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Reconstruction of the dynamics of hydrothermal explosions based on ballistic ejecta characterization - Insights from Pocket Basin, Yellowstone National Park

Bettina Scheu 1, Anna E. Freudenstein1,2, Lauren N. Harrison3,4, Arjun Sreekumar1, Shaul Hurwitz2, Shane Cronin5, Ingrid A. Ukstins5, Cristian Montanaro1

  • Affiliations: 1 Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Munich, Germany; 2 Earth and Planetary Sciences, ETH Zürich, Zürich, Schweiz; 3 Volcano Science Center, U.S. Geological Survey, Menlo Park, CA, USA; 4 Department of Geosciences, Colorado State University, Fort Collins, CO, USA; 5 School of Environment, University of Auckland, Auckland, New Zealand. 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 44

  • Programme No: 3.8.28

  • Theme 3 > Session 8

Abstract

Phreatic and hydrothermal explosions are sudden, violent events driven by the explosive vaporisation of near-boiling, pressurised water, which form characteristic craters, breccia deposits and ballistic strew-fields. Such explosions are controlled by the properties of the host rocks and any secondary alteration. Studying the lithologies in hydrothermal explosion breccias can allow to reconstruct the pre-explosive conditions in the subsurface reservoir and, combined with laboratory experiments, help decipher the explosion dynamics. Yellowstone National Park attracts more than 3 million visitors annually with its diverse geothermal manifestations. The Yellowstone Volcanic Field also hosts the largest number of hydrothermal explosions globally, ranging from small to the largest ever documented. However, much is still unknown about the triggers and reoccurrence intervals of these explosions. Pocket Basin is a large (400×800m) hydrothermal explosion crater that likely formed around 13--14ka. We mapped the distribution of ballistics and studied their variably hydrothermally altered lithologies applying a combined approach based on petrophysical, experimental, and analytical techniques to gain insights into the explosion dynamics and conditions that primed the explosion. Our results show that the prevailing silicic-argillic alteration of Pocket Basin rocks reduces porosity and permeability and increases the fragmentation threshold. Textural elements such as lithic clasts, crossbedding, and pre-explosion brecciation further influence petrophysical properties and fragmentation behaviour. The high pressure-temperature conditions required to induce fragmentation in saturated samples support the presence of a lake above the pre-explosive reservoir that would have increased confining pressure on the system. Rapid draining of the lake may have triggered the explosion.