Report on Ruapehu (New Zealand) — April 1982
Scientific Event Alert Network Bulletin, vol. 7, no. 4 (April 1982)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Explosive activity declines
Please cite this report as:
Global Volcanism Program, 1982. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 7:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198204-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
NZGS personnel observed increased explosive activity from Crater Lake in mid to late March, but no explosions occurred during visits to the summit area 15 and 21 April. The temperature of the lake water declined from 47°C on 23 March to 39° on 15 April, then increased slightly to 42° six days later. On 15 April, lake-surge deposits could be seen on 6-day-old snow as much as 2 m above the lake surface, but on 23 April there was no evidence of additional surges or recent ash emission. Deformation surveys indicated that about 12 mm of inflationary expansion had occurred across Crater Lake 23 MarCH-15 April, but 10 mm of contraction of the same line was measured on 23 April. However, this line remained 15 mm longer than it had been a year earlier.
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: A. Cody and I. Nairn, NZGS, Rotorua; P. Otway, NZGS, Wairakei.