Hydrothermal eruptions, apparently accompanied by felt earthquakes, were observed 8 and 9 February. On 8 February at 1202, ranger Lisle Irwin saw a vigorous steam column rise > 150 m above Crater Lake and heard loud roaring from his vantage point 2 km to the NE. Inside Dome Shelter, ~350 m NNW of the lake, Rob McCallum felt what he described as a slight earthquake during the eruption. Irwin noted no fresh ejecta deposits around the lake after the eruption. At 1517, he observed a second apparently smaller eruption that produced a steam column and audible water surges over the lake's outlet channel. While flying over the crater the next day at 0719, ranger Paul Dale saw a small column of muddy water rising to ~5 m before it was obscured by steam. Between 1300 and 1330, two climbers saw a "high bubbling circle in the middle of the lake and at the same time felt the ground shake."

When geologists visited the crater 11 February, lake temperature had risen to 46°C (from 29°C on 14 January) and there was evidence of surging of 1 vertical meter or more within the previous 12-24 hours. Strands of sulfur were floating on the lake and there was a strong odor of SO₂. Activity was similar on 14 February, and there appeared to have been additional but somewhat weaker surging in the preceding 24 hours. Deformation surveys 11 and 14 February suggested minor deflation since a small inflation peak was measured in mid-January. The lake temperature had dropped to 34°C by 26 February and there was no evidence of further eruptions.

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: NZGS, Rotorua.