Report on Ruapehu (New Zealand) — August 1999
Bulletin of the Global Volcanism Network, vol. 24, no. 8 (August 1999)
Managing Editor: Richard Wunderman.
Ruapehu (New Zealand) Visit on 17 September discloses Crater Lake's passive steaming
Please cite this report as:
Global Volcanism Program, 1999. Report on Ruapehu (New Zealand) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 24:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199908-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
On 17 September a group of scientists visited Ruapehu's Crater Lake in response to observed steam plumes and recent increases in volcanic tremor levels. They found the lake a dirty gray color, caused by considerable convective water movement. The lake contained a large amount of suspended material, and sulfur slicks were present on the surface. The group measured a maximum temperature of 58°C, similar to that of 27 August. The steam plumes were therefore consistent with both the morning's clear, cool weather and the warm temperatures of the lake. They saw no evidence of any eruptive activity at the lake.
Volcanic earthquakes and tremor levels have stabilized at levels higher than those prior to the major 1995-96 eruptions. From 1997-99 there have been periods when both volcanic tremor has intensified and minor hydrothermal eruptions have occurred at Crater Lake. Volcanic tremor levels were elevated during 6-21 August, culminating in a sequence of volcanic earthquakes on 21 August, but no eruptive activity. Since 13 September, volcanic tremor levels have risen again.
The elevation of volcanic tremor levels during the first 2-3 weeks of September indicates a slight increase in the probability of further minor activity occurring at Ruapehu, especially in the form of hydrothermal eruptions in the Crater Lake. The Alert Level remains at Level 1.
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: Brad Scott, Wairakei Research Center, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).