Report on Ruapehu (New Zealand) — March 1988
Scientific Event Alert Network Bulletin, vol. 13, no. 3 (March 1988)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Small phreatic eruptions
Please cite this report as:
Global Volcanism Program, 1988. Report on Ruapehu (New Zealand) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 13:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198803-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Small phreatic eruptions were reported 20-27 March, and lake temperature rose from 18.5°C on 1 March to 31.8°C on the 22nd. On 20 March at about noon, hikers heard a rumbling, avalanche-like sound, then noted that the surface of Crater Lake was agitated and partially obscured by steam. Between 1255 and 1300, two steam clouds appeared above the crater and quickly dissipated. Another phreatic eruption was photographed at about 1315. Some apparent high-frequency tremor had been detected on a nearby low-gain seismic instrument during the previous several hours. On 22 March at 1410, geologists observed an eruption from Crater Lake. A jet of muddy water rose 15 m or more above the surface, waves roughly 1-2 m high surged onto the lake's shore, and a steam cloud reached ~200 m height. No tremor was evident. A considerably smaller eruption was photographed at 1547. Another small eruption was reported on 27 March at about 1300.
Seismic data, available 21-24 March, showed semi-continuous, low-amplitude, 2-Hz tremor. Strong tremor was also reported on 24 February, and 3 and 10-13 March. B-type earthquakes were also recorded on 13 March. A tilt survey on 22 March showed little change since the last measurements in February 1987.
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: P. Otway, NZGS Wairakei.