Hydrothermal eruption dynamics reflecting vertical variations in host rock geology and geothermal alteration, Champagne
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RESEARCH ARTICLE
Hydrothermal eruption dynamics reflecting vertical variations in host rock geology and geothermal alteration, Champagne Pool, Wai-o-tapu, New Zealand Anna Gallagher 1 & Cristian Montanaro 1,2
&
Shane Cronin 1 & Bradley Scott 3 & Donald B. Dingwell 2 & Bettina Scheu 2
Received: 6 June 2020 / Accepted: 15 October 2020 / Published online: 14 November 2020 # The Author(s) 2020
Abstract Hydrothermal eruptions are characterised by violent explosions ejecting steam, water, mud, and rock. They pose a risk to tourism and the operation of power plants in geothermal areas around the world. Large events with a severe destructive threat are often intensified by the injection of magmatic fluids along faults and fractures within volcanotectonic rifting environments, such as the Taupo Volcanic Zone. How these hydrothermal eruptions progress, how craters form and the scale of ejecta impacts, are all influenced by the local geology and reservoir hydrology. By analysing breccia lithology, undisturbed strata proximal to the explosion sites, and conducting tailored decompression experiments, we elucidate the eruption sequence that formed Champagne Pool, Wai-o-tapu, New Zealand. This iconic touristic site was formed by a violent hydrothermal eruption at ~ 700 years B.P. Samples from undisturbed drill cores and blocks ejected in the eruption were fragmented in shock-tube experiments under the moderate pressure/temperature conditions estimated for this system (3–4 MPa, 210–220 °C). Our results show that this was a two-phase eruption. It started with an initial narrow jetting of deep-sourced lithologies, ejecting fragments from at least a 110-m depth. This event was overtaken by a larger, broader, and dominantly shallower eruption driven by decompression of much more geothermal fluid within a soft and porous ignimbrite horizon. The second phase was triggered once the initial, deeper-sourced eruption broke through a strong silicified aquitard cap. The soft ignimbrite collapsed during the second-phase eruption into the crater, to repeatedly choke the explosions causing short-term pressure rises that triggered ongoing deeper-sourced eruptions. The eruption spread laterally also by exploiting a local fault. These results are relevant for hydrothermal eruption hazard scenarios in environments where strong vertical variations in rock strength and porosity occur. Keywords Hydrothermal eruptions . Geothermal . Champagne pool . Experimental . Eruption dynamics
Introduction Editorial responsibility: G. Lube Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s00445-02001414-3. * Cristian Montanaro [email protected] 1
School of Environment, Science Centre, University of Auckland, Building 302, 23 Symonds Street, Auckland Central, New Zealand
2
Ludwig-Maximilians-Universität München, Theresienstrasse 41, 80333 Munich, Germany
3
GNS Science, 114 Karetoto Rd., Wairakei 3377, New Zealand
Steam-driven hydrothermal eruptions commonly occur in bot
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