IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A2_1E-O10

Room A3

Date/Time: July 21 11:15-11:30

Understanding phreatic & hydrothermal explosion dynamics: insights into the energy
conversion based on lab experiments
Bettina Scheu, Donald B Dingwell
Earth and Environmental Sciences, LMU Munich, Germany
E-mail: b.scheu@lmu.dePhreatic eruptions are amongst the most common and most diverse eruption types on earth. Often they areprecursory to another type of volcanic activity but they can stand as well on their one. Phreatic explosions occur
for instance when subsurface water or water on the surface is heated by magma, lava, hot rocks, or fresh volcanic
deposits and result in craters, tuff rings and debris avalanches. Another wide and important field affected by steam
explosions are hydrothermal areas; here hydrothermal explosions might occur every few months creating explosion
craters and resemble a significant hazard to hydrothermal power plants as well as visitor parks. Despite of theirhazard potential, phreatic explosions have so far been overlooked by the field of experimental volcanology. A partof their hazard potential in owned by the fact that phreatic explosions are hardly predictable in occurrence time andsize as they have manifold triggers. The diversity of phreatic and hydrothermal eruptions arises from the variety of
host rocks, ways to seal possible degassing pathways, and to alter this material depending on the composition ofvolcanic gases and the hydrothermal fluids.

We conduct rapid decompression experiments on a variety of natural samples from igneous rocks to sedimentaryrocks; consolidated and unconsolidated. The setup used can be operated with gas and/or vapour overpressureof up to 20 MPa and in a temperature range from range from 20°C to 400°C, further we vary the degree ofwater saturation of the samples. The experiments are monitored with a set of pressure and temperature sensors,synchronized to high-speed video recording of the particle ejection. The resulting particles are recovered andtheir grain size distribution are analysed. This together allows us to constrain the fragmentation behaviour and theefficiency of the fragmentation in phreatic explosions. We observe that the threshold for fragmentation is slightly
lowered whereas the propagation speed of the fragmentation process through the sample is comparable to dry
magmatic experiments. A clear contrast is seen for the ejection speed of the clasts: phreatic clasts can be ejected
at almost double the speed as their magmatic counterparts under similar overpressure conditions. Further theejection speed depends on the ratio of superheated water to vapour and argon gas. This ratio controls also theefficiency of fragmentation as well as the grain shapes.

Our experiments and results enable us to better understand the dynamics of phreatic and hydrothermal
explosions and associated risks. Here we present insights into the energy conversion during phreatic eruptions,based on various starting scenarios. This will provide vital information for the hazard assessment of hydrothermally
active areas.

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