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Modeling the Evolution of Effusive Silicic Eruptions



Figure 2: Fit of the model to data from Mount St. Helens. Left: dome-growth data. Right: GPS data from seven stations. Further work should allow us to better fit the GPS timeseries.

This project focuses on the geophysics of effusive silicic volcanic eruptions, with a special emphasis on the 2004-08 eruption of Mount St. Helens, Washington. Mount St. Helens began to erupt in 2004 with very little precursory seismicity or ground deformation, which gave public officials little time to prepare for a potential eruption. We hope that by better understanding these events we can improve our ability to forecast eruptions at other silicic volcanoes around the world.

Present work focuses on the development of a unified model of the evolution of an effusive silicic eruption through time, involving the rise of magma from a deep chamber through a conduit to the surface and growth of a lava dome. As the magma rises to the surface its pressure drops and gas bubbles are exsolved from the melt. This, along with the formation of crystals, results in a dramatic increase in the viscosity of the magma, and the formation of a stiff plug of rock which is extruded from the volcano like very hot toothpaste from a tube. Modeling this behavior, and relating it to deformation of the surrounding host rock, requires solving coupled equations representing mass and momentum balance, with magma rheology and gas exsolution. We run the model many hundreds of thousands of times using different combinations of initial conditions (such as chamber pressure and size) in order to better understand how different model parameters affect the evolution of the eruption.

One of the advantages of such a model is that we can constrain it using many different kinds of geophysical data; for example, since this model predicts rates of dome growth as well as ground deformation simultaneously, it can be constrained by both types of data, and this allows us to make better inferences about subsurface magma properties. Also, since the model couples realistic magma physics and geometries to surface deformation, we are able to move beyond some of the simple deformation source models currently in use. This is becoming increasingly important as advances in our ability to monitor ground deformation at erupting volcanoes reveal the limitations of our existing models.

Preliminary results for Mount St. Helens indicate a magma chamber volume of approximately 5-15 km3, of which ?0.1 km3 erupted in the interval 2004-2008. The model also helps us to answer the question of whether or not there was recharge into the chamber during the eruption; current results suggest that there may have been, although this is not yet well constrained. Future work will help us to better answer these and other important questions about the eruption of Mount St. Helens, as well as at other volcanoes around the world.