Report on Ruapehu (New Zealand) — April 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 4 (April 1983)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Crater lake green; low pH of river water
Please cite this report as:
Global Volcanism Program, 1983. Report on Ruapehu (New Zealand) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 8:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198304-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
When W.W. Chadwick visited the volcano on 18 April, the lake was mostly green, with extensive floating yellow and gray sulfur slicks. Minor upwelling, marked by a light gray patch, was occurring over the N vents; none was observed over the central vent. There was no evidence of recent eruptions or surging. Lake temperature measured at the outlet was 20°C, 3° lower than on the previous visit, 17 March. . . .
On 25 March the New Zealand Railways Communications Section, Taumaranui, reported an abnormally low pH for the Whangaehu River, which drains Crater Lake. Records showed a pH of 3.5 on 3 March, 4.0 on 11 March, 3.5 on 18 March, but 1.5 on 25 March. Chadwick interpreted the data as showing that Crater Lake was overflowing continuously during March and that a substantial increase in flow occurred toward the end of the month.
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 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 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 (Te Wai a-moe), 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 3,000 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: W. Chadwick, NZGS, Wairakei.