Increased thermal activity began in early January, peaked around 23-26 February, and concluded about 8 March. Several minor phreatic eruptions were observed beginning on 8 February, including several since 11 February fieldwork. Steam clouds were observed over Crater Lake at 1500-1600 on 12 February by Dept of Conservation staff. On 18 February, airplane pilots (Darren Kirkland and Ross Bateup) reported a phreatic eruption generating a steam column and waves 0.6-0.9 m high at around 1100, and two large "bubbles" and a pulse of steam at 1822. During a visit on 20 February, Dr. Ockens (Univ of Sydney) heard an airplane engine-like noise, followed by steam emission from the lake and a sulfur odor. Kirkland observed an eruption that produced large waves at 1200 on 23 February, and after a week of relative quiet, reported the lake to be vigorously active at 1600-1800 on 6 March, producing dense steam clouds and large waves. Steam clouds were noted rising from the lake that day at about 1330, by Roger Smith (from near the lake shore) and Keith McKenzie (from a helicopter, at a distance). Smith also reported the appearance of sulfurous, yellow-green patches in the lake.

During fieldwork on 24 March, Crater Lake appeared pale gray, with upwelling (three or four cells) and yellow slicks over the N vent area. New snow was visible at the water's edge and there was no evidence of recent activity. The temperature data logger recorded a peak of 47°C on 23 and 26 February, then temperature fell, plateauing briefly at ~44°C on 2-8 March, and reaching 31.5°C (direct measurement) on 24 March (figure 12). Water analyses indicated no significant changes in Mg or Cl concentrations during the February-March activity.

EDM measurements showed moderate inflation associated with the February/March activity. Seismicity has remained at background levels since 11 February, with low-level volcanic tremor or small volcanic earthquakes recorded on most days. No signals related to reported activity were seen. Geologists noted that the February/March activity was similar to the minor events that occurred every 5-10 months between 1985 and January 1990.

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, DSIR Wairakei.