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 Ruapehu's Crater Lake. During the ea rlier phreatic and phreatomagmatic eruptions, lahars were generated by expulsion of waters from the Lake, but once the lake had emptied, lah ars were formed by remobilisation of seasonal snowpack laden with satu rated freshly erupted tephra. Four types of lahars occurred during the eruptive sequence: (1) Initial snow-slurry lahars, composed of granul ar snow and ice incorporated by eruptively expelled Crater Lake waters which left behind frozen deposits with 2.5-20% elastic sediment. (2) Large dilute lahars, generated as the volumes of ejected lake water in creased and removed much of the readily available snow. At least one t hird of the pre-eruption Crater Lake volume was expelled during one da y producing the largest lahars of the series. These lahars were hyperc oncentrated flows for up to 84 km from source, leaving extensive depos its along the channel margins. (3) Concentrated lahars; smaller volume lahars generated as the frequency of eruptions and volumes of expelle d water declined. These lahars were able to maintain high sediment con centrations, measured at 46-52% by volume suspended sediment at 42 km from source. Their high sediment concentrations were maintained by ero sion and incorporation of sand from the deposits of earlier flows whic h were lining the channel margins. (4) Remobilised tephra lahars, gene rated following the two largest tephra eruptions of the sequence. Seas onal snowpack was covered by water-saturated tephra. Warmer spring tem peratures and heavy rainfall events caused collapse and remobilisation of snow and tephra, producing several lahars in catchments draining e astern Ruapehu.