Report on Ruapehu (New Zealand) — August 1987
Scientific Event Alert Network Bulletin, vol. 12, no. 8 (August 1987)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Crater lake temperature increase; convecting lake water
Please cite this report as:
Global Volcanism Program, 1987. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 12:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198708-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Crater Lake temperature had increased from 11.5 to 24.5°C between visits by geologists on 3 July and 17 August. Convection of lake water was occurring at both the central and N vents on 17 August. Outflow had increased from 30 l/s to >200 l/s, and total heat flow from the lake had increased by a factor of 4 since the July visit, reaching ~180 MW.
No recent eruptions appear to have occurred and no significant deformation or seismicity has accompanied the lake heating. Lake temperature had been relatively stable after dropping rapidly between November and December 1986, but reached a low in July (figure 7).
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.