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.

Geological Summary. 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 NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: P. Otway, NZGS Wairakei.