A recent volcanic eruption near Tonga in the southwest Pacific created a new island, giving scientists a rare opportunity to explore the volcanic record of this remote region.
On 25 September 1995, phreatomagmatic explosions through Crater Lake at Ruapehu volcano, New Zealand, generated a closely spaced sequence of lahars. From direct observations of the flows and timely description of their deposits, we infer these debris flows transformed to hyperconcentrated streamflows not by dilution with incorporated water they overran, as previously proposed. Rather, the described debris flows diluted by selective deposition of their coarse clasts as they thinned and decelerated while spreading >700 m laterally over the Whangaehu fan. Deposits recording this transformation are veneering (<100 mm thick) layers of muddy sandy gravel interspersed with many boulders and cobbles. Downstream of their transformation to hyperconcentrated streamflows, ephemeral near-channel deposits indicate the flows were vertically stratified. A new depositional model for these hyperconcentrated streamflows includes a basal, coarse, sediment-concentrated "channel flow" that emplaced transitory near-channel sediment wedges. The near-channel sediment was bouldery, massive, and poorly sorted, like debris-flow deposits elsewhere in the Whangaehu catchment. The upper and marginal parts of the lahars (the surface layer) were diluted, finer hyperconcentrated flows that left voluminous overbank deposits. The overbank sediment is poorly sorted gravelly sand, with some degree of horizontal bedding, like other hyperconcentrated flow deposits elsewhere in the catchment. The rapid erosion of channelflow deposits within days to months of the events indicates that geologic records will only preserve lateral-flow deposits of such lahars. Hence, long after an event, interpretation of hyperconcentrated streamflow mechanisms from geologic deposits can be misleading without the near-channel record.
Steam-driven eruptions are caused by explosive vaporization of water within the pores and cracks of a host rock, mainly within geothermal or volcanic terrains. Ground or surface water can be heated and pressurized rapidly from below (phreatic explosions), or already hot and pressurized fluids in hydrothermal systems may decompress when host rocks or seals fail (hydrothermal eruptions). Deposit characteristics and crater morphology can be used in combination with knowledge of host-rock lithology to reconstruct the locus, dynamics, and possible triggers of these events. We investigated a complex field of >30 craters formed over three separate episodes of steam-driven eruptions at Lake Okaro within the Taupo volcanic zone, New Zealand. Fresh unaltered rock excavated from initially >70 m depths in the base of phase I breccia deposits showed that eruptions were deep, “bottom-up” explosions formed in the absence of a preexisting hydrothermal system. These phreatic explosions were likely triggered by sudden rise of magmatic fluids/gas to heat groundwater within an ignimbrite 70 m below the surface. Excavation of a linear set of craters and associated fracture development, along with continued heat input, caused posteruptive establishment of a large hydrothermal system within shallow, weakly compacted, and unconsolidated deposits, including the phase I breccia. After enough time for extensive hydrothermal alteration, erosion, and external sediment influx into the area, phase II occurred, possibly triggered by an earthquake or hydrological disruption to a geothermal system. Phase II produced a second network of craters into weakly compacted, altered, and pumice-rich tuff, as well as within deposits from phase I. Phase II breccias display vertical variation in lithology that reflects top-down excavation from shallow levels (10−20 m) to >70 m. After another hiatus, lake levels rose. Phase III hydrothermal explosions were later triggered by a sudden lake-level drop, excavating into deposits from previous eruptions. This case shows that once a hydrothermal system is established, repeated highly hazardous hydrothermal eruptions may follow that are as large as initial phreatic events.
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