - such as magma temperatures, size of the chamber and crystal concentration inferred from the study of magmas - the team was able to pretty much reproduce the time intervals between these warning signals and the eruptions.
For Pinatubo, for instance, the mathematical model predicted that the underlying chamber could be reactivated within 20 to 80 days, whereas conventional theory gave a figure of 500 years. In reality, there was a gap of two months between the tremors and the eruption of the volcano.