HR: 0800h
AN: V21D-0629    [Abstracts]
TI: Dilatancy and Failure in Basalt From Mt Etna Under Triaxial Compression
AU: * Zhu, W
EM: zwei@ic.sunysb.edu
AF: Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794 United States
AU: Baud, P
EM: Patrick.Baud@eost.u-strasbg.fr
AF: Laboratoire de Physique des Mat"Ýriaux, Institut de Physique du Globe de Strasbourg (CNRS/ULP),5 rue Ren"Ý Descartes, Strasbourg Cedex, F-67084 France
AU: Wong, T
EM: Teng-fong.Wong@stonybrook.edu
AF: Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794 United States
AB: The recent history of Mt Etna volcano was marked by several flank eruptions from fractures that opened and feeded lava flow towards the eastern flank of the volcano. In Mt Etna as in most volcanic systems pervasive fracturing of rocks whether it is the lava dome or the surrounding rocks is a dominant feature. Understanding how the strength of volcanic rock varies with stress state, pore fluid content and pressure, damage (content and anisotropy) is fundamental to understanding the dynamics of volcanic systems and in particular modeling the progressive transport of magma towards Earth's surface that leads to eruptions. In this study, we investigated the micromechanics of failure in Mt Etna's basalt. Our block of basalt had a nominal connected porosity (measured by water saturation) of 5% and was composed mainly of pyroxene, olivine and feldspar. Microstructural observations of the intact material revealed the presence of thin cracks (probably formed during the rapid cooling of the lava) and quasi-spherical voids formed during degassing. Some of those cavities appear isolated suggesting that the total porosity of this rock could be significantly higher than the connected porosity. Under hydrostatic conditions however, significant compaction was observed even after closure of the cracks up to 450 MPa of effective pressure. We performed around 20 conventional triaxial experiments on water saturated samples in drained conditions at confining pressures between 10 and 150 MPa and with 10 MPa of pore pressure. Dilatancy and brittle faulting were observed in all samples. Below 150 MPa of effective pressure, a single shear band oriented at 30 degree cut through the samples. At 150 MPa of effective pressure, a somewhat different failure mode involving conjugate shear bands was observed. Up to 50 MPa of effective pressure, Young's modulus increased linearly with pressure and dilatancy was not accompanied by any increase in acoustic emission (AE) activity before the macroscopic failure of the sample. Beyond 50 MPa of effective pressure, the failure envelope became nonlinear and AE activity increased significantly after the onset of dilatancy. Several experiments were stopped at different stages of the deformation after dilatancy at 10 and 50 MPa of effective pressures. Petrophysical thin sections of the deformed samples were prepared. Microstuctural observations revealed the micromechanism leading to dilatancy before and after closure of pre-existing cracks in Mt Etna basalt.
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms and flow emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005