Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 5, May 1995

Ruapehu (New Zealand)  Several phreatic eruptions from hot Crater

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

After a period of heating and minor eruptions in January-February,
Crater Lake commenced a cooling phase in late February (Bulletin v.
20, nos. 1 & 4). A new heating phase that began in mid-April was
continuing through May. This latest episode has been unusually
vigorous, and at least one phreatic eruption (on 25 April) appears
to have been larger than the generally minor activity seen during
the last decade, with the exception of a vent-clearing eruption in
late 1988. Significant changes in the lake chemistry indicated
changed vent conditions, and tremor bursts have been particularly
strong. The alert level was raised to Stage 2, indicating a
significant change in ongoing activity and an increase from the
usual weak eruptions.

During a visit by IGNS scientists on 19 April there was no
upwelling visible over the central vent, but some upwelling was
seen over the N vents and small wisps of steam were rising from the
lake surface. Lake level had risen slightly since 2 March, when it
was 0.15 m below overflow, resulting in diffuse outflow (~10 l/s).

The 13-year lake surface temperature high of 55 deg C was recorded
on 12 February, but by 19 April it was 31 deg C. Temperatures
measured at 20-m depth (Argos satellite relayed) showed a similar
trend, falling to stabilize around 35 deg C during the first part
of April after being above the 47.5 deg C threshold level prior to
13 March. A slow increase in lake temperature began on 16 April,
and became more rapid, though irregular, on the 24th. Low-frequency
acoustic noise detected by the Argos system reached one of the
highest values recorded in the past several years on 25-26 April,
immediately following a temperature jump above 47.5 deg C. Three
phreatic eruptions were reported on 27 April; a burst of
medium-frequency noise was also recorded. A period of strong noise
on 29-30 April may have indicated another eruption.

When IGNS geologists visited again on 4 May, surface water
temperature had risen to 46 deg C, while the water level had fallen
to ~5 cm below overflow. There was also evidence of high flows, and
wave erosion that extended to 2.5 m above lake level. Equipment
along the shore of the lake had been moved by wave action, possibly
caused by an eruption and large wave associated with the acoustic
noise on 25-26 April. A small eruption heard on 19 April only
caused a small fluctation in the lake level, and moderate noise was
recorded by the Argos system. Steam clouds ~500 m above the crater
were observed intermittently throughout the first half of May.

Although Mg/Cl levels had declined steadily since 1990, in May they
increased abruptly to reach early 1994 levels (table 1). Absolute
levels of aqueous Cl have increased by ~12% since December 1994,
consistent with the increased discharge of HCl-bearing steam into
the lake during the last two heating events. A recent increase in
Mg was taken to indicate that either unaltered andesitic material
was exposed to the lake water circulating through the vent, or
previously equilibrated vent fluids had been expelled during the
May-April phase of activity.

Volcanic tremor has dominated seismic records since mid-April, and
during four episodes lasting as long as 4 days, it was particularly
strong and centered around 2 Hz. Owing to its similarity to wind
noise, higher frequency tremor has remained difficult to quantify;
volcano-tectonic earthquakes remained very rare. A 19 April
distance survey carried out under good conditions failed to show
volcanically significant deformation.

Information Contact: P.M. Otway, IGNS (see White Island).

Table 1. Ruapehu Crater Lake water analyses and temperatures at
Outlet, 7 December 1994-4 May 1995. Courtesy of IGNS.

Date            Mg      Cl    Mg/Cl    Temp xC
               ppm     ppm
07 Dec 1994    239    6451    0.037    22.0
13 Jan 1995    241    6652    0.036    41.5
18 Jan 1995    237    6662    0.035    46.5
29 Jan 1995    235    6719    0.035    51.4
02 Mar 1995    243    7018    0.035    45.5
19 Apr 1995    226    6989    0.032    31.0
04 May 1995    278    7235    0.038    46.0