Crater Lake has continued cooling since a minor heating event in early June, which occurred without eruptions. Observations through late August indicated a possible reversal of this cooling trend: minor convection, slightly enhanced acoustic signals, and an increase in volcanic tremor.

On 12 August the crater lake was pale gray with an indistinct slick over the central vent. The N vent area was not observed. Snow was present almost to the water's edge with no evidence of surging. Lake temperature at Logger Point was 16°C on 12 August. The battery for the ARGOS temperature logger was replaced on 12 August and a lake temperature of 18°C was recorded. The lake had a similar appearance on 27 August, but there was weak upwelling in the N vent area. Rafts of yellow sulfur were stranded on the shoreline. Lake temperature at Outlet was 17°C. In late August, ARGOS temperatures began displaying significant diurnal variation, and were not much higher than at Outlet. This may indicate that either the sensor had drifted closer to the surface or that surface temperature variations penetrated deeper into the lake. Outflow was ~25 l/s during both visits.

Volcanic tremor remained at slightly elevated levels during June, and during July the tremor levels varied. The dominant frequency remained at 2 Hz, implying only one source region but a periodic variation in output strength. Tremor levels were low in early August, but rose slightly during the month. Volcano-seismic activity was last reported on 7 July. . . .

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.