During a visit by geologists on 10 December, the crater lake was a battleship-gray color with no upwelling or slicks around the main (central) vent. The N vent area exhibited moderate upwelling, possibly slightly stronger than seen in recent months, with yellow slicks. Little or no steam was present, and there was no indication of surging or other eruptive activity. Except for exposed rock near Outlet, the entire lake remained surrounded by snow. A tour operator reported similar conditions following a visit on 4 December, although the N vent area was not observed. A commercial pilot reported that on the morning of 19 December there was extensive ash staining on the snow down the E side of the mountain, though no steam was visible from the crater lake. A guided party that visited the crater lake later that day observed no evidence of recent activity. The most likely source for the staining is old ash exposed by the recent thaw that was blown across the snow. This phenomenon has been observed frequently at this time of year in the past.

The lake temperature measured at Outlet on 10 December was 26.0°C (see table 3), a drop of 11°C since 4 November. The lake level was estimated to be at least 20 mm below overflow level with evidence of a recent retreat (fresh strandlines). By the next day, scientists repaired the ARGOS satellite telemetry station at the crater lake, which had been inactive since 3 July. The crater lake appears to have been cooling rapidly after approximately 3 months of above-average heatflow. However, temperatures telemetered from the ARGOS station since 10 December indicated minor renewed heating of the lake (an increase of 1.2°C between 11 and 19 December). The recent heating follows a minor increase in volcanic tremor in late November, so it is unclear whether the cooling phase will continue. Lake water analyses (see table 3) show that Mg and Cl concentrations appear to have stabilized following a minorincrease between June and November.

Tremor power output declined to very low levels on 2 October before rising again into the 10-70 W range for the rest of the month. Late in November the power output rose above background levels, reaching 200 W on 21 November. Several small earthquakes, mostly A-type, but some B-type events, were recorded by the Dome seismograph, especially in late November.

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, IGNS Wairakei.