Fragmentation mechanisms associated with explosive lava–water interactions in a lacustrine environment

Abstract

Rootless cones form when partially outgassed lava interacts explosively with external water. The explosions represent an end-member system that can elucidate mechanisms of explosive magma–water interactions in the absence of magmatic fragmentation induced by outgassing. The proportion of finely fragmented ejecta (i.e., ash), generated in rootless explosions, may contribute significantly to the energy of the explosion even if the ash volume is small relative to coarser ejecta. Laboratory experiments indicate that the degree of melt–water mixing and energy release are proportional to the abundance of blocky grains, fragmented by brittle disintegration, which effectively contribute thermal energy to the system. To constrain the mechanisms and dynamics of rootless explosive activity, we assess the nature and modes of fragmentation and ejecta characteristics through morphological, textural, and density analysis of rootless tephra associated with a pāhoehoe lava flow in a lacustrine (lake basin) environment. We observe strong correlations between the mean grain size and the mass percentage of both blocky (negative power law trend) and fluidal (positive logarithmic) tephra clasts of all sizes. We interpret these trends as scale-dependent fragmentation behavior due to the decreasing efficacy of hydrodynamic fragmentation as it occurs over finer scales, especially over the ash size range. Additionally, all analyzed beds contain fine ash-sized blocky and mossy clasts, which are thought to be diagnostic of a high transfer rate of thermal to mechanical energy, characteristic of molten fuel–coolant interactions. These results agree with a recent model of rootless cone formation, prior fragmentation theory, and scaled laboratory experiments and therefore provide a field-based analog for future experimental and modeling efforts.

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Acknowledgements

We would like to thank the Icelandic Institute of Natural History for generous access to the study area. We are grateful to Gary Huss, Ryan Ogliore, Aurelien Thomen, Elizabeth Koeman, and Myriam Telus at the Hawai‘i Institute of Geophysics and Planetology for help with sample preparation and assistance with SEM and EDS analysis. We also thank Bruce Houghton, Scott Rowland, John Sinton, Maria Janebo, and Samantha Isgett at the University of Hawai‘i Department of Geology and Geophysics for helpful discussions. This work was supported by NSF EAR-1119648 HIGP publication number 2240 and SOEST publication number 9876.

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Correspondence to Erin P. Fitch.

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Fitch, E.P., Fagents, S.A., Thordarson, T. et al. Fragmentation mechanisms associated with explosive lava–water interactions in a lacustrine environment. Bull Volcanol 79, 12 (2017). https://doi.org/10.1007/s00445-016-1087-3

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Keywords

  • Rootless cones
  • Phreatomagmatic
  • Ash
  • Molten fuel–coolant interaction
  • MFCI
  • Fragmentation