Report on Ruapehu (New Zealand) — 28 November-4 December 2001
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 28 November-4 December 2001
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2001. Report on Ruapehu (New Zealand). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 28 November-4 December 2001. Smithsonian Institution and US Geological Survey.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Seismicity returned to normal levels on 25 November after a moderate-to-large volcanic earthquake occurred at Ruapehu on 21 November. Observations on 25 November revealed no signs of eruptive activity. Scientists found that upwelling sediment in Ruapehu's crater lake caused the lake to change from its normal blue-green color to dark gray. In addition, the temperature of the lake was relatively low (22°C in comparison to 21°C in September), which further supported the theory that no eruptive activity occurred after the earthquake. The volcano remained at Alert Level 1 (on a scale of 0-5).
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.