Report on Ruapehu (New Zealand) — December 1994
Bulletin of the Global Volcanism Network, vol. 19, no. 12 (December 1994)
Managing Editor: Richard Wunderman.
Ruapehu (New Zealand) New heating episode in crater lake begins after a burst of acoustic noise
Please cite this report as:
Global Volcanism Program, 1994. Report on Ruapehu (New Zealand) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 19:12. Smithsonian Institution.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Crater Lake appears to have commenced a new heating episode following a burst of acoustic noise on 25 November, and has been accompanied by a significant increase in volcanic tremor. The last heating episode, in June 1994, was short-lived and apparently not accompanied by eruptions. The crater lake on 7 December was generally pale gray with a blue-green tinge in some places. An area of dark gray water, signifying upwelling, was located over the central vent and streaming NE towards the shore. At least five upwelling cells in the N vents area, one of which was moderately vigorous, were associated with local discoloration and yellow slicks. There was no evidence of surging around the shoreline. Snow around the S half of the lake was discolored by sulfur sublimate stains and newly exposed ash due to recent snow recession.
Lake temperature had increased ~4°C since 27 October: to 21.3°C at Logger Point and 22.0°C at Outlet. Outflow was measured at ~80 l/s in the early afternoon of 7 December, but had increased noticeably since late morning, presumably due to increased snow-melt during daylight. A replacement solar cell was installed at the ARGOS station on Logger Point. The increased voltage on subsequent days was accompanied by a 2-2.5°C increase in recorded temperature, confirming the suspicion that recent temperatures have been in error by this amount. Previously recorded trends, however, are believed by IGNS to be reliable. ARGOS temperatures since 27 October remained low at ~15°C (~17-18°C true) until around 25 November, when a steady temperature rise coincided with a burst of 2-Hz acoustic noise. By 7 December, the temperature had risen 5°C and had reached 25°C (true) by 12 December (0.4°C/day). The manually recorded temperature at Logger Point also rose from 18°C on 27 October to 21.3°C on 7 December.
Volcanic tremor remained at low levels (~200 watts) through October to mid-November. Tremor levels began rising on 17 November, with a peak of 12,600 watts on the 25th. This high level closely followed the peak of 2-Hz acoustic noise. Tremor decreased again in early December.
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: P. Otway, IGNS Wairakei.