Report on Ruapehu (New Zealand) — July 1985
Scientific Event Alert Network Bulletin, vol. 10, no. 7 (July 1985)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Hydrothermal activity ceases; lake temperatures drop
Please cite this report as:
Global Volcanism Program, 1985. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 10:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198507-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Geologists returned to Crater Lake on 11 July and 5 August to monitor trends in lake temperature and crater deformation following June's increased activity.
In July, the lake was the normal battleship-gray color with some sulfur slicks at the central vent area, unchanged from late June. The lake temperature was 31.5°C (a 5.5° decline since late June) and the air carried a strong gas odor toward one lake outlet. The lake was about 0.5 m below overflow level and there was no evidence of recent large surges. No hydrothermal eruptions cccurred during the 4.5-hour observation period.
By 5 August, the lake color had changed to light gray and its temperature was nearly unchanged. There was no sign of convection at either of the two vents, and snow and ice were within 0.5 m of the lake surface. The lake had begun overflowing at 3 l/s.
Only a 15 ± 10 mm expansion across the N rim of the crater was reported on the 11 July inspection. Rapid but minor deformations were occurring in July, but their significance was uncertain.
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: I. Nairn, NZGS, Rotorua; P. Otway, NZGS, Wairakei.