Inclusions Provide Clues to Deep Processes

Crystallization of ascending magma may affect the style of volcanic activity. Pockets of melt incorporated into crystals provide windows on processes that occur several kilometres below the Earth's surface.

Scientists studying Mount St Helens lavas say they have gained new insights into the trigger mechanism of explosive eruptions. Their analysis of tiny glassy features trapped in ejected rocks provides novel information on how molten material behaves deep underground. Writing in Nature, Dr. Jon Blundy of Bristol University, Dr. Madeleine  Humphreys of Cambridge, and Dr. Katharine Cashman from the University of Oregon, Eugene, looked at a range of material thrown out from Mount St. Helens in the 1980s, and from more recent events at Shiveluch in Russia. The scientists examined tiny inclusions (phenocrysts) to determine what conditions must have been like when they formed deep underground. The samples contained small glassy inclusions - minute droplets of once molten material that came up with the rising magma. This work has established that as magma approaches the surface and pressure decreases, crystallization occurs, along with a substantial increase in temperature, up to 100 degrees centigrade (212 F). This is surprising, because crystallization is traditionally assumed to be associated with cooling. "It is the novel twist on this study: that magma, as it rises up, crystallizes and gets hotter. That's something which could have been anticipated from thermodynamic ideas but has never previously been shown" said Dr. Blundy. 

Most volcanic activity on land occurs above subduction zones, where one tectonic plate dives beneath another. When a subduction-zone volcano erupts, magma containing abundant dissolved H₂O ascends from a reservoir 8–14 km below the Earth's surface: it is the fate of H₂O vapour bubbles boiling out of the rising melt that largely determines whether the magma emerges with a bang or a whimper. Mount St. Helens displayed an impressive diversity of eruptive styles in the early-to-mid 1980s. Magma intrusion produced a bulge on the volcano's flank between March and May of 1980; eruption intensity peaked on 18 May with a sustained, explosive eruption of ash and pumice; and intermittent brief explosions and non-explosive lava extrusions characterized activity for the subsequent half-dozen years.  This variety of eruptive products made the current study possible by providing lavas erupted under a variety of conditions. 

Dr. Blundy said that had magma that was thrown out in an eruption a month before the spectacular May 18, 1980 eruption of Mount St Helens been analyzed using this technique, it would have revealed that the magma was under exceptional pressure.  This information could have been used to inform civil authorities of the potential for an extremely violent eruption.   

If ascending magma is able to heat itself up by crystallizing it may provide an important trigger for eruption without the need to invoke an extraneous heat source such as a shot of hotter magma from deep below the surface. The new findings also suggest the possibility that volcanic crystals grow in response to decompression on an unexpectedly short timescale of several years rather than centuries, a time period during which volcanoes can be effectively monitored.

Based on: Nature 443, 76-80 (7 September 2006)  and Geology 33, 793–796 (2005). 

For a very readable interpretive piece based on the Blundy et al.  Geology paper see:

Julia E. Hammer, Nature 439, pages 26-27 (5 January 2006)