Oscillating “plug” of magma causes tremors that forecast volcanic eruptions: UBC research

Media Release | Feb. 23, 2011

Oscillating “plug” of magma causes tremors that forecast volcanic eruptions: UBC research

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Published this week in the journal Nature, the new model developed by UBC researchers is based on physical properties that most experts agree are common to all explosive volcanic systems, and applies to all shapes and sizes of volcanoes.

“All volcanoes feature a viscous column of dense magma surrounded by a compressible and permeable sheath of magma, composed mostly of stretched gas bubbles,” says lead author Mark Jellinek, an associate professor in the UBC Department of Earth and Ocean Sciences.

“In our model, we show that as the center ‘plug’ of dense magma rises, it simply oscillates, or ‘wags,’ against the cushion of gas bubbles, generating tremors at the observed frequencies.”

“Forecasters have traditionally seen tremors as an important – if somewhat mysterious – part of a complicated cocktail of observations indicative of an imminent explosive eruption,” says Jellinek, an expert in Geological Fluid Mechanics. “Our model shows that in systems that tend to erupt explosively, the emergence and evolution of the tremor signal before and during an eruption is based on physics that are uniform from one volcano to another.”

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Jellinek and Bercovici's model offers alternative explanations for many of the features used to formulate these other models. An especially welcome contribution is that the main frequencies produced by wagging are in the 1–5-Hz range, exactly the dominant frequencies observed for most tremor. Importantly, these frequencies are caused by the apparent stiffness of the gas annulus (the spring) and are not related to the dimensions of the conduit. This marks a fundamental distinction between this and previous efforts.

The model also returns similar frequencies for reasonable choices of input parameters such as conduit length, shape or diameter, and is not sensitive to magma composition (andesite, dacite, rhyolite and so on). It also demonstrates that higher frequencies of tremor, up to 7 Hz or more, are produced during explosive eruptions. During eruptions, fragmentation and flow of gases occur in the annulus, causing it to be thinner and stiffer, and hence producing higher wagging frequencies. Such an increase in the frequency of tremor is observed for many eruptions.

There are several limitations to Jellinek and Bercovici's formulation. It may explain only one type of tremor — that during eruptions —and is unlikely to be applicable to deep tremor emanating from around 40 km depth, or tremor caused by hydrothermal boiling. And it does not explicitly address how the wagging system is coupled to the surroundings. Furthermore, the model is simplified to include mainly linear effects: nonlinear effects such as feedback may be relevant in some cases.

Nonetheless, this work provides a fresh perspective on an important and long-standing problem. The basic elements of the model may also provide testable elements to provoke the next generation of field observations.