Report on Ruapehu (New Zealand) — April 1980
Scientific Event Alert Network Bulletin, vol. 5, no. 4 (April 1980)
Managing Editor: David Squires.
Ruapehu (New Zealand) Small explosions through the crater lake continue
Please cite this report as:
Global Volcanism Program, 1980. Report on Ruapehu (New Zealand). In: Squires, D. (ed.), Scientific Event Alert Network Bulletin, 5:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198004-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Geologists visited Ruapehu on 27 March, and 12-13 and 16 April. During the 27 March visit, three small phreatic explosions took place through Crater Lake in 4 hours and 40 minutes, continuing the activity that probably began in late January. The largest explosion produced 20-m-high water jets and a few waves big enough to overtop the lake outlet, sending very small pulses of water down the Whangaehu River. A strong H2S odor was noted after this explosion. The lake water temperature near the outlet was 40°C, 2.5° higher than on 14 March, but close to the average value recorded since 20 February. Strong upwelling occurred near the center of the lake, but was less distinct in the N area. There was no evidence of recent ashfall around the crater.
Activity was similar during the 12-13 April inspection, with 6 explosions in about 23 hours. One, with a muffled booming sound, ejected jets of water 30 m high, formed 1-2 m waves, and sprinkled dark ash on the observers. There was a strong H2S odor downwind from all of the explosions. The lake color remained battleship gray, but lake level had risen more than 0.5 m since 27 March and was overflowing at about 20 liters/sec down the Whangaehu River. The lake temperature at the outlet had dropped 3°C to 37°C since 27 March. A leveling survey indicated that less than 1 µrad of tilt had occurred since mid-March. There has been minimal deformation during the last 2.5 months.
No explosions took place during a 2.5-hour visit on 16 April, but a thin layer of ash had fallen on snow NNE of the crater, perhaps from an eruption cloud seen from a distance early 14 April.
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.