Abstract
On 27 April 2016, White Island erupted in a multi-pulse, phreatic event that lasted for ~ 40 min. Six, variably sized pulses generated three ballistic ejections and at least one pyroclastic surge out of the inner crater and onto the main crater floor. Deposit mapping of the pyroclastic surge and directed ballistic ejecta, combined with numerical modelling, is used to constrain the volume of the ejecta and the energetics of the eruption. Vent locations and directionality of the eruption are constrained by the ballistic modelling, suggesting that the vent/s were angled towards the east. Using these data, a model is developed that comports with the field and geophysical data. One of the main factors modifying the dispersal of the eruption deposits is the inner crater wall, which is ~ 20 m high. This wall prevents some of the pyroclastic surge and ballistic ejecta from being deposited onto the main crater floor but also promotes significant inflation of the surge, generating a semi-buoyant plume that deposits ash high on the crater walls. While the eruption is small volume, the complexity determined from the deposits provides a case study with which to assess the relatively frequent hazards posed by active volcanoes that host hydrothermal systems. The deposits of this and similar eruptions are readily eroded, and for complete understanding of volcanic hazards, it is necessary to make observations and collect samples soon after these events.
Highlights
Phreatic eruptions are generated by either ascending fluids heated by magma (Browne and Lawless 2001), generally in a volcano hydrothermal system, or by the release and ascent of magmatic gas into a sealed or partially sealed hydrothermal system (e.g. Jolly et al 2010)
We focus on the 27 April 2016 event to unpick the eruption dynamics so that we have a better understanding of these types of phenomena at White Island, and as an analogue phreatic eruption that was well monitored and where field visits were made soon after to collect potentially perishable sample data
The extent of the dominantly ash deposits is readily identified for its distinctive yellow–brown alteration caused by the oxidation of sulphur-rich fluid within the deposits
Summary
Phreatic eruptions are generated by either ascending fluids heated by magma (Browne and Lawless 2001), generally in a volcano hydrothermal system, or by the release and ascent of magmatic gas into a sealed or partially sealed hydrothermal system (e.g. Jolly et al 2010). Jolly et al 2010) This style of eruption is difficult to forecast (despite recent attempts, e.g. Chardot et al (2015); de Moor et al (2016); Girona et al (2018)), partly because the driving mechanisms can be complicated and involve potentially rapid onsets. To compound poor forecasting, these areas are frequently popular with tourists (Fitzgerald et al 2017) and eruption hazard footprints may be poorly. With careful fieldwork, modelling and experimental work, the range of processes involved in phreatic eruptions can be better understood and the potential to forecast with adequate warning will improve. In water-bearing hydrothermal environments, the hazard footprint of a phreatic eruption is dominated by ballistic ejecta, pyroclastic surges and fallout of ash from steam plumes. Cases of phreatic eruptions where many volcanological disciplines have come together are rare and include the 2007 eruption of Ruapehu (Christenson et al 2010; Jolly et al 2010; Kilgour et al 2010), 2012 eruption of Te Maari, Tongariro (e.g. Crouch et al 2015; Jolly et al 2014) and 2014 eruption at Ontake, Japan (e.g. Kaneko et al 2016; Tsunematsu et al 2016)
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