Report on Ruapehu (New Zealand) — 19 September-25 September 2007
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 19 September-25 September 2007
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2007. Report on Ruapehu (New Zealand). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 19 September-25 September 2007. Smithsonian Institution and US Geological Survey.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
An eruption of Ruapehu that occurred on 25 September prompted GeoNet to raise the Alert level to 2 (on a scale of 0-5). Pilots reported that an eruption plume rose to an altitude below 4.6 km (15,000 ft) a.s.l. Further reports from ski field operators and the Eastern Ruapehu Lahar Alarm and Warning System (ERLAWS) indicated that lahars traveled down the Whakapapa ski field and possibly E in the Whangaehu river valley, and other areas.
On 26 September, aerial observations revealed that the summit area was covered with ash and mud, mostly directed N and reached 2 km from the crater lake. Impact craters caused by falling blocks over 1 m in diameter were also evident.
According to news articles, the eruption prompted evacuations at several ski lodges and caused train service to be temporarily suspended. A boulder crashed through the roof of a hut and injured one person.
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.