HR: 11:50h
AN: V42A-07    [Abstracts]
TI: The Effect of Degassing Efficiency on the Fragmentation Behavior of Volcanic Rocks
AU: * Mueller, S
EM: mueller@min.uni-muenchen.de
AF: Earth and Environmental Sciences, LMU Muenchen, Theresienstr. 41, Muenchen, 80333 Germany
AU: Scheu, B
EM: scheu@min.uni-muenchen.de
AF: Earth and Environmental Sciences, LMU Muenchen, Theresienstr. 41, Muenchen, 80333 Germany
AU: Spieler, O
EM: spieler@min.uni-muenchen.de
AF: Earth and Environmental Sciences, LMU Muenchen, Theresienstr. 41, Muenchen, 80333 Germany
AU: Dingwell, D B
EM: dingwell@lmu.de
AF: Earth and Environmental Sciences, LMU Muenchen, Theresienstr. 41, Muenchen, 80333 Germany
AB: The degassing efficiency of volcanic rocks is a decisive measure for the eruptive style and thus the explosivity of a volcano, since it directly affects magma fragmentation behaviour. Vesicles in ascending magma may bear overpressure if the relevant magma viscosity entails a relaxation time scale which is significantly larger than the time scale of ambient pressure reduction due to magma ascent. As long as this overpressure does not overcome the tensile strength of the magma, the system is in a structurally stable state, eventually degassing quiescently via an interconnected pore network. However, if a decompressive event (e.g. sector collapse) disturbs this stable pressure situation, two possible scenarios are conceivable: (1) An interconnected pore network has been established whose permeability is sufficiently high, so vesicle overpressure can be reduced efficiently by gas filtration. (2) The permeability of the network (or cluster of isolated pores, respectively) is low and gas overpressure can not be reduced within the required time scale. In this case the expansion of the pressurized gas may cause bubble wall failure and magma fragmentation into pyroclasts. This study compares experimentally derived fragmentation threshold values of volcanic rock samples, determined with a shock-tube based setup, to unsteady-state permeability values of the same sample sets. In order to cover a wide range in rock properties, we analysed samples from a broad variety of volcanic deposits. Among the treated volcanoes were Colima (Mexico), Bezymianny (Russia), Krakatoa and Merapi, (both Indonesia), Unzen (Japan), Lipari and Campi Flegrei (both Italy), Pinatubo (Philippines), and Santorini (Greece). The correlation of extensive databases of both investigated parameters revealed that permeabilities above a transition zone between 10^-13 and 10^-12 m² shift the fragmentation threshold towards higher values. By means of this dataset the influence of the permeability on fragmentation processes has been shown and quantified.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
DE: 8434 Magma migration and fragmentation
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005