By Jonathan Amos
Science reporter, BBC News, Norwich
UK-US scientists studying the Mount St Helens volcano say they have gained new insights into what might trigger the most explosive eruptions.
The ability of scientists to forewarn of eruptions is limited
Their analysis of tiny glassy features trapped in ejected rocks has provided novel information on how molten material behaves deep underground.
Combined with surface monitoring, such information could warn if a simmering cone is likely to blow its top.
The joint Bristol-Oregon team's work is reported in the journal Nature.
"We can give a range of probable timescales for eruptions and I know that may be unsatisfactory for those living in the shadow of a volcano, but I would hope that in the future we will be able to refine and combine our techniques to reduce the error-bars on those estimates," said Professor Jon Blundy, from Bristol's department for earth sciences.
The team's study looked at a range of material thrown out from the US mountain in the 1980s, and from more recent events at Shiveluch in Russia.
The samples contained small glassy inclusions - minute droplets of once molten material that came up with the rising magma but whose contents remained unaltered.
The scientists probed these tiny volcanic blobs to determine what conditions must have been like deep underground.
The work has established that as a magma rises and the pressure falls, crystallisation occurs. There is also a substantial increase in temperature.
This is quite a surprise, the researchers say, because crystallisation is usually associated with cooling.
"It is the novel twist on this study: that magma, as it rises up, crystallises and gets hotter. That's something which could have been anticipated from thermodynamic ideas but has never previously been shown."
Analysis of the glasses betrays not only the crystal content, pressure and temperature conditions of the rising magma, but also the chemical composition and water content.
Putting all this information together will allow researchers to model better the complex interplay factors that drive eruptions.
Water content in particular is key. Explosive events are fuelled by the escape of water from the liquid rock to form bubbles.
"If the magma is stored at high pressure, it contains quite a lot of water and has the potential to form a lot of bubbles - more bubbles, a more explosive eruption," explained Professor Blundy.
"If the magma is stored at low pressure, it can contain less water - it has less explosive potential."
Currently, volcanologists will monitor active volcanoes from the surface to follow, for example, their earthquake behaviour and the gases they release.
The droplets record information about conditions underground
This Bristol-Oregon work opens up the possibility for cross-matching that information with data from any later ejected rocks. This might then permit scientists to know what was occurring in the subterranean world just from looking at surface signals.
"For me that's the most exciting new research development for the next 10 years," said Professor Blundy.
"I'd like to work with those who monitor active volcanoes, look at erupted products and try to link those with monitoring signals that were acquired before that magma was erupted."
Professor Blundy was explaining his work here at the British Association's Science Festival. His co-researchers were Dr Madeleine Humphreys, formerly of Bristol University; and Katharine Cashman from the University of Oregon, Eugene.