Assessment of the Role Played by Neutral Buoyancy on the Failure of Shallow Magma Reservoirs

Details Assessment of the Role Played by Neutral Buoyancy on the Failure of Shallow Magma Reservoirs Assessment of the Role Played by Neutral Buoyancy on the Failure of Shallow Magma Reservoirs

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p13a1035h&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P13A-1035

Physics

0560 Numerical Solutions (4255), 5480 Volcanism (6063, 8148, 8450), 6225 Mars, 6295 Venus, 8439 Physics And Chemistry Of Magma Bodies

Scientific paper

During its ascent from the depth where it forms, magma often stalls at the horizon of neutral buoyancy (NB), i.e. the depth at which the magma and immediate surrounding host rock have equal densities. If magma supply rate and other conditions promote formation of a stable magma reservoir, then subsequent inflation, for instance in response to periodic injection of magma from below, can cause the reservoir walls to fail in tension, leading to lateral or vertical intrusion and the possibility of surface eruption. Constraining the overpressure required to induce failure of an ellipsoidal reservoir, and identifying the location along the wall where initial rupture occurs, has been the subject of a great deal of previous research. Most published work (e.g., Parfitt et al., J. Volc. Geotherm. Res., 55, 1993) indicates that the balance of stresses normal to the reservoir wall, largely determined by depth-dependent variations in magma and host rock density structure, is the predominant factor controlling the rupture process and location. Recent work, however, points out that the relative change in the wall-parallel component of the lithostatic stress, measured from the crest to the base of the reservoir, is far greater in magnitude than the change in normal stress across the wall for the same depth range (Grosfils, J. Volc. Geotherm. Res., in press). We thus predict that, while magma NB likely dictates where a reservoir will form, the relative density structures in the reservoir and host rock with depth will have little effect on either the conditions required to induce failure or the failure location. Using FEM techniques, we test this hypothesis by examining the failure of magma reservoirs under NB and non-NB conditions. The reservoirs range in size from 0.2-4 km, in depth from 0.3-20 km, and in magma density from 175(gas)-3500 kg/m3; the host rock is defined by uniform, two-layer and smoothly varying density structure with depth. Our results demonstrate that the relative density structure of the magma and host rock contributes only negligibly to the overpressure required to induce failure and the initial rupture location, i.e. that magma NB is not an important consideration when evaluating reservoir failure.

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