Tephra, snow and water: complex sedimentary responses at an active snow-capped stratovolcano, Ruapehu, New Zealand

A feature of small-scale explosive volcanism at stratovolcanoes is the rapi d destruction of primary near-vent pyroclastic deposits by sedimentary proc esses. A protracted series of explosive eruptions of moderate volume from S eptember 1995 until July 1996 at Mount Ruapehu in New Zealand, its largest eruptive episode this century, afforded an opportunity to study these remob ilisation processes in detail. All significant sub-plinian eruptions occurr ed in mid-winter, forming metre-thick tephra accumulations on steep slopes covered with perennial ice and seasonal snow. Subsequent events demonstrate d the variety and complexity of the erosion processes that remobilise prima ry pyroclasts in such a setting. These processes arose from the complex int eractions of tephra with snow and ice, and liquid water in varying proporti ons, and were very diverse in nature and scale. Their effectiveness can be gauged from the fact that there is almost no stratigraphic record of any of the >40 eruption episodes recorded in the past 100 years at Ruapehu. Syn-e ruptive remobilisation processes included the generation of eruption-trigge red lahars by the ejection of hot water from the Crater Lake. Post-eruptive interactions mainly remobilised fall deposits from proximal areas, and inc luded rain-triggered lahars, which were among the largest and most hazardou s events with the greatest distal impacts.