Crater Lake temperatures had risen to 25°C (at the Outlet) and 27° (at the logger site) on 22 August, compared to 20 and 21°C respectively during 20 July fieldwork. Convection above the lake's center was indicated by dark slicks, while yellow slicks were present over the N vents; the rest of the lake was battleship gray. Clearer visibility on 29 August confirmed central vent convection that produced dark yellow-green slicks. Steam rising from the lake formed an impressive column by 1330, when it was reported to the Dept of Conservation as a possible eruption by the crew of a commercial airliner. Lake water collected 22 August showed declines in Mg and Cl contents of 1.4% and 7.2% respectively since 20 July. The Mg/Cl ratio continued to drop (to 0.051) indicating continued steam discharge into the lake with little or no liquid phase input from the vent.

After a month with little or no tremor, amplitude rose to low-moderate levels for ~2 weeks beginning 1 August; after the 15th, only low-amplitude tremor has been recorded. A series of small to moderate volcanic earthquakes (maximum ML 2.1) was recorded, mainly between 2 and 5 August, and a short episode of low-frequency tremor occurred 10 August. Deformation measurements showed extensions of as much as 21 mm across Crater Lake between 20 July and 29 August.

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. Scott, NZGS Rotorua; P. Otway, DSIR Wairakei.