Report on Ruapehu (New Zealand) — July 1990
Bulletin of the Global Volcanism Network, vol. 15, no. 7 (July 1990)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Tremor declines after buoyant block eruption; Crater Lake temperature drops
Please cite this report as:
Global Volcanism Program, 1990. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Bulletin of the Global Volcanism Network, 15:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199007-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Crater Lake temperature (measured by a continuously recording Temtrac data logger) rose 3° to 27.5°C during the seven days that followed the ejection of hot buoyant blocks on 17 June, then fell steadily, reaching 21° on 20 July. Tremor amplitude decreased after 17 June, and tremor had disappeared from seismic records by 1 July.
During fieldwork on 20 July, Crater Lake was steam-free but turbid (battleship gray), with upwelling at the N vent and some slight upwelling at the lake's center. The lake's Mg/Cl ratio showed a slight increase (to 0.054), perhaps associated with the increased late June heat flow. However, the Mg/Cl data did not suggest significant lava extrusion onto the lake floor around the time of the 17 June activity. EDM data indicated reversal of deformation trends noted 17 June, and cumulative changes 10 May-20 July were generally small.
In BGVN 15:06 we cited a 1970 bathymetric survey that gave depths of 60-80 m to vents in Crater Lake. However, 1982 soundings yielded depths exceeding 180 m in a cone-shaped central eruptive area (Nairn and others, 1982). Nairn notes that the pressure change from lake bottom to surface would exceed 40 bars, requiring slow rise rates if blocks such as those that rose to the lake surface on 17 June are to survive without fragmenting from internal gas pressure. Such floating blocks would therefore be diagnostic of very low-energy, low-velocity phreatic events that only just reach the lake surface.
Reference. Nairn, I.A., Scott, B.J., Otway, P.M., and Cody, A.D., 1982, Depth measurements in Ruapehu Crater Lake: Volcano News, no. 12, p. 3.
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.