HR: 08:30h
AN: V41I-03    [Abstracts]
TI: A Relationship Between Fragmentation Energy and Fragmentation Speed: Evidence for two Mechanisms of Magma Failure?
AU: * Scheu, B
EM: betty@min.uni-muenchen.de
AF: Earth & Environmental Sciences, LMU Muenchen, Theresienstr. 41, Muenchen, 80333 Germany
AU: Dingwell, D B
EM: dingwell@lmu.de
AF: Earth & Environmental Sciences, LMU Muenchen, Theresienstr. 41, Muenchen, 80333 Germany
AB: The investigation of volcanic eruptions and magmatic processes is becoming increasingly systematic, quantitative and rigorous. The scientific study of volcanic phenomena cannot rely solely on field data, as direct observations of eruption processes is limited to those parts of an eruption that are accessible. Fragmentation is the process of disintegrating magma into pyroclasts. Various models have been proposed for fragmentation. In this study we solely focus on the fragmentation of vesicular magma due to rapid decompression. In most cases silicic, highly viscous magma will disrupt in a brittle manner, as the process occurs close to the glass transition. During fragmentation the potential energy stored as overpressure in bubbles and melt is used to fracture the magma, creating new surface. The surplus is transformed into kinetic energy of ejected particles. To enhance our quantitative underpinning of mechanisms of magma failure, we performed rapid decompression experiments with a shock tube-like apparatus. Samples from different volcanoes were investigated for this study. The open porosity ranges from 2.5 vol.% to 67 vol.%, resulting in a range of 2.5 - 30.0 MPa for the fragmentation threshold. These threshold values are in good agreement with the threshold curve of Spieler et al. (2004). Experiments were carried out in the pressure range between 2 MPa and 40 MPa resulting in values of fragmentation speed of up to 150 m/s. The energy, which drives the fragmentation process, is largely provided by the expansion of the pressurized gas located in the pore space of the samples. In general, we observed a logarithmic increase of the fragmentation speed with the energy density (fragmentation energy standardized to a unit volume) as soon as a certain ``energy threshold'' is overcome. However, some highly porous samples with high permeability values deviate from the trend towards higher energy values. The densest samples of this study deviate clearly to lower energy values. These observations lead us to the suggestion, that magma fragmentation is driven by two distinct physical mechanisms: (1) At moderate and high porosities gas expansion is the determining process, leading to a vesicle bursting, which is widely accepted as the common process. (2) Whereas at low porosity samples the fracture process due to the unloading wave appear to become increasingly important. We also infer a transition zone, in which both mechanisms occur jointly.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005