Fieldwork on 17 February revealed little change compared to previous visits. The lake was pale gray-green with a large area of brownish discoloration over the N vent area, where several convection cells were apparent. Blue-green meltwater around the N and W margins increased in area during the day. There was no shoreline evidence of recent surging. Following a decline in January, the lake temperature had increased from 21°C (measured on 12 January) to 24°C. Over the same period the lake outflow decreased from 100 l/s to 90 l/s. The Mg/Cl ratio in the lake water has remained essentially constant since December (table 3), after reaching a low of 0.042 in September.

Table 3. Temperature, outflow measurements, and water analyses from the crater lake of Ruapehu, 21 December 1992 to 10 December 1993. Discharge is in liters/second (l/s). Discharge of "0" indicates a lake level below overflow stage. A dash (--) signifies no measurement. (The low Mg and Cl values on 11 January were due to excessive meltwater dilution. Analyses done by M.E. Crump). Courtesy of IGNS.

EDM measurements revealed no long-term movements of volcanic significance. A key benchmark, buried by snow since July, was exhumed. Its burial prevented detection of any short-term inflationary changes.

The crater was revisited 28 February-2 March to install telemetry equipment. The lake initially appeared unchanged from 17 February, but by 2 March it was a uniform gray. Outflow was estimated to be 140 l/s. Lake temperatures of 28-29°C measured near the telemetry equipment were similar to the 29.2°C telemetered via ARGOS on 4 March.

Seismicity was generally low with some volcanic earthquakes recorded. Strong tremor was recorded 9-10 January and moderate tremor on 25-28 February.

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.