HR: 0800h
AN: V21D-0630    [Abstracts]
TI: Evaluation of Microcracks orientation at Stromboli volcano using a Magnetic Ferrofluid and the Method of Anisotropy of Magnetic Susceptibility
AU: Lewis, O
EM: olivia.lewis@ucl.ac.uk
AF: University College London, Gower Street, London, ON WC1E 6BT United Kingdom
AU: * Benson, P M
EM: p.benson@ucl.ac.uk
AF: University College London, Gower Street, London, ON WC1E 6BT United Kingdom
AU: Vinciguerra, S
EM: vinciguerra@ov.ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano 328, Naples, ON 80124 Italy
AU: Meredith, P G
EM: p.meredith@ucl.ac.uk
AF: University College London, Gower Street, London, ON WC1E 6BT United Kingdom
AB: Most crustal rocks are anisotropic. In volcanic areas, anisotropy primarily results due to preferred directions of microcracks as magma cools. This effect is, in turn, enhanced due to local stress fields during deposition. The combined effects of these processes may thus give rise to a complex anisotropic fabric. Such fabrics can play crucial roles when enhancing the formation of slip surfaces which can lead to sector collapses of volcanic edifices, as is the case of Stromboli volcano (Italy) which experienced 4 sector collapses in the past 13ka. However, the rapid analysis of anisotropic microcrack fabrics (in terms of magnitude and principal direction) remains non-trivial. Current methods range from time consuming microcrack analysis of thin sections to the preparation of oriented cores for elastic-wave velocity measurement. To further our understanding of how microcrack fabrics influence the bulk properties of volcanic basalt, we employ a novel method which rapidly evaluates the 3-D microcrack orientation using technique of Anisotropy of Magnetic Susceptibility (AMS). First, we determine the rock matrix AMS (mAMS) using standard methods (via a Agico KLY-4 Kappabridge). Samples are then saturated with a magnetic ferrofluid, filling the microcrack network with a magnetically susceptible suspension of microscopic (10nm) magnetite particles. The AMS is then re-measured, with the matrix susceptibility values subtracted from these readings to yield the average 3-D pore space shape, size and orientation (pAMS). We describe the use of this method using basalt from Stromboli and comparing to a granite (Takidani) from the Japanese Alps in order to verify the technique and to investigate the relationship between the basalt microcrack geometry and field scale observation. For Takidani granite we find the structural anisotropy formed by the void space, as measured by pAMS, is well described by elastic wave velocity measurement; exhibiting anisotropy values of 19.1% and 7.6% for P-waves and S-waves respectively. Stromboli basalt possesses a weaker anisotropy of 4.7% and 3.0% (P-wave and S-wave velocity). We relate our pore space AMS measurements to the layering observed in Stromboli basalt on the flanks of the volcanic edifice; and infer that the microcrack network is both formed by this deposition and active tectonics as well as providing a key control on its physical properties. Such data has crucial significance upon the accurate assessment of flank stability, with consequences to hazard assessment for the surrounding area.
DE: 5109 Magnetic and electrical properties (0925)
DE: 5112 Microstructure
DE: 5194 Instruments and techniques
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005