HR: 1340h
AN: V43D-1443    [Abstracts]
TI: Decompression Profiles During Magma Fragmentation.
AU: * Scheu, B
EM: betty@min.uni-muenchen.de
AF: Earth and Environmental Sciences, LMU Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Melnik, O
AF: Institute of Mechanics, Moscow State University , 1-112b, Michurinsky prosp., Moscow, 119192 Russian Federation
AU: Mueller, S
AF: Earth and Environmental Sciences, LMU Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Spieler, O
AF: Earth and Environmental Sciences, LMU Munich, Theresienstr. 41, Munich, 80333 Germany
AU: Dingwell, D B
AF: Earth and Environmental Sciences, LMU Munich, Theresienstr. 41, Munich, 80333 Germany
AB: The dynamics of magma fragmentation likely exert a strong influence on the explosive behavior and thus the eruptive style of a volcano. The speed of fragmentation for instance is likely to be directly affected by the pressure distribution within a volcanic conduit or dome. We use a shock-tube fragmentation apparatus to analyze the speed of fragmentation of samples covering a wide range of porosity. The results show that the speed of fragmentation depends in first order on the potential energy available for the fragmentation process. This energy results from the gas volume within the sample and the applied pressure. The pore structure (size, shape, and orientation) of the analyzed samples is of second order importance to the fragmentation behavior. For highly porous volcanic rocks this structure achieves the strongest influence on the permeability and thus the steepness of the pressure gradient, which built up while rapid decompression. We observed that a fast degassing of a sample (leading to a flat pressure gradient) shifts the fragmentation threshold to higher values. To elucidate the influence of this pressure gradient on the fragmentation behavior, we reconstructed the pressure profiles within different porous samples undergoing rapid decompression. Therefore, we performed a set of decompression experiments with a single sample shortened stepwise from 60 mm down to 1.8 mm. These experiments were conducted at a constant initial pressure value below the fragmentation threshold. The experimentally derived pressure profiles were compared to numerically modeled pressure profiles based on a 1D filtration code and showed good agreement. Further experiments were conducted above the fragmentation threshold to investigate the influence of the sample length on the speed of fragmentation. The results of differently porous samples at three different lengths (15 mm, 30 mm, and 60 mm with constant diameter of 25 mm) showed that the speed of the fragmentation wave seems to remain constant over the whole sample length. We developed a numerical flow model considering the different properties of gas and matrix skeleton to reproduce the experimentally derived pressure drop curves. Our results represent one contribution to a better understanding of the physical processes controlling the initiation, speed, and cessation of a fragmentation event.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting