Report on Ruapehu (New Zealand) — December 1982
Scientific Event Alert Network Bulletin, vol. 7, no. 12 (December 1982)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Moderate inflation; lake temperature lower
Please cite this report as:
Global Volcanism Program, 1982. Report on Ruapehu (New Zealand) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 7:12. Smithsonian Institution.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
A NZGS deformation survey on 15 December showed a moderate degree of inflation. The distance between two stations on opposite sides of the 600-m-wide crater was 15-20 mm longer than on 17 September. About half the increase had occurred since 9 November.
The temperature of Crater Lake as measured at the outlet was 13°C, the lowest of the year. The lake temperature had been falling since the eruptive activity in January. Some intermittent upwelling was observed in the N part of the lake, but little or none in the center. Due to meltwater inflow, the lake was greener and less turbid than in October, and the magnesium and chlorine concentrations were lower. The Mg:Cl ratio was 0.115 on 21 October, 0.113 on 9 November and 0.109 on 15 December. The decline was attributed to lack of interaction between lake water and lava (lower Mg values), but some continued fumarolic input of Cl.
The NZGS team noted that similar periods of cooling lake water in 1979, 1980, and 1981 ended with rapid lake temperature increase accompanied by hydrothermal eruptions. They also noted that the inflation was moderate but probably significant, indicating that Ruapehu may enter a phase of renewed activity within the next few months.
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.