Changes in Whangaehu river lahar characteristics during the 1995 eruption sequence, Ruapehu volcano, New Zealand

During the 1995 Ruapehu eruptive sequence multiple lahars occurred in the Whangaehu river, which drains Ruapehuʹs Crater Lake. During the earlier phreatic and phreatomagmatic eruptions, lahars were generated by expulsion of waters from the Lake, but once the lake had emptied, lahars were formed by remobilisation of seasonal snowpack laden with saturated freshly erupted tephra. Four types of lahars occurred during the eruptive sequence: (1) Initial snow-slurry lahars, composed of granular snow and ice incorporated by eruptively expelled Crater Lake waters which left behind frozen deposits with 2.5–20% clastic sediment. (2) Large dilute lahars, generated as the volumes of ejected lake water increased and removed much of the readily available snow. At least one third of the pre-eruption Crater Lake volume was expelled during one day producing the largest lahars of the series. These lahars were hyperconcentrated flows for up to 84 km from source, leaving extensive deposits along the channel margins. (3) Concentrated lahars; smaller volume lahars generated as the frequency of eruptions and volumes of expelled water declined. These lahars were able to maintain high sediment concentrations, measured at 46–52% by volume suspended sediment at 42 km from source. Their high sediment concentrations were maintained by erosion and incorporation of sand from the deposits of earlier flows which were lining the channel margins. (4) Remobilised tephra lahars, generated following the two largest tephra eruptions of the sequence. Seasonal snowpack was covered by water-saturated tephra. Warmer spring temperatures and heavy rainfall events caused collapse and remobilisation of snow and tephra, producing several lahars in catchments draining eastern Ruapehu.