NZGS personnel visited Ruapehu on 7 May. Although occasional small phreatic explosions had occurred through Crater Lake during previous visits beginning in late January, no explosions were observed on 7 May. There was no ash on snow that had fallen around the summit area on 29 April.

Large, yellow-green sulfur slicks floated at the N end of the battleship-gray lake. Upwelling at the center of the lake was only intermittent, in contrast to the continuous upwelling seen during earlier visits. Lake water temperature was at 39°C at the outlet, 2° higher than on 13 April, but within the range of temperatures recorded since mid-February.

The seismometer recorded continuous low-level tremor [on 7 May, as on many other days].

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: B. McG. Simpson, NZGS, Rotorua.