Report on Ruapehu (New Zealand) — September 1987
Scientific Event Alert Network Bulletin, vol. 12, no. 9 (September 1987)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Small hydrothermal eruptions
Please cite this report as:
Global Volcanism Program, 1987. Report on Ruapehu (New Zealand) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198709-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Small hydrothermal eruptions were reported on 24 August as 1-m-high bubble-like updomings in the lake center. The upwelling generated waves and steam plumes. Steam plumes were also observed 29-30 August when several eruptions threw muddy water jets to >10 m above the lake surface. On 31 August lake temperature was measured at 40°C, up from 24.5°C on 17 August. During a 14 September inspection the lake was steaming strongly. The observed hydrothermal events did not appear to correlate with seismicity, which continued at generally low levels. The activity was regarded by NZGS personnel as part of the normal cycle of heatflow rather than precursory to a major eruption.
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 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 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 (Te Wai a-moe), 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 3,000 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.