Fragmentation of basaltic melt in the course of explosive volcanism

B. Zimanowski, R. Buettner, V. Lorenz, H.-G. Haefele
Physikalisch-Vulkanologisches Labor, Institut fuer Geologie, Universitaet Wuerzburg, Pleicherwall 1, D-97070 Wuerzburg, Germany

Abstract:

With the aim to enhance interpretation of fragmentation mechanisms during explosive volcanism from size and shape characteristics of pyroclasts experimental studies have been conducted using remelted volcanic rock (olivine-melilitite). The melt was fragmented and ejected from a crucible by the controlled release of pressurized air volumes (method 1) or by controlled generation of phreatomagmatic explosions (Molten Fuel Coolant Interaction, i.e. MFCI; method 2). Both methods were adjusted in such way that the ejection history of the melt was identical in both cases. The experiments demonstrate that exclusively during MFCI angular particles in the grain-size interval 32 to are generated, that show surface textures dominated by cracks and pitting. The physical process of their generation is described as a brittle process acting at cooling rates , at stress rates well above , and during approx. . In this time period the emission of intense shock-waves in the MHz-range was detected, releasing kinetic energy > 1000 J. By both experimental methods three further types of particles were produced, which could be identified and related to the accelleration and ejection history of the melt: spherical particles, elongated particles, and Pele's hair. Abundance and grain-size distribution of these particles was found to be proportional to the rate of acceleration and the speed of ejection, but not influenced by the experimental method used. Pele's hair occurred at ejection speeds > 75 m/s.

AGU Index Terms: 8414 eruption mechanisms; 8145 Physics of magma and magma bodies; 8194 Instruments and techniques
Keywords/Free Terms: Magma physics, fragmentation experiments, genesis of pyroclasts

JGR-Solid Earth 96JB02935
Vol. 102 , No. B1 , p. 803