Heatflow in the crater lake continued at low levels in June and July but increased significantly in August. Little change was observed in the pale gray color of the lake during fieldwork on 3 June, 18 June, and 3 July. Snow was present along the lake's edge and there was no shoreline evidence of surging on any of the field dates. Moderate upwelling was observed with associated yellow slicks over two sites in the N vent area on 3 and 18 June. Some weak convection and a faint short-lived dark slick were visible above the central vent on 18 June. Yellow sulfur globules (hollow spheres with long tails) were also seen floating and stranded on the shore near the outlet on 18 June. Weak upwelling from the N vents on 3 July produced a sulfur slick extending nearly 150 m E of the outlet on the S shore to Logger Point, but no slicks were seen from the central vent. Convection had recommenced from the central vent by 6 August and produced khaki-yellow slicks on the lake surface. The calculated August heatflow was ~3x that of July. Temperature and discharge from the outlet (at the S end of the lake) had increased since June, while the temperature at Logger Point declined (see table 3).

EDM distances measured on 3 June showed a minimal change of -7 mm overall since the last leveling survey on 20 April, which is unlikely to have any volcanic significance. No distance measurements were made in July. Changes since 3 June were generally below detection limits (<5 mm ) with the exception of the shortening of two lines to a station SW of the crater lake (station E), continuing the trend since February. Two other stations moved ~18 mm closer to station E. None of these movements is thought to have any significance in terms of future eruptive activity. Snow was still an obstacle during the 6 August survey with deep drifts around stations on the W side of the crater lake.

Detailed seismic monitoring in June was hindered by the failure of the Dome seismograph, which was struck by lightning in early April. Very little tremor was recorded by the Chateau seismograph. The signal from Dome was restored on 18 June and some very small A-type events were recorded on many days. During a 6-minute period on 3 July, a sequence of five events with a dominant frequency of 2-3 Hz was recorded, indicating low-frequency volcanic earthquakes or short duration tremor. Some very small local high-frequency events were also recorded in early July and were attributed to probable ice-induced quakes. No records were obtained from Dome during 12-20 July, and when recording recommenced a 2-Hz volcanic tremor of variable amplitude was occurring. The tremor was continuous up to the time of the 6 August report, with the dominant frequency rising from 2 to 3 Hz on 27-28 July. Small high-frequency events continued to occur in early 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: P. Otway, B. Scott, and A. Hurst, IGNS Wairakei.