Published online 16 September 1999 | Nature | doi:10.1038/news990916-11

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A new angle on volcanoes

A large number of the world's volcanoes are located near the boundaries between tectonic plates, where one part of the Earth's outer crust slides beneath another. In many places, where pieces of oceanic crust override each other, chains of volcanoes are formed. For example, the volcanoes of Japan have formed where the Pacific and Philippine plates slide beneath the Asian plate. Volcanoes such as Mount Fuji have their origins 80-150 km beneath the surface, where rocks from the floor of the Pacific Ocean descend into the denser mantle rocks beneath. Kenji Mibe of the University of Tokyo, Tokyo, and colleagues have duplicated in the laboratory the temperatures and pressures found at the root of these volcanoes, to try and understand what makes them form where they do.

Analysis of rocks from these volcanoes shows that they arise from partial melting of dense mantle rocks, not of the ocean crust descending into the Earth. So, why do the mantle rocks melt to form these volcanoes? One leading hypothesis is that the oceanic crust carries water down into the mantle. Wet rocks melt at lower temperatures than dry, so the water carried down with the oceanic rocks enables the mantle to melt, forming the molten magma which rises to the surface, creating a volcano.

However, this still leaves unresolved questions. In particular, why are the volcanoes confined to a band parallel to, but separated from, the zone where the two plates meet on the surface? The onset of volcanism happens at a more-or-less uniform distance from the plate boundary, so the melting must be triggered by something that takes place at a uniform depth.

Writing in the 16 September issue of Nature1, Mibe and colleagues suggest that the controlling factor is the ability of water to move through the rock. To check this hypothesis, they sealed mixtures of magnesium silicate and water in platinum tubes and subjected them to temperatures between 800 and 1,000 °C and pressures of 3-5 gigapascals (30,000-50,000 atmospheres). At these temperatures and pressures, the magnesium silicate re-crystallized into forsterite, a mineral typical of the mantle.

The water remains at the boundaries between grains of forsterite, forming either isolated droplets or a network of channels rather like the water in foam. In other words, the water is either trapped in droplets, or can move freely through a network through the rock. Whether droplets or a network forms depends on the temperature and pressure, which alter the properties of the rock-water mixture just as soap changes the properties of air-water mixtures.

The researchers measured the angle at corners of water pockets with an electron microscope. This angle depends on the surface properties that control the formation of droplets or fluid networks. The switch from a network to droplets takes place when the angle exceeds 60°.

Mibe and colleagues show that water cannot move at low temperatures and pressures, but becomes mobile at higher temperatures and pressures. The water begins to move at temperatures and pressures corresponding to those in the region where the lavas of island arc volcanoes have their origin. They suggest that the opening of these pore networks allows water to move from the oceanic block into the rocks of the mantle, lowering their melting point so that volcanic lava is formed. Great volcanoes may have their origins in the opening of minute channels in rocks deep beneath the ground. 

  • References

    1. Mibe, K. et al. Control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge. Nature 401, 259 - 262 (2001). | Article | ChemPort |