HIGH-ENERGY LAVA–WATER EXPLOSIONS: A CASE STUDY FROM HAWAI‘I

Reconstructing the energetics of explosive interactions between magma and water during phreatomagmatic eruptions is complicated by the competing effects of juvenile degassing-induced fragmentation. During explosive interactions between surface lava flows and water, however, only external water drives explosive activity. Explosive melt–water experiments and previous work on lava–water explosions indicate that the energy release during an explosion is proportional to the median grain size of the ejecta and the degree of fine-scale fragmentation, especially the abundance of ash-sized grains. Therefore, more energetic explosions result in a larger abundance of ash-sized grains that contribute significant energy, even if the total mass of the ash-size portion is small relative to coarser ejecta. Previous work has focused on relatively low-energy lava–water explosions, resulting primarily in bomb-lapilli dominated beds making up a cone of ejecta (i.e. rootless cone) with a modal dispersal diameter of 60 m. Since an increase in explosion energy generally produces more widely dispersed deposits, we investigated the characteristics of tephra from a littoral (near shore) rootless cone on the island of Hawai‘i with a modal dispersal diameter of 280 m, to identify processes occurring on the higher-energy end of the spectrum. In general, we find that coarser-grained beds contain a larger total abundance of fluidal grains (i.e. molten spatter) than finer-grained beds, consistent with previous work. Beds of the 280 m cone are lapilli- to ash-dominated and contain twice the mass percent of ash-sized grains as beds of the 60 m cone that have similar grain-size distributions. The degree of abrasion and abundance of grains having undergone brittle fragmentation are also substantially higher in tephra of the 280 m cone. Additional analysis will allow us to constrain the effects of lava rheology and abrasion on the production of ash-sized grains and therefore provide estimates of the fragmentation energy expended during an explosion. We intend to use carefully constrained parameters derived from field-based data as inputs for energy modelling with the aim of applying these results to the study of explosive magma–water interactions.