FRAGMENTATION OF BASALTIC MELT IN THE COURSE OF EXPLOSIVE VOLCANISM

With the aim to enhance interpretation of fragmentation mechanisms dur ing explosive volcanism from size and shape characteristics of pyrocla sts experimental studies have been conducted using remelted volcanic r ock (olivine-melilitite). The melt was fragmented and ejected from a c rucible by the controlled release of pressurized air volumes (method 1 ) or by controlled generation df phreatomagmatic explosions (Molten Fu el Coolant interaction (MFCI); method 2). Both methods were adjusted s o that the ejection history of the melt was identical in both cases. T he experiments demonstrate that exclusively during MFCI, angular parti cles in the grain size interval 32 to 130 mu m are generated that show surface textures dominated by cracks and pitting. The physical proces s of their generation is described as a brittle process acting at cool ing rates of >10(6) K/s, at stress rates well above 3 GPa/m(2), and du ring similar to 700 mu s. In this time period the emission of intense shock waves in the megahertz range was detected, releasing kinetic ene rgy of >1000 J. By both experimental methods, three more types of part icles were produced in addition, which could be identified and related to the acceleration and ejection history of the melt: spherical parti cles, elongated particles, and Pele's hair. Abundance and grain size d istribution of these particles were found to be proportional to the ra te of acceleration and the speed of ejection but were not influenced b y the experimental method used. Pele's hair occurred at ejection speed s of >75 m/s.