HR: 0800h
AN: V11A-0365    [Abstracts]
TI: Deformation of Magma-Filled Bodies during Solidification
AU: * Gaffney, E S
EM: edgaffney@earthlink.net
AF: Gaffney Associates. Inc., 747 Simms Avenue, Council Bluffs, IA 51503, United States
AU: Damjanac, B
EM: branko@itascacg.com
AF: Itasca Consulting Group, 111 Third Avenue South Suite 450, Minneapolis, MN 55401, United States
AB: As magma or lava solidifies, volatiles are concentrated in the residual liquid. The result will be expansion (including venting) or pressurization. The mechanism behind this is well-described. A rough hand calculation indicates that an alkali basalt with 4 wt% volatiles would attain attain 12 MPa with 50% crystallization at constant volume. Such pressures would easily be enough to break through the roof of a typical lava tube. If confined in a tunnel deeper in the ground, even in a relatively weak rock, crystallization would be virtually isochoric. However, in a sill at depths of only a few hundred meters, expansion could result in more nearly isobaric crystallization. In either event, before cooling enough to become a brittle solid, the outer portions of the magma would reach a viscoplastic state that could seal in any remaining vapor phase. This would allow pressures to increase further as solidification progressed. Using PELE, a computer code developed to calculate the progress of solidification (Boudreau, 2005), we calculate isochoric and isobaric equilibrium crystallization of alkali basalt and obtain pressures and viscosities as a function of temperature. For an initial pressure of 6 MPa and 0.85 weight percent water, the liquidus is 1433 K. The isochoric pressure reaches 11 MPa at 1293 K with 57% of the mass crystallized; the bulk viscosity is about 3 MPa-s, but that of the residual liquid is only 1 kPa-s. At the same temperature, the isobaric path results in 60% crystallization and a viscosity on the order of 10 kPa-s. A tabular body with these properties would be easily deformed by sagging of the roof if the viscoplastic seal were breached, resulting in a saucer shape. With 91% of the mass crystallized, the isochoric pressure exceeds 28 MPa at 1173 K. By that time, the bulk viscosity of the nearly crystallized mass is on the order of 10²⁵ Pa-s, effectively solid, and the viscosity of the residual liquid (there is also a vapor phase) is about 50 kPa-s. Combining these results with those of a calculation of the cooling of magma in a horizontal cylinder, we find that the interior of the magma is at high-pressure (>10 MPa) and quite fluid (μ < 1 kPa-s) for long enough times to allow considerable deformation of any structures that might be entombed in magma in a deep tunnel. Reference: Boudreau, 2005, http://www.nicholas.duke.edu/people/faculty/boudreau/DownLoads.html
DE: 8429 Lava rheology and morphology
DE: 8430 Volcanic gases
DE: 8434 Magma migration and fragmentation
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting