Using block and ash flow deposits to infer lava dome collapse dynamics and resultant hazards

 

PhD Candidate: Thomas Johnson

Supervisors: Dr Claire Harnett, Dr Pete Rowley (University of Bristol), Dr Nick Varley (Universidad de Colima)

Funded by: Irish Research Council Postgraduate Studentship

Abstract:

Volcanic domes grow when lava is too viscous to flow away from the vent [1]. They are inherently unstable structures that can undergo collapse, leading to block and ash flows (BAFs) that devastate communities surrounding a volcano, as well as affecting eruptive dynamics. These currents are a subset of pyroclastic density currents (PDCs), and carry large blocks or fragments of the lava dome within a fine ash-laden flow. These hot and voluminous currents can ignore topography and obstacles, and are too fast to escape from [2].

The differences in deposits between BAFs and other PDCs result from differences in transport dynamics, and that research into wider PDC behaviour typically does not apply to BAFs. Whereas other PDCs are typically fed by fountaining ash columns, BAFs are supplied from a rockfall-like process, with a lack of explosive energy, and a starting material which is not saturated with gases [3]. Specifically, we do not understand the origin of fine ash in these flows, or how such large blocks of dome material are transported by the current. One key question is whether ash comes from simple abrasion of larger blocks, or instead from dissolved gases from deeper within the dome. How the grain size distribution evolves during the flow will fundamentally control the velocity and runout of the block and ash flow, determining hazard to surrounding populations. Our inability to perceive the internal dynamics of these currents in the field is a major obstacle to reducing associated risks.

By combining laboratory experiments, field work, and particle analysis, this project explores the relationship between deposits and dome collapse mechanisms. This will allow us to (i) explore BAF current dynamics; (ii) infer important characteristics of growing domes from resultant collapse deposits; and (iii) create more accurate hazard assessment and mitigation applicable to volcanic domes worldwide.

References:

Calder, E.S., Lavallée, Y., Kendrick, J.E. and Bernstein, M., 2015. Lava dome eruptions. In The encyclopedia of volcanoes, (pp. 343-362). Academic Press.

Branney, M.J. and Kokelaar, B.P., 2002. Pyroclastic density currents and the sedimentation of ignimbrites. Geological Society of London.

Varley, N.R., 2019. Monitoring the recent activity: understanding a complex system. In Volcán de Colima: Portrait of a Persistently Hazardous Volcano, pp.159-193. Springer