Report on Ruapehu (New Zealand) — 19 September-25 September 2007
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 19 September-25 September 2007
Managing Editor: Sally Sennert.
Please cite this report as:
Global Volcanism Program, 2007. Report on Ruapehu (New Zealand) (Sennert, S, 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.
Geological Summary. 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 NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.