HR: 08:00h
AN: V51B-01    [PDF]
TI: A fragmentation threshold for the initiation and cessation of explosive eruptions
AU: Spieler, O
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Kennedy, B
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Kueppers, U
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: * Dingwell, D B
EM: Dingwell@lmu.de
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Scheu, B
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Taddeucci, J
EM:
AF: University of Munich, Theresienstr. 41, Munich, 80333 Germany
AB: A fragmentation threshold for magma may be defined as the critical gas overpressure required to fragment volcanic materials when decompressed rapidly. There are several potential causes of pressure differentials between bubbles and melts in magma. In fact, bubble growth results itself from the action of bubble overpressure on the melt walls of the bubble. Perturbations of the pressure differential may additionally arise due to sudden decompression, flow pressure gradients in variably viscous magmas in conduits, or external influences such as landslides or gravitational dome collapse. Overcoming such a threshold provides a volcanic system with the possibility to change its behaviour from that of a relatively harmless effusive eruption into a potentially highly destructive explosive eruption The magnitude of the fragmentation threshold is likely to be a controlling factor in initiation and cessation of explosive eruptions. Pressure differentials commonly arise between the gas in bubbles and the ambient pressure of the surrounding melt. If the stress perturbations involved call for viscous strain rates higher than the relaxational strain rate (1/relaxation time) of the melt, brittle fragmentation may ensue. Here, we show results of fragmentation experiments at 850$\deg$C performed with volcanic rocks varying widely in porosity, permeability, crystallinity and chemical composition. Increasing porosity decreases the value of the fragmentation threshold. At high porosity values, the influence of permeability becomes increasingly important in defining the threshold. Chemical composition and temperature appear to have relatively minor effects. In contrast with existing models the present parameterization implies that the fragmentation threshold is proportional to the tensile strength of the porous magma in glassy response.
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting