Littoral blasts: Pumice-water heat transfer and the conditions for steam explosions when pyroclastic flows enter the ocean

[1] Steam explosions, or littoral blasts, generated when pyroclastic flows interact with seawater may be a common, although rarely documented, phenomena. The development of steam explosions rather than passive steam production is related to the rate of thermal energy transfer from hot pyroclasts to water. We conduct a series of laboratory experiments to quantify the heat transfer and steam production rates when hot pyroclasts encounter water. Hot pumice (> 200 degrees C) rapidly ingests water while remaining at the surface, producing measurable amounts of steam during the process. Approximately 10% of the thermal energy of the pumice particles is partitioned into the production of steam, and smaller particles have greater steam production rates. The laboratory experiments are used to develop a subgrid model for steam production that can be incorporated into a multiphase numerical framework. We use this model to study the critical steam production rates required to initiate explosive events. For conditions typical of many pyroclastic flows, particles smaller than similar to 1-5 mm are required to initiate a littoral blast. A second set of two-dimensional numerical simulations is conducted to simulate the 12-13 July Soufriere Hills dome collapse event that reached the sea. The simulations predict that the focus of the blast is likely generated several hundred meters offshore and although the landward directed base surge is primarily dry (< 15% water vapor), the area immediately above the blast is steam-rich and may be a likely site for the production of accretionary lapilli.