Evolution of a small hydrothermal eruption episode through a mud pool of varying depth and rheology, White Island, NZ

Abstract

White Island volcano, New Zealand was a host to multiple hydrothermal eruptive episodes within a mud-sulphur pool in 2013. Although hydrothermal activity is common at White Island, past events have largely gone undescribed in favour of the larger phreatomagmatic and magmatic eruptions. Here, we detail the first and longest hydrothermal episode of 2013, lasting from 15 January to 7 February using video and photo analysis from tour operators and staff responsible for monitoring the volcano. Differences in the dominant bubble burst style across this episode led to the classification of four distinct eruption regimes: (1) multiple irregular bursts on the pool surface, (2) larger distinct symmetric hemispheres with starbursts and/or followed by mud heaves, (3) no initial pool surface deformation but a vertical steam jet followed by a sometimes large directed mud heave and (4) no lake and continuous pulsating dry ash and block venting. The progression through these regimes is associated with a lowering lake level and a concomitantly increasing viscosity of the pool, which initially comprises a low viscosity muddy water, and partially evaporates to yield a shallow layer of high viscosity mud that ends with the complete drying up of the mud pool. Formation of primary mud hemispheres or gas jets is followed by heaves or secondary upheaval events. The heights of these heaves are used as a measure of explosivity. Heights increase from ∼8 m during regime 1 on 15 January to ∼102 m during regime 3 on 28 January. Venting of dry mud during regime 4 developed on 29 January before a regression back to regime 1 took place on 7 February as the pool re-established. Through observations of the shapes of ejected mud clots, we propose that the increasing explosivity of higher number regimes is primarily due to increasing slug bubble lengths teamed with increasing mud pool viscosity. We attribute a lesser control to the decreasing depth of the pool during its progressive desiccation, which may in turn influence the bubble burst depth. Occasionally, visible yellowing of the steam/gas plume led us to suggest that elemental sulphur may also be present in the conduit and may also play a role in regulating bubble release dynamics. Although, evidence for magmatic/phreatomagmatic eruptions was present during eruptions later in 2013, we found no evidence for juvenile magma in the January-February eruption episode described here. However, we concur with other investigators that magma was probably intruded to shallow levels and may have driven heat and gas flux. Our explanation for the correlation of pool depth, mud viscosity and eruption regime is based on a conceptual model in which a pool is perched above a two phase hydrothermal system and is sensitive to changes in the heat and gas flux from shallow magma. The variable release of gas and thermal perturbations in the course of the January-February eruptive episode impacted the pool level, the water to sediment ratio in the pool, and thus its viscosity, and in turn modulated the eruption regime. The varying degree of explosivity throughout this episode calls for a new consideration of pool properties in assessing eruption hazards at this frequently visited volcano. We additionally emphasise that ballistic hazards from small eruptions exist coupled with a range of seismic signals and that the hazard was greatest during infrasound tremor.