HR: 08:00h
AN: V41I-01 INVITED     [Abstracts]
TI: Experimental Studies of Lava Dome Fracture
AU: * Smith, R
EM: rosanna.smith@ucl.ac.uk
AF: Benfield Hazard Research Centre, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: * Smith, R
EM: rosanna.smith@ucl.ac.uk
AF: Mineral, Ice and Rock Physics Laboratory, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Sammonds, P R
EM: p.sammonds@ucl.ac.uk
AF: Benfield Hazard Research Centre, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Sammonds, P R
EM: p.sammonds@ucl.ac.uk
AF: Mineral, Ice and Rock Physics Laboratory, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Kilburn, C R
EM: c.kilburn@ucl.ac.uk
AF: Benfield Hazard Research Centre, Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT United Kingdom
AB: Renewed extrusion at andesitic to dacitic lava domes and collapses of these domes are usually preceded by fracturing and frictional sliding of material in and around the lava dome and magma conduit. This is observed through the occurrence of shallow high frequency earthquakes. Samples of andesite from Mount Shasta in the Cascades, a typical material for both lava domes and shallow underlying country rock, have been deformed in compression and tension, at temperatures of up to 900°C, and under confining pressures of up to 70MPa. During these tests the axial load, sample deformation and acoustic emissions were recorded, in order to compare the results with field observations of deformation and short period seismicity at lava domes. Typical strengths at room temperature and pressure were 6MPa in tension, and 100MPa in compression. Increased temperatures increased the tensile strength, but reduced the compressive strength, whereas both strengths increased with increasing confining pressure. There were ~10 times more acoustic emissions at room temperature than at maximum test temperatures, indicating that increased temperatures favour ductile, rather than brittle, failure. These results suggest that young, hot lava domes may collapse or erupt with little precursory short period seismicity, whilst older, cooler domes are likely to exhibit stronger short period seismic precursors. However, hotter material is likely to exhibit more recognisable deformation precursors. This is consistent with the seismicity observed after the 18 May 1980 climactic eruption at Mount St Helens, where there was ~100 times more seismicity prior to eruptions in 1985 and 1986 than there was prior to eruptions in 1980 and 1981. During these later eruptions, the interior of the dome would still have been ductile due to its temperature and the overburden weight acting as a confining pressure, but the large amount of pre-failure deformation in this zone could drive fracturing of the cooler outer lava dome and surrounding rocks. The low compressive and tensile strength of andesite found in these experiments compared with the strengths of intrusive igneous rocks such as granite, whose properties are well known and often used in lava dome models; indicate that this fracturing could be more important in controlling lava dome growth and stability than suggested by existing models.
DE: 5104 Fracture and flow
DE: 5194 Instruments and techniques
DE: 8429 Lava rheology and morphology
DE: 8488 Volcanic hazards and risks
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005