Lahar-Triggering Mechanisms and Hazard at Ruapehu Volcano,New Zealand

Late Holocene volcanic activity at Ruapehu has been characterizedby the generation of small (<10⁵ m³) to very large (>10⁷ m³) lahars and repeated,small to medium (VEI 1-3) tephra-producing eruptions. The Onetapu Formation groupsall lahar deposits that accumulated during the last 2,000 years on the southeastern Ruapehu ring plain. The andesitic tephras are grouped within the Tufa Trig Formation and are intercalated within the laharic sequence. By correlating these two formations with new radiocarbon ages obtained on interbedded paleosols, we reconstruct a detailed volcanic history of Ruapehu for this period.Clast assemblages identified in the laharic sequences record thelithologies of synchronous tephras and rocks within the source region. These assemblages suggest a strong genetic link between the development of Crater Lake, the variation in eruptivestyles, and the production of lahars.Lahar-triggering mechanisms include: (1) flank collapse ofhydrothermally altered and unstable portions of the cone; (2) phreatic and phreatomagmatic eruptions favoring the generation of snow-rich slurries and hyperconcentrated stream flows; (3) suddenCrater Lake rim collapse, releasing large amounts of water inducing debris flows; and (4) eruptions that generate large volumes of tephra on snow-covered slopes, later remobilized by heavy rain.Two major lahars in the Onetapu sequence had a volume 4 × 10⁷ m³, roughly 1 to 2 orders of magnitude larger than the 1953event leading to the Tangiwai disaster (151 casualties). One of these lahars crossed over a lowinterfluve currently separating the Whangaehu River from a stream feeding the Tongariro River,sometime since peat accumulated between AD 1400 and AD 1660. A repetition of such a large-scaleevent would have devastating consequences on the infrastructure, economy and environment withinthe distal areas of the two catchments. The 1995–1996 eruptions were a timely reminder ofthe hazards posed by the volcano.