Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 4, April 1995

Ruapehu (New Zealand)  Crater lake temperature drops 10 deg C from
      13-year high

North Island, New Zealand
39.28S, 175.57E; summit elev. 2,797 m

The following was extracted from the IGNS Ruapehu Immediate Report
(RUA 95/02). Peaks on the crater lake temperature versus time curve
have often correlated to small vent-clearing eruptions (see figure
in Bulletin v. 20, no. 1).
"Crater Lake has been in a heating phase since late November,
reaching the highest temperature (55 deg C) in 13 years by 12
February, but a 10 deg C decline since then and a reduction in
volume suggest this phase has peaked. Minor phreatic eruptions have
been occurring since early January but appear to have become
infrequent, or may have even ceased, during February. Despite the
relatively high heat output, the recent activity has so far
followed the cycle of heating and cooling typical of Ruapehu since
at least 1985."
There were several reports of steam clouds and other phenomena
after 20 January. A hiker on 24 January described the crater lake
seen through the clouds as "a seething surface" that made "roaring
sounds" lasting 1 to 2 minutes.
Two or more observers on 29 January described the crater lake,
which was visible for almost 2 hours, as "pale gray, almost white"
and two, 1.5 m (or smaller) upwelling and splashing episodes were
seen. The report also mentioned "pure yellow styrofoam-sulfur"
littering the Outlet area. The water temperature, measured with two
calibrated thermometers, was 51.4 deg C.
Hikers in cloudy weather on 30 January witnessed a "small
hydrothermal eruption up to 10-20 m." Hikers in cloudy weather on
5 February heard sloshing noises from the crater lake followed by
two "loud explosions." On 15 February observers saw a 3 km tall,
stationary steam plume over the crater lake; on 25 and 27 February
observers also saw steam clouds. These clouds were undoubtedly
steam, but they may have arisen from "atmospheric enhancement" due
to a rise in relative humidity rather than from definite eruptions.
Their interpretation thus remains ambiguous. A ground inspection on
2 March failed to confirm any significant surging took place around
the shore of Lake Wade.
In the interval 31 January-early March there were few discrete
earthquakes and mainly background tremor was detected on the
volcano's Dome seismograph. On the other hand, there were short
intervals of strong, high frequency tremor, an unusual occurrence
for Ruapehu.
Although in the latest crater visit on 2 March all deformation
survey stations were accessible and clear of snow, most of the
length changes seen since 13 January were insignificant (<= 5 mm).
Station I (see map, Bulletin v. 19, no. 12) appeared to have moved
18 mm ENE relative to all other stations since May 1994--a motion
consistent with moderate deflation seen in the past 10 months, but
also possibly due to displacement by local snow loading or other
Mg and Cl analyses of lake water were made on 18 and 29 January,
and on 2 March, but showed relatively change. The Mg/Cl ratio
changed only about 4% (shifting downward from an 18 January value
of 0.036 to a 2 March value of 0.035). The Mg/Cl ratios were
interpreted to indicate that the heating event was driven by
convective flow of lake water through the upper portion of the
vent. Thus, the heating event was regarded as mainly due to fluid
flow rather than heat input from magmatic sources within the
Information Contact: P.M. Otway, Institute of Geological and
Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.