QUANTITATIVE EXPERIMENTS ON PHREATOMAGMATIC EXPLOSIONS

An experimental set-up for controlled generation of thermal explosions (Fuel Coolant Interactions) has been built by an interdisciplinary research group of volcanologists, physicists and engineering scientists: The TEE-Haus (Thermal Explosion Experiment). During the last 3 years more than 500 experimental runs were performed. Aim of the project was not to build a "mini volcano" but to produce ignitable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects. Such small scale mixtures, however, most probably also occur during phreatomagmatic events in nature. Mixtures of water and magma on a large scale may be modelled by combination of small elements. Earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively. Therefore, a carbonate melt (approx. 0.3 kg of 1:1 mass ratio Na2CO3/K2CO3) Was used for initial experimental series because of easy handling and more "magma similarity". By injection of water into this melt explosive mixtures were produced and ignited (self-triggering). Explosive interactions occurred in a wide melt-temperature range (720 to 1040-degrees-C). At melt temperatures exceeding 1000-degrees-C, CO2 increasingly degassed. Presence of non-condensible gas bubbles in the melt strongly reduced the explosivity. The intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities, because of more intensive mixing. A minimum injection velocity was found to be approx. 0.7 m s-1. Experiments with silicate melts, derived from diverse remelted volcanic rocks were carried out with a slightly modified set-up. Explosions were ignited by an additional trigger: a shock wave of approx. 8 J. In volcanic systems, shock waves of this magnitude are abundant (volcanic tremor, etc.). Strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750-degrees-C. The produced explosions exerted repulsion forces of up to 24,000 N with a typical pulse duration of 1 ms (carbonate melt) resp. 1.5 ms (silicate melts). The ejection velocity was found to be in a range between 200 and 400 m s-1. Explosion energy values up to 500 J were calculated. The maximum explosion pressure was calculated to be in the range between 10 and 100 MPa. Only part of the melt (so-called interactive melt) reacts explosively with injected water. Most of the melt is passively ejected by and after the thermal explosion. Different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other. Particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20-mu-m to 180-mu-m. The amount of interactive melt is proportional to the intensity of the explosion. The maximum amount of water vapourized at the explosive interaction (so-called interactive water) was estimated. Thus, the water/melt mass ratio for explosive interactions could be calculated. The values range between 1/25 and 1/35 (carbonate melt) resp. between 1/6 and 1/25 (silicate melts).