On 17 September a group of scientists visited Ruapehu's Crater Lake in response to observed steam plumes and recent increases in volcanic tremor levels. They found the lake a dirty gray color, caused by considerable convective water movement. The lake contained a large amount of suspended material, and sulfur slicks were present on the surface. The group measured a maximum temperature of 58°C, similar to that of 27 August. The steam plumes were therefore consistent with both the morning's clear, cool weather and the warm temperatures of the lake. They saw no evidence of any eruptive activity at the lake.

Volcanic earthquakes and tremor levels have stabilized at levels higher than those prior to the major 1995-96 eruptions. From 1997-99 there have been periods when both volcanic tremor has intensified and minor hydrothermal eruptions have occurred at Crater Lake. Volcanic tremor levels were elevated during 6-21 August, culminating in a sequence of volcanic earthquakes on 21 August, but no eruptive activity. Since 13 September, volcanic tremor levels have risen again.

The elevation of volcanic tremor levels during the first 2-3 weeks of September indicates a slight increase in the probability of further minor activity occurring at Ruapehu, especially in the form of hydrothermal eruptions in the Crater Lake. The Alert Level remains at Level 1.

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: Brad Scott, Wairakei Research Center, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).