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

Abstract.

A feature of small-scale explosive volcanism at stratovolcanoes is the rapid destruction of primary near-vent pyroclastic deposits by sedimentary processes. A protracted series of explosive eruptions of moderate volume from September 1995 until July 1996 at Mount Ruapehu in New Zealand, its largest eruptive episode this century, afforded an opportunity to study these remobilisation processes in detail. All significant sub-plinian eruptions occurred in mid-winter, forming metre-thick tephra accumulations on steep slopes covered with perennial ice and seasonal snow. Subsequent events demonstrated the variety and complexity of the erosion processes that remobilise primary pyroclasts in such a setting. These processes arose from the complex interactions of tephra with snow and ice, and liquid water in varying proportions, 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-eruptive remobilisation processes included the generation of eruption-triggered lahars by the ejection of hot water from the Crater Lake. Post-eruptive interactions mainly remobilised fall deposits from proximal areas, and included rain-triggered lahars, which were among the largest and most hazardous events with the greatest distal impacts.