Abstract
Breccia-filled eruption conduits are dynamic systems where pressures frequently exceed critical thresholds, generating earthquakes and transmitting fluids. To assess the dynamics of breccia-filled conduits, we examine lava, ash tuff, and hydrothermal breccia ballistics with varying alteration, veining, fractures, and brecciation ejected during the 27 April 2016 phreatic eruption of Whakaari/White Island. We measure connected porosity, strength, and permeability with and without tensile fractures at a range of confining pressures. Many samples are progressively altered with anhydrite, alunite, and silica polymorphs. The measurements show a large range of connected porosity, permeability, and strength. In contrast, the cracked samples show a consistently high permeability. The cracked altered samples have a permeability more sensitive to confining pressure than the unaltered samples. The permeability of our altered ballistics is lower than surface rocks of equivalent porosity, illustrating that mineral precipitation locally blocked pores and cracks. We surmise that alteration within the conduit breccia allows cracks to form, open and close, in response to pore pressure and confining pressure, providing a mechanism for frequent and variable fluid advection pulses to the surface. This produces temporally and spatially variable geophysical and geochemical observations and has implications for volcano monitoring for any volcano system with significant hydrothermal activity.
Highlights
IntroductionThe behaviour of breccia-filled conduits remains a potentially controlling factor on the pressurization timescales and total pressure build-up
The tragic events on November 2019 at Whakaari (White Island; New Zealand), which killed over 20 people, highlight the need for a better understanding of the controls on phreatic eruptions.In particular, the behaviour of breccia-filled conduits remains a potentially controlling factor on the pressurization timescales and total pressure build-up
We focus our analysis on relatively unaltered lava from the surface, ballistics, and sulfur cemented tuff from surface fumaroles [13]
Summary
The behaviour of breccia-filled conduits remains a potentially controlling factor on the pressurization timescales and total pressure build-up. Geosciences 2020, 10, 138 phreatic/hydrothermal eruption that is responsible for clearing a pathway for magma to reach the surface [1]. Shallow intrusions and economically significant mineral resources are frequently associated with hydrothermal breccia sheets and pipes [2]. The physical properties of these breccia filled conduits is critical to volcano monitoring and economic mineral exploration. The physical properties of the altered materials that fill hydrothermal conduits directly control (1) magma outgassing efficiency, (2) the build-up of pressure that can lead to explosive eruptions [3], and (3) subsequent fluid flow, hydrothermal alteration, and mineralization. Past studies of magmatic conduits and breccias reveal insights into magma flow, outgassing, and fragmentation processes [4,5,6,7]
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