Report on Ruapehu (New Zealand) — August 1986
Scientific Event Alert Network Bulletin, vol. 11, no. 8 (August 1986)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Crater Lake temperature rises after 5-month decline
Please cite this report as:
Global Volcanism Program, 1986. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 11:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198608-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Crater Lake had cooled 6.5°C/month since February, reaching 10.8°C by 31 July. Four similar periods of cooling have occurred during the last 6 years, three of them followed by minor hydrothermal eruptions 2-3 months later.
On 19 and 31 July, weak upwelling was occurring at the N end of Crater Lake. Yellow sulfur slicks appeared to emanate from the N vent area, with dark green slicks in the center of the lake. By the next visit, on 26 August, lake temperature had risen to 14.5°C. Lake appearance was similar to that of late July, with some weak upwelling over the central vent.
Deformation measurements on 31 July recorded only slight changes since the 8 May survey, including 13 mm of apparent shortening across the crater. Little additional change was measured 26 August.
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. Otway, NZGS Wairakei; I. Nairn, NZGS Rotorua.