HR: 17:45h
AN: V24B-08    [Abstracts]
TI: The Influence of Microcracks on the Physical and Mechanical Properties of Etna and Iceland Basalt
AU: * Meredith, P
EM: p.meredith@ucl.ac.uk
AF: University College London, Department of Earth Sciences , London, WC1E 6BT United Kingdom
AU: Lewis, O
EM:
AF: University College London, Department of Earth Sciences , London, WC1E 6BT United Kingdom
AU: Vinciguerra, S
EM:
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, 00143 Italy
AU: Trovato, C
EM:
AF: D'Appolonia S.p.A., Via S. Pasquale a Chiaia 62, Naples, 01110 Italy
AU: Benson, P
EM:
AF: University College London, Department of Earth Sciences , London, WC1E 6BT United Kingdom
AB: We report results of measurements of the physical and mechanical properties of typical basalts formed in different enviromments. We considered material from extruded lava flows from Mt. Etna volcano (Italy) and fresh columnar basalts formed in an intrusive environment, from Seljadur (Iceland). Laboratory measurements of ultrasonic wave velocities and fluid permeability have shown that Etnean basalt has much lower velocities and much higher permeability than Seljadur basalt. Optical and scanning electron microscopy shows clearly that the Etnean basalt contains a much higher level of pre-existing crack damage than the Seljadur basalt. Therefore, in order to verify that the differences in physical and transport properties are due to the higher crack density, we thermally stressed samples of both basalts at temperatures up to 900°C. Following the thermal stressing, the wave velocities of Seljadur basalt dropped and its permeability increased to values similar to those observed in the untreated Etnean basalt. By contrast, the values for thermally stressed Etnean basalt remained essentially unchanged. We ascribe these changes to the significantly increased level of crack damage caused by the thermal stressing, which is again revealed by microscopic observation. We also present complementary measurements of the strength and deformability of the two basalts, both before and after thermal stressing. As expected, both the static and dynamic moduli, and the strength, of the basalt samples containing high levels of crack damage (untreated Etna basalt and thermally stressed Seljadur basalt) are very low compared with the values for samples with low levels of crack damage (untreated Seljadur basalt). We consider such data to be of fundamental importance in reliably interpreting large scale in-situ geophysical measurements and modeling volcanic processes. For example, interpretations of tomographic data and estimates of volcano instability need to use physical and mechanical rock parameters appropriate to the specific conditions being studied.
DE: 1822 Geomechanics
DE: 3902 Creep and deformation
DE: 3909 Elasticity and anelasticity
DE: 5102 Acoustic properties
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005