Sj. Cronin et al., CHANGES IN WHANGAEHU RIVER LAHAR CHARACTERISTICS DURING THE 1995 ERUPTION SEQUENCE, RUAPEHU VOLCANO, NEW-ZEALAND, Journal of volcanology and geothermal research, 76(1-2), 1997, pp. 47-61
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.