Report on Ruapehu (New Zealand) — August 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 8 (August 1983)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Upwelling in crater lake; slight inflation
Please cite this report as:
Global Volcanism Program, 1983. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 8:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198308-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Inspections by NZGS personnel revealed no significant changes. On 15 and 21 July, and 16 August, Crater Lake appeared its usual battleship gray color. Moderate upwelling was observed over the vents at the N end of the lake, none over the central vent. Water temperature measured at the outlet was 18.7°C in July (2.2° higher than on 15 June), and 18.3° in August. Concentrations of magnesium (778 and 744 ppm) and chlorine (7,310 and 7,470 ppm) were virtually unchanged for the two months. Horizontal deformation surveys showed that by August the distance between two stations on opposite sides of the 600 m-wide crater had increased 16 mm from a 2-year minimum value measured in June.
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: P. Otway, NZGS, Wairakei; B. Scott, A. Cody, NZGS, Rotorua.