Research reported in Science this week shows that rocks erupted from the Mount St Helens volcano in 1980 preserve a remarkable record of the goings-on beneath the volcano in the period prior to that eruption.

Professor Jon Blundy and his PhD student Kim Berlo from the Earth Sciences Department at Bristol University demonstrate that monitoring the isotopic content of gases being emitted from the volcano right now might predict whether the next eruption will be so catastrophic, and when it might occur.

The team identified a subterranean reservoir full of magma at around 7 km depth that had been steadily shedding gas for at least five years prior to the eruption. Some of this gas then accumulated in a more shallow and short-lived reservoir around 4 km beneath the volcano. It is the expansion of these gas bubbles as they rise up that ultimately drives volcanic eruptions.

Blundy and his team showed that the magma which erupted explosively in May 1980 came largely from both the deep and shallow reservoirs, while subsequent, more gentle eruptions in 1980 came exclusively from magma trapped at shallow levels. Clearly there is a link between the storage depth of the magma and its eruptive style.

Furthermore, using short-lived radioactive isotopes, they demonstrate that the process of gas transfer from deep to shallow reservoirs must have occurred just weeks before the eruption.

Mount St. Helens recently cleared its throat with a series of small steam and ash eruptions. But it is unclear whether this activity will evolve into a more substantial eruption such as the explosion in 1980 which removed 400 metres off the top of the volcano and spewed ash over an area of more than 56,000 square kilometres.

Professor Blundy said: 'We have shown there is a link between the storage depth of magma and the explosiveness of an eruption. This suggests that monitoring the abundance of short-lived radioactive isotopes above restless volcanoes could be a useful tool for predicting the style of the next eruption. It might also provide clues as to when the next eruption will occur'.

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