ELSEVIER Journal of Volcanology and Geothermal Research 76 (1997) 47-61 Changes in Whangaehu river lahar characteristics during the 1995 eruption sequence, Ruapehu volcano, New Zealand Shane J. Cronin *, V.E. Neall, J.A. Lecointre, A.S. Palmer zyxwvutsrqponmlkjihgf Department zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Soil Science, Massey Unioersity, Private Bag I1 222, Palmerston North, New Zealand Received 7 June 1996; accepted 17 September 1996 zyxwvutsrqponmlkjihgfedcbaZYXWVUTS Abstract During the 1995 Ruapehu eruptive sequence multiple lahars occurred in the Wbangaehu river, which drains Ruapehu’s 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% elastic 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 Mars were hyperconcentrated flows for up to 84 km from source, leaving extensive deposits along the channel margins. (3) Concentrated Mars; smaller volume Mars 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 Mars in catchments draining eastern Ruapehu. Keywords: lahar: Ruapehu volcano; Crater lake; phreatomagmatic eruption; debris flow; hyperconcentrated streamflow; Whangaehu river 1. Introduction The 1995 Ruapehu eruptive sequence began on September 18 with a small phreatomagmatic erup- tion through Crater Lake which generated a lahar in the Whangaehu river on the eastern flank of the volcano (Fig. 1). Several more phreatic and phreatomagmatic eruptions through Crater Lake dur- * Corresponding author. ing September and October eventually displaced all of the lake water, producing multiple Mars, mostly within the Whangaehu river. Lahars in other catch- ments during this time did not travel beyond 3 km from source. From September 18 until the lake basin was empty on October 12, twenty-six Mars were recorded with a total gauged volume of 10.2 x lo6 m3 at 56.5 km from source. Once Crater Lake had emptied, the frequency of Mars decreased over the next two weeks. However, on October 28 renewed lahar activity commenced in the Whangaehu and 0377-0273/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO377-0273(96)00064-9