Report on Ruapehu (New Zealand) — September 1988
Scientific Event Alert Network Bulletin, vol. 13, no. 9 (September 1988)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Coldest recorded crater lake temperature
Please cite this report as:
Global Volcanism Program, 1988. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 13:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198809-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
When geologists visited Ruapehu on 15 September, Crater Lake's temperature was 10°C, a decrease of 4° since August and the lowest temperature on record. Geologists attributed this decrease to continuing low heat flow aided by recent cold weather. No central convection was evident in the lake . One of at least three remaining convection cells in the N portion of the lake produced a strand of yellow sulfur. Since mid-l986, Mg/Cl ratios have remained near 0.06-0.07, suggesting heating of the lake by steam rather than by direct magma-water interaction.
Geologists noted that similar slow declines in activity over the past 8 years have usually been followed closely by rapid lake reheating.
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.
Information Contacts: P. Otway, NZGS Wairakei.