Thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from Molten Fuel Coolant Interaction experiments

Abstract

[1] Thermohydraulic explosions were produced by Molten Fuel Coolant Interaction (MFCI) experiments using remelted shoshonitic rocks from Vulcano (Italy). The fragmentation history and energy release were recorded. The resulting products were recovered and analyzed with the scanning electron microscope. Fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode. These clasts relate to the thermohydraulic phase of the MFCI, where most of the mechanical energy is released; they are here called ''active'' particles. The total surface area of such particles is proportional to the energy of the respective explosions. Other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime. These fragments, called ''passive'' particles, form after the thermohydraulic phase, during the expansion phase of the MFCI. In order to verify thermohydraulic explosions in volcanic eruptions, we compared experimental products with samples from phreatomagmatic base-surge deposits of Vulcano. Ash particles from the experiments show features similar to those from the deposits, suggesting that the experiments reproduced the same fragmentation dynamics. To achieve discrimination between active and passive particles, we calculated shape parameters from image analysis. The mass of active particles in base-surge deposits was calculated. As the material properties for the natural samples are identical to the experimental ones, the energy measurements and calculations of the experiments can be applied. For a single phreatomagmatic eruption at Vulcano, a maximum mechanical energy release of 2.75 Â 10 13 J was calculated, representing a TNT analogue of 6.5 kt.

Rapid injection of particles and gas into non-fluidized granular material, and some volcanological implications

“…Therefore this driving pressure represents the pressure of superheated steam formed after thermohydraulic explosions (Zimanowski 1998), and the gas pressures used for the current experiments are of the same order of magnitude. Laboratory molten fuel-coolant interactions are themselves energetically scaled relative to natural phreatomagmatic explosions, since they generate identical pyroclasts (Büttner et al 2002).…”

Generation of pyroclastic flows by explosive interaction of lava flows with ice/water-saturated substrate

“…Wohletz, 1983;Heiken and Wohletz, 1985;Büttner et al, 1999;Morrissey et al, 2000;White and Houghton, 2000;Belousov and Belousova, 2001;Büttner et al, 2002). Voluminous lahars, simultaneously outpoured from the locations of explosions, indicate that water was available in large amount in the moment of the PF generation.…”

“…Commonly these products have higher clast density than products simultaneously erupted from the primary vent (Lockwood and Hazlett, 2010). Presence of high percentage of fine-grained particles having surface features that result from melt fragmentation in brittle mode (blocky morphology with chip-off marks) is an evidence of an MFCI type of explosion (Büttner et al, 2002). Although some of those secondary explosions produced weak base surges, no evidence for pyroclastic flows able to travel several kilometers from the source has been described until now.…”

Eifel maars: Quantitative shape characterization of juvenile ash particles (Eifel Volcanic Field, Germany)

“…smooth surfaces at small scales) (e.g. Heiken, 1974;Schmincke, 1977;Wohletz, 1983;Fisher and Schmincke, 1984;Heiken and Wohletz, 1985;Zimanowski et al, 1991;Dellino and La Volpe, 1996;Büttner et al, 2002), which are larger (mainly 250-63 μm) than the particles formed by vapor film collapse (Wohletz, 1983).…”