Minor phreatic activity . . . continued through mid-April. During a 5-hour visit by geologists on 12 April, one small eruption was observed, generating 1-2 m of updoming and a 100-m steam plume. Lake temperature was 38.5°C, 7° higher than three weeks earlier. No significant deformation was detected. Data from one seismic station starting 30 March showed low-amplitude 2-Hz tremor through 3 April. None was detected 4-5 April, but similar tremor resumed late 6 April. Tremor stopped for 2.5 hours early 7 April, then resumed and gradually built to a large 4.5-minute volcanic earthquake that saturated the instrument. Low-amplitude tremor continued for ~8 hours. Little tremor was recorded the next two days, but low-amplitude tremor was again recorded 9-11 April.

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.