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Report on Ruapehu (New Zealand) — May 1994


Bulletin of the Global Volcanism Network, vol. 19, no. 5 (May 1994)
Managing Editor: Richard Wunderman.

Ruapehu (New Zealand) Cooling trend in crater lake ends in early May; no recent activity

Please cite this report as:

Global Volcanism Program, 1994. Report on Ruapehu (New Zealand) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 19:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199405-241100


New Zealand

39.28°S, 175.57°E; summit elev. 2797 m

All times are local (unless otherwise noted)

Heatflow during April remained low (table 4), but evidence of convection (dark slicks from the central vent) on 6 May indicated some recent increase. Lake temperature at 20 m depth continued to decline from 47°C on 18 February to 23.6°C on 6 May. Two bursts of strong tremor, on 5 and 8 May, corresponded to a renewed steady temperature rise to 24.9°C by 11 May. As with the previous heating phase, this activity occurred several weeks after strong low-frequency acoustic signals were recorded.

Table 4. Temperature, outflow measurements, and water analyses from the crater lake of Ruapehu, 18 January 1994 to 27 August 1994. Discharge of "0" indicates a lake level below overflow stage. A dash (--) signifies no measurement. Courtesy of IGNS.

Date Outlet (°C) Logger Point (°C) Discharge (l/s) Mg (ppm) Cl (ppm) Mg/Cl
18 Jan 1994 25.2 -- 230 255 6642 0.038
28 Jan 1994 32.7 -- <=200 278 7140 0.039
10 Feb 1994 36 39 -- 253 6646 0.038
18 Feb 1994 39 40 0 271 7118 0.038
26 Feb 1994 38.5 39.5 0 -- -- --
06 Mar 1994 32 36.5 0 -- -- --
12 Mar 1994 31.6 -- 0 273 7198 0.038
28 Mar 1994 25.0 -- low 277 7195 0.038
18 Apr 1994 23.0 -- 40 272 7150 0.038
06 May 1994 19.0 -- 110 270 7128 0.038
04 Jul 1994 -- -- -- 262 7029 0.037
12 Aug 1994 -- 16 ~25 -- -- --
27 Aug 1994 17 -- ~25 -- -- --

On 18 April the lake was a uniform battleship gray color with no evidence of upwelling, although the N vents were not fully visible from the observation point. No signs of surging were seen around the shoreline or at Outlet. A dark khaki-green slick emanating from the central vent area on 6 May drifted slowly onto the SE shore, but no upwelling was observed. Broken yellow slicks originating from several weak upwelling cells in the N vent area were also present over the N half of the lake. The general color of the lake was the same as in April, and there was no sign of recent activity. Prior to the heating episode in February, the ratio of Mg to Cl in the lake water decreased slightly from 0.042 in late 1993 to 0.038 in January (table 4), due mainly to a decrease in Mg. This ratio had remained stable at least through 18 April.

Inspection of photographs taken during the reported steam eruption on 1 March revealed an apparently passive steam cloud, a common atmospheric effect at the crater lake. The rising cloud was most intense over an area of discolored water, and may have been caused by vigorous convection or a minor phreatic event shortly beforehand. This incident is a reminder that even reports from reliable eyewitnesses should be treated with caution; reports of possible eruptions in February-April should be regarded as unproven.

The only deformation change of possible volcanic significance detected on 6 May was a reversal of the 9 mm contraction of the crater width indicator line recorded between 12 and 28 March. This suggested a return to the mildly inflated level first recorded in January. It is not yet known if the evidence of minor inflation is significant. A leveling survey on 18 April indicated 21 µrad of tilt towards the crater (deflation) at the Dome location over the past year, the largest tilt since 1981. Because this follows a period of slow apparent deflation (0.7 µrad/year), the measurement may not be reliable. Southern benchmarks may have been lowered by downhill creep of a lava slab. However, large systematic apparent tilts of

Geological Summary. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: P. Otway, IGNS Wairakei.