Quaternary lahar stratigraphy of the western Ruapehu ring plain, New Zealand

Abstract

One of the most widespread hazards associated with explosive activity at Mt Ruapehu is the production of potentially destructive lahars. Volcanic activity at this andesite stratovolcano has produced numerous lahars throughout Quaternary time, with the resulting deposits being preserved principally on the lower slopes of the composite cone and its ring plain. This paper describes the stratigraphy of volcaniclastic sequences on the western Ruapehu ring plain, in order to improve the record of known events. The lithologic record is principally one of lahar deposits interbedded with datable andesitic tephras, loesses, paleosols, and rhyolitic marker beds, allowing a chronological reconstruction of volcanic events in the region during the last 120 000 yr. The main laharic surfaces include the extensive Porewan‐aged (>65 000 yr B.P.) uplifted Erua plateau (Waimarino Formation, new) and voluminous Ratan‐Ohakean (10 000–65 000 yr B.P.) sequences of stacked diamictons east of the Waimarino Fault (Horopito Formation, new, and Te Heuheu Formation). A marked change in eruptive style occurred around 10 000 yr B.P. when pumiceous pyroclastic flows were emplaced to the east and the west of the Mt Ruapehu cone (Taurewa Eruptive Episode). At about this time, river entrenchment into the Ohakean surface began. Holocene lahar deposits are preserved only in the Mangaturuturu and Mangawhero catchments where they are confined to riparian strips. In the Mangawhero catchment a mid‐Holocene debris flow unit (Turoa diamicton) contains numerous jigsaw‐jointed bombs and vitric fragments, indicative of a coeval vulcanian eruption. In historical time, only lahars in 1975 left recognisable deposits in the Mangaturuturu catchment. Thus, the potential for long‐term preservation of modern, small‐size sandy diamictons is strictly limited on andesite stratovolcanoes such as Ruapehu.