Abstract

The first lahars of the 1995 Ruapehu eruptive sequence were generated by explosively ejected Crater Lake water, sediment, and juvenile material, which incorporated snow and ice to form “snow slurry” lahars. On September 18, small amounts of Crater Lake water were ejected, which mobilized large amounts of snow. This was because unstable seasonal snow pack was present in the lahar flow path. Five days later, a larger volume of water was ejected but proportionally less snow was mobilized. Beyond the volcanic cone, the initially homogenized snow slurries segregated into watery flows with a floating snow-slurry cap. Frozen deposits were emplaced as far as 45 km from the source and 1840 m lower in altitude. More than 50% of the sediment within the flow was derived from the initial eruption; the remainder was entrained before it reached 9.5 km. Beyond 9.5 km from the source, the flows progressively lost their gravel and sand.

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