HR: 0800h
AN: NS31B-04    [Abstracts]
TI: The Importance of the Free Surface to Magmatic Plumbing Features
AU: * Gaffney, E S
EM: edgaffney@earthlink.net
AF: Gaffney Associates, 111 N. Walnut St, Glenwood, IA 51534, United States
AU: Keating, G N
EM: gkea@lanl.gov
AF: Los Alamos National Lab, PO Box 1663, Los Alamos, NM 87545, United States
AU: Krier, D
EM: krier@lanl.gov
AF: Los Alamos National Lab, PO Box 1663, Los Alamos, NM 87545, United States
AU: Valentine, G
EM: gav@lanl.gov
AF: Los Alamos National Lab, PO Box 1663, Los Alamos, NM 87545, United States
AU: Damjanac, B
EM: branko@itascacg.com
AF: Itasca Consulting Group, 111 Third Ave S Suite 450, Minneapolis, MN 55401, United States
AU: Wohletz, K
EM: wohletz@lanl.gov
AF: Los Alamos National Lab, PO Box 1663, Los Alamos, NM 87545, United States
AB: We wish to call the attention of the near-surface community to the need for geophysical sensing methods that can evaluate rock properties in the upper ~100 m to aid in the understanding and prediction of volcanic eruptions. Both field observations and numerical models show that decrease of confining stress as the free surface is approached can interact with different host rock properties to have a profound effect on the style of the early stages of a volcanic eruption. For example, at Paricutin where the host rocks were relatively strong basalts, eye witness accounts of the initiation of the eruption indicate that the crack tip that led the ascent of magma was a single, long fracture filled with gases well ahead of magma. On the other hand, observations from a slightly exhumed Miocene basaltic neck in southern Nevada show anastomosing of the magma with the country rock as it approached the surface through a weak and fractured rhyolitic tuff, resulting in a widening of the dike at 50-100 m depths and flaring at shallower depths. In the absence of observations of the initial eruption at the Nevada site, and of subsurface geology at Paricutin, any contrast in subsurface styles must still be speculative. Still, a contrast is supported by numerical models that show magma distributing itself among multiple paths to the surface as it rises through a mass of blocks. Models of conduit erosion by overpressured multiphase flow show similar flaring at the surface. Geophysical sensing methods that could penetrate beneath the scoria cones of historical eruptions to identify such structures would be very helpful in understanding how pre-eruptive geology controls near-surface magmatic plumbing. Sensors that could determine near-surface mechanical properties such as fracturing, low-frequency elastic moduli and porosity would be useful in predicting initial eruptive styles.
DE: 0905 Continental structures (8109, 8110)
DE: 5104 Fracture and flow
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8486 Field relationships (1090, 3690)
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly