Visits by F. Greenhall on 19 October and P. Otway on 20 October revealed a thin layer of pale gray ash in a 400 m-wide area S of Crater Lake. The ash appeared to be composed entirely of lake sediments and contained no coarse particles. It had been deposited in the upper layers of snow known to have fallen between 17 October and the late afternoon of 18 October. Only a minor wave surge of less than 1 m appeared to have been associated with the ash ejection.

J.H. Latter reported that a period of low-frequency volcanic seismicity occurred on 18 October at 1435, reaching a maximum magnitude of 2.5. [Similar activity occurred 13 and 15 September and 3 November without associated ash emission.] The temperature of the lake on 20 October was 31°C, a 6° increase in 13 days. The lake was a turbid gray, with large slicks of dark sulfur floating near its center and much steam rising from the surface. Upwelling near the center appeared strong, although steam partially obscured this area.

Occasional explosions from the crater lake began in late January and continued through mid-April. No evidence of additional activity had been observed since 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; J. Latter, DSIR, Wellington; F. Greenhall, Ohakune.