Volcanology

High Temperature Fracture Mechanics of Dome Lava [Standard NERC grant]

Volcanoes that erupt highly-viscous lava domes are amongst the most hazardous, intensely studied and newsworthy of geological phenomena, as the past twenty-five years of activity at Mount St Helens, USA have demonstrated. A key challenge in geological science is how to predict reliably the sudden changes in behaviour that are typical of such domes, from gentle lava effusion to devastating explosive eruptions. Much progress has been made in recent years in studying eruptive behaviour, with the realisation that the material response of magma to applied stresses largely controls eruption mechanisms and the rheology of dome lava changes greatly during shallow degassing and crystallisation. A goal of the current generation of models for dome growth is to identify thresholds in behaviour (e.g. explosive/effusive eruptions, endogenous/exogenous dome growth) that are closely related to the fundamental physical behaviour of whether deforming magma will fracture or flow.

To meet these goals, Dr Rosanna Smith is conducting experiments in the fracture mechanics laboratory with Prof Peter Sammonds, Dr Hugh Tuffen, Prof Philip Meredith, Mr Neil Hughes and Mr Steve Boon (2006-2009) in collaboration with the Cascades Volcano Observatory. They are deforming lava dome material from the recent eruption of Mt St Helens under simulated volcanic conditions (stress, pressure, temperature, and pore fluid conditions) whilst recording acoustic emissions (AE) and stress-strain relationships. These experiments address the role of fracture, friction and the brittle-ductile transition during volcanic lava dome growth and in magma conduit dynamics. Characteristics of AEs recorded during these laboratory experiments are compared to those of seismic events recorded during the emplacement of the lava dome from which the samples were taken in order to aid interpretation of volcanic seismicity. These laboratory studies will provide direct input into the development of the current generation of models of volcanic dome behaviour, which hinge upon whether deforming magma will flow, slip or fracture.

This project builds on earlier work on rates of rock fracture before eruptions, seismogenic fracture of obsidian magma, and fracturing of basalt lava flows.

Experimental reconstruction and characterisation of Long-Period harmonics with application to volcanic hazard prediction: The laboratory volcano [EU Marie-Curie Outgoing International Research Fellowship (EU FP6)]

Dr Philip Benson(RIPL, UCL and Lassonde Institute, University of Toronto) with Prof Philip Meredith (Co-PI), Dr Sergio Vinciguerra (INGV, Rome), and Dr Chris Kilburn (Benfield UCL Hazard Research Centre).

Europe includes some of the most volcanically active regions on Earth, hosting about 6% of the 600 volcanoes known to have erupted in historical time and, of those, 2-3 are normally in eruption each year. Some 4-5 million people live within sight of an active European volcano, and ~10% of the EU population is economically vulnerable. Seismicity and ground deformation are the precursory phenomena most frequently seen before eruption, as the Earth's crust is distorted by magma moving to the surface, and as fluids (magma / gas / hydrothermal fluid) move within faulted rock. Final approach to eruption is commonly preceded by accelerating rates in the rate of low magnitude volcano-tectonic (VT) earthquakes and of long-period (LP) events (seismic signals unique to volcanoes and associated with fluid movement). Although the association of LP events with volcanic activity is not new, the specific mechanisms for LP generation is poorly understood. This project is, for the first time under in-situ conditions, generating unique, well-constrained laboratory data under simulated volcanic conditions of stress and temperature. Using state-of-the-art acoustic emission recording systems, microseismic events due to brittle failure and fluid movement (in a manner analogous to LP events at field scale on a volcanic edifice) have been recorded in the rock physics laboratory. By comparing this data to published field monitoring and theoretical data, this work is contributing to improved methods for investigating short-term precursors before volcanic eruptions.

Recent results include the full-waveform recording and event location of Etna basalt deformation in real-time, and – for the first time – without the need to artificially slow the failure process using AE-feedback servo control. In addition, frequency analysis of brittle deformation AE events shows marked differences to pore fluid movement and decompression events. Future experimental plans will investigate these data with reference to field seismic data, and expand the laboratory deformation studies to true-triaxial conditions in order to explore the complex stress components acting during volcanic flank collapse.

Dr Rosanna Smith studied fracturing rates before volcanic eruptions for her PhD with Dr Chris Kilburn and Prof Peter Sammonds (2002-2006) in the Benfield UCL Hazard Research Centre and Rock & Ice Physics Laboratory. This involved triaxial, uniaxial, and tensile deformation of andesites under simulated volcanic conditions in the fracture mechanics laboratory. Acoustic emission rates during these experiments were compared with rates of volcano-tectonic earthquakes before volcanic eruptions.

Dr Hugh Tuffen deformed obsidian at temperature and strain rates within the brittle-ductile transition in the fracture mechanics laboratory whilst recording acoustic emission rates (2005-2006). This work identified the mechanical properties of obsidian in the glass transition in compressive stress regimes, highlighting that the behaviour is different to that expected from extrapolating results from tensile experiments on synthetic glasses (often used in volcanology). The high AE rates at conditions very close to the glass transition also showed that earthquakes can occur in hot magma, with such small changes in extrinsic conditions necessary for the transition from ductile flow to seismogenic fracture that they may occur concurrently.

Dr Valentina Rocchi studied fracture of basalts under simulated lava flow conditions for her PhD with Prof Peter Sammonds and Dr Chris Kilburn (1998-2002) in the Rock & Ice Physics Laboratory and the Benfield UCL Hazard Research Centre . The high temperature triaxial deformation cell was designed as part of this project.

Selected Recent Publications

Benson P. M., B. D. Thompson, P. G. Meredith, S. Vinciguerra, R. P. Young (2007), Imaging slow failure in triaxially deformed Etna basalt using 3D acoustic-emission location and X-ray computed tomography, Geophys. Res. Lett., 34, L03303, doi:10.1029/2006GL028721

Smith, R., Kilburn, C. R. J., and Sammonds, P. R., 2007, Rock Fracture as a Precursor to Lava Dome Eruptions at Mount St Helens from June 1980 to October 1986: Bulletin of Volcanology, 69(6) 681-693

Vinciguerra, S., Trovato, C., Meredith, P., Benson, P., De Luca, G., Troise, C. and De Natale, G., 2006, Understanding the seismic velocity structure of Campi Flegrei caldera (Italy): from the laboratory to the field scale: Pure and Applied Geophysics, 163, Issue 10, 2205-2221.

Vinciguerra, S., Trovato, C., Meredith, P. and Benson, P., 2005, Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts: Int. J. Rock. Mech., 42, 900-910.
doi:10.1016/j.ijrmms.2005.05.022

Kilburn, C. R. J. and Sammonds, P. R., 2005, Maximum warning times for iminent volcanic eruptions: Geophys. Res. Lett., 32, Art. No.-L24313.

Rocchi, V., Sammonds, P. R., and Kilburn, C. R. J., 2004, Fracturing of Etnean and Vesuvian rocks at high temperatures and low pressures: Journal of Volcanology and Geothermal Research, 132, 137-157.

Rocchi, V., Sammonds, P. R., and Kilburn, C. R. J., 2002, Flow and fracture maps for basaltic rock deformation at high temperatures: Journal of Volcanology and Geothermal Research, 120, 25-42.