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Report on Ruapehu (New Zealand) — May 1995

Bulletin of the Global Volcanism Network, vol. 20, no. 5 (May 1995)
Managing Editor: Edward Venzke.

Ruapehu (New Zealand) Several phreatic eruptions from hot Crater Lake

Please cite this report as:

Global Volcanism Program, 1995. Report on Ruapehu (New Zealand). In: Venzke, E. (ed.), Bulletin of the Global Volcanism Network, 20:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199505-241100.

Volcano Profile |  Complete Bulletin


Ruapehu

New Zealand

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

All times are local (unless otherwise noted)


After a period of heating and minor eruptions in January-February, Crater Lake commenced a cooling phase in late February (BGVN 20:01 and 20:04). 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°C was recorded on 12 February, but by 19 April it was 31°C. Temperatures measured at 20-m depth (Argos satellite relayed) showed a similar trend, falling to stabilize around 35°C during the first part of April after being above the 47.5°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°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°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 6). 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.

Table 6. Ruapehu Crater Lake water analyses and temperatures at Outlet, 7 December 1994-4 May 1995. Courtesy of IGNS.

Date Mg (ppm) Cl (ppm) Mg/Cl Outlet Temp (°C)
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

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.

Geologic Background. 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 110 km3 dominantly andesitic volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake, is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.

Information Contacts: P.M. Otway, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.