School of Earth Sciences

Magma microtextures and fragmentation

Supervisors: Kathy Cashman and Alison Rust

The havoc caused by the 2010 eruption of Eyafjallajokull in Iceland highlights the importance of understanding the generation of volcanic ash, and reliable methods of determining the location and concentration of volcanic ash in the atmosphere. Of particular importance is the formation and dispersal of the fine ash as it remains suspended for longest and so has the most distal effects; also the finest ash can cause respiratory and other health problems in addition to aviation hazards. Explosive eruptions and ash formation are driven by rapid expansion of gas bubbles that exsolve from magma as it rises and decompresses inside volcanoes. However, the details of how the magma breaks apart and what controls the size distribution of the fragments are still poorly understood and will be addressed in the proposed doctoral studentship. The focus is determining how the size and abundance of bubbles and crystals affect magma fragmentation by detailed examination of natural ash and pumice from several volcanic eruptions, and by laboratory experiments of fragmentation of analogue magma.

Secondary electron image of a micropumice clast from the distal fall deposit of Mount St. Helens. Scale bar is 10 µm. Ash sample is from Steve Carey.

Schematic diagram of fragmentation within a volcanic vent (left). Graph on right shows the total grain size distribution for Mount St. Helens (Carey and Sigurdsson 1982) and, in red dots, the bubble size distribution of a pumice clast from the same deposit (Klug and Cashman, 1994)

References

• Carey, S. N., and H. Sigurdsson (1982), Influence of Particle Aggregation on Deposition of Distal Tephra from the May 18, 1980, Eruption of Mount St-Helens Volcano, J Geophys Res, 87(Nb8), 7061-7072.
• Kaminski, E. and Jaupart, C., 1998. The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions. Journal of Geophysical Research 103, 29759-29779.
• Klug, C., and Cashman, K.V., 1994. Vesiculation of May 18, Mount St. Helens pumice. Geology 22, 468-472.
• Rust, A.C. and Cashman, K.V. in press. Permeability controls on expansion and size distributions of pyroclasts, doi:10.1029/2011JB008494