Eruptions explosive and effusive

Some rhyolitic volcanos erupt abruptly and violently, while others are far more sedate in their eruptive behavior. Ludwig-Maximilians-Universitaet (LMU) in Munich geoscientist Professor Donald Dingwell has now discovered why that is so. The chemistry of rhyolitic magmas is the key.

Many volcanic eruptions are highly dramatic, others are comparatively unspectacular. So-called effusive eruptions spew out relatively fast-flowing lava which contains little gas and little crystalline material, and exits the vent at a comparatively stately rate. Then there is the other kind – the explosive eruptions, which are far more impressive, and potentially much more destructive. In this case, the lava is much more viscous and is normally rich in crystalline materials and gases. The latter classes of constituents together generate high pressures within the magma chamber, which are generally released abruptly in a sudden explosion. This type of eruption can propel huge rock fragments from the vent, and expansion of the superheated gases can thrust enormous amounts of ash kilometers high into the atmosphere, where they can remain for long enough to influence global climate.

So-called rhyolitic magma, which is particularly viscous, is thought to be responsible for the most powerful volcanic eruptions worldwide. Rhyolitic magma is the volcanic equivalent of, has a similar chemical composition to granite. Rhyolite magmas, however, have one puzzling characteristic: They can take on a highly viscous form, producing violent explosions, but can also emerge as much less viscous lava in eruptions follow a much less threatening and more protracted course. A team of researchers led by Professor Donald Dingwell, Director of the Department of Earth and Environmental Sciences at LMU, has now uncovered the mechanism that determine the viscosity of rhyolitic magmas and therefore governs the type of eruption they give rise to. The new findings are described in a paper that has just appeared in the leading journal Nature.

Generally, volcanologists can explain the differing viscosities of magmas on the basis of their temperatures and their relative contents of crystalline phases, gas and water. However, this model fails in detail in the case of rhyolitic magmas, as their viscosities vary independently of the values of the above-mentioned parameters.

Dingwell and his colleagues have now shown experimentally that the viscosity of rhyolitic lavas is directly dependent on their overall chemical composition to an unexpected degree which collates with global eruption styles. The Munich researchers analyzed rhyolite magmas that represented the complete spectrum of rhyolitic rocks found in the Yellowstone volcanic system, which provides paradigmatic examples of both effusive and explosive rhyolite-based volcanism. The results showed that minor variations in the ratios of particular chemical constituents had a surprising impact on the viscosity of the magma – and therefore on the destructiveness of its eruptive behavior.

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