Friday, July 25, 2008

Volcanoes may not be fed by magma 'mushrooms'

A new simulation shows that magma rising under the surface of a volcano may not be mushroom-shaped

THE plumes of hot magma that fuel the volcanism of "hotspots" like Hawaii and Iceland have long been thought to be efficient conduits of Earth's fiery contents. Yet it seems they can be rather lacklustre on their way to the surface.

We traditionally picture the plumes of hot magma that rise through the mantle as mushroom-shaped with a thin stalk feeding a bulbous head, or hotspot, beneath the crust. However, seismic imaging in Iceland reveals a patchy structure without a stalk, leading some researchers to suggest there are no plumes at all.

Ichiro Kumagai and colleagues at the Paris Institute of Earth Physics in France reckon they can explain these patchy structures. They created plumes by heating the base of a tank containing sugar syrups of varying densities, to simulate the composition of the mantle. The densest material was heated just enough to rise and create the core of the plumes, but as it rose it also cooled, so its density increased once more to match or slightly exceed that of the surrounding material. This caused the core to either stall or sink, while the less dense material separated and continued rising (Geophysical Research Letters, in press).

John Brodholt of University College London says the model "seems to explain some of the observations used to argue that plumes do not exist".

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