Danger reassessment of some dormant volcanoes needed
It has traditionally been accepted that once a volcano's magma chamber had cooled down this was the sign that it had become dormant and that it was likely to remain so for many centuries.
But research published in the latest edition of Nature casts doubt on this hypothesis. A theoretical model developed by Dr Alain Burgisser of the Orleans Institute of Earth Sciences, and Professor George Bergantz from the US, predicts that reawakening of a magma chamber could occur within a period of just a few months. When this theory was put to the test on two major eruptions, the old hypothesis was completely overturned.
In order to understand the theory it is necessary to understand what happens beneath a volcano. Several kilometres below the volcano is a large reservoir of molten rock known as magma. According to accepted theory this magma cools down to form an extremely viscous mush until such time as fresh magma from deep inside the Earth reawakens it and heats it up through thermal contact.
Since many of these magma chambers can be as large as several hundred cubic kilometres, it can take several hundred or even thousand years to wake a volcano from its dormant state.
According to the mathematical model developed by Dr Burgisser and Professor Bergantz, this process of reheating actually takes place in three stages:
I. Fresh molten magma rises from below and arrives beneath the chamber and melts the viscous magma at the base of the reservoir.
II. The freshly molten magma therefore becomes less dense and starts to rise through the chamber, forcing the rest of the viscous mush to mix.
III. The mixing process enables the heat to spread through the chamber a hundred times faster than had previously been predicted.
As a result, depending on the size of the chamber and the viscosity of the magma it contains, activity can be rekindled in as little as a few months.
The two researchers tested the validity of their model against both the eruption of Mount Pinatubo in the Philippines in March 1991, which caused 100 fatalities and displaced two million people, and the ongoing eruption of the Soufriere Hills volcano on the Caribbean island of Montserrat. In both cases the eruptions had been preceded by seismic shocks, indicating the arrival of fresh magma below the cooling reservoir.
By taking account of various known physical parameters of the two volcanoes, such as magma temperatures, size of chamber and crystal concentration inferred from the study of magmas, the two scientists succeeded in approximately reproducing the time intervals between these warning signals and the eruption that followed.
In Pinatubo's case, according to conventional theory it would have taken 500 years to remobilise the underlying chamber. The mathematical model predicted that 20 to 80 days were sufficient and in reality there was a gap of two months between the first tremors and the eruption.
This research has led to calls for a reassessment of the danger of some of these dormant volcanoes that experts have previously regarded as being completely benign.
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