School of Earth Sciences

Magmatic Degassing: Bubbles, Crystals and Eruption Dynamics

Supervisors: Dr H M Mader, Professor K V Cashman and Professor J D Blundy

Figure 1: Mount St Helens volcano two years after its May 1980 eruption. The blast in that eruption was so powerful that it destroyed the top 400m of the volcano, leaving a crater over 3km wide. This photograph (courtesy Lyn Topinka, USGS) shows the crater with the growing lava dome over the vent from which a steam plume is rising.

Background

As magmas rise to the surface during an eruption the reduction in pressure causes degassing which in turn leads to the growth of bubbles and crystals. Significant progress has been made on understanding the evolution of the bubble and crystal assemblage during decompression (see e.g. Cashman and Blundy, 2000). These syn-eruptive effects have a pronounced effect on the rheology (i.e. the viscosity) of the erupting magma which will change with depth as the bubble and crystal assemblage evolves. Recent work allows us to quantify the effect of bubbles and crystals on the rheology (see e.g. Llewellin et al 2002, Mueller et al 2010).

Figure 2: Backscattered electron image of the Mt St Helens 18th May 1980 gray pumice. Glass and plagioclase microlites are gray, silica phases are dark gray, pyroxene and oxide microlites are white, and vesicles are black. Scale bar = 10 µm. Ex: Cashman and Blundy (2000).

The Project

In this project, we will study the evolution of the rheology with depth. We will do this by considering the kinetics of degassing at Mt St Helens as a specific case study. This volcano has been well-studied and so a lot of background information is available. From our knowledge of the petrology we can determine the degassing and hence the bubble and crystal growth. This information can then be used to generate a rheological model. The implications of the changing rheology with depth will be explored by including it in existing numerical models of explosive volcanic eruptions.

Training

The student will become involved in experimental petrology and rheological measurements and numerical modelling of eruption dynamics. One or more fieldtrips for sample collection is envisaged. The balance of the different components can be altered to some extent to suit the background of the student.

Candidate

This project would suit a student with a first degree in a physical science or mathematics and a desire to develop a range of different skills (field, lab, numerical).

References

Cashman and Blundy (2000) ‘Degassing and crystallization of ascending andesite and dacite.’ Philosophical Transactions of the Royal Society A 358, 1487-1513, doi:10.1098/rsta.2000.0600.

Mueller, Llewellin and Mader (2010) ‘The rheology of suspensions of solid particles.’ Proceedings of the Royal Society A, 466, 1201-1228, doi:10.1098/rspa.2009.0445, 2010.

Llewellin, Mader and Wilson (2002) ‘The rheology of a bubbly liquid.’ Proceedings of the Royal Society, Series A, 458, 987-1016.