Report on Ruapehu (New Zealand) — 20 April-26 April 2011
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 20 April-26 April 2011
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2011. Report on Ruapehu (New Zealand). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 20 April-26 April 2011. Smithsonian Institution and US Geological Survey.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
On 18 April, GeoNet reported that the temperature of Ruapehu's summit Crater Lake was slowly cooling and decreased to 33-34 degrees Celsius from a peak of 41 degrees in March. A general decline of activity had been noted since 4 April, including lower carbon dioxide gas flux, less seismicity, modest change in the Crater Lake water chemistry, and cessation of lake overflow accompanying the start of the cooling trend. The Aviation Colour Code remained at Yellow and the Volcanic Alert Level remained at 1 (signs of volcano unrest).
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.