Report on Ruapehu (New Zealand) — August 1989
Scientific Event Alert Network Bulletin, vol. 14, no. 8 (August 1989)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Small phreatic eruption
Please cite this report as:
Global Volcanism Program, 1989. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 14:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198908-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
The first reported phreatic eruptions from Crater Lake since January-February 1989 began in early July. Small steam explosions up to 200 m high were observed 1-3 July (the first at 1300 on the 1st) by climbers and two conservation officers. The explosions followed a week of volcanic tremor that first appeared 25 June and remained strong until declining in early July. Preliminary examination of seismic records showed no eruption signals.
During geologic field work on 24 July, Crater Lake appeared gray with dark surface slicks. A small phreatic eruption, accompanied by a relatively sharp detonation, occurred at about 0942. Twenty seconds later, the lake level fell 1 m, then passively rose to ~0.5-0.6 m above overflow and receded a similar amount. A 1-1.8 m wave soon washed onto the shore, succeeded by several smaller waves in the next 5 minutes. Ten minutes after the eruption, the lake temperature was 33.7°C, and by 1240, it had risen to 35.4°C, a sharp increase from 13.8°C measured on 14 June.
Deformation measurements indicated no significant change in the crater diameter. The Cl content of lake water collected 24 July was 694 mg/kg, higher than in samples collected 14 June, interpreted by geologists to indicate an increase in fumarolic activity but no apparent contact of lake water and fresh lava. The present activity appeared typical of at least three earlier episodes over the past two years in which a sudden 10-fold increase in the heatflow through the vent caused rapid lake heating and phreatic eruptions (figure 9). Due to generally poor weather, it is not known if minor eruptive activity has occurred continuously since early July.
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: B. Scott, NZGS Rotorua; P. Otway and S. Sherburn, DSIR Wairakei.