Overall volcanic activity has been at an unusually low but stable level in recent months. Deformation and seismicity have remained at background levels, and lake chemistry has shown little change. No evidence of eruptive activity was observed during fieldwork on 23 March or 20 April. The weather on 23 March was clear with a light wind and a temperature of -1°C. The crater lake was pale gray with a slight upwelling and yellow slicks at two or three sites in the N vent area. Small rafts of floating yellow sulfur were observed near the shoreline. There was no upwelling visible over the main vent. Blue-green meltwater was present in some areas around the N shore, but there was no evidence of recent surging.

Measurements in late March and April show that the lake is now undergoing slow cooling. The mid-day lake temperature measured manually on 23 March at 15 cm depth was 23.0°C at the Outlet (S end of the lake) and 24.5°C at Logger Point (~150 m E of the outlet on the S shore). A thermistor measurement directly offshore of Logger Point recorded a temperature of 28.0°C at the same time, at a depth of at least 10 m. No change was apparent in the lake on 20 April, although the meltwater had disappeared. Lake temperatures had dropped to 21.0°C at Outlet and 21.5°C at Logger Point. The temperature telemetered from the thermistor was 25.0°C. The 3.5°C temperature difference observed on both days at Logger Point is thought to reflect stratification between the relatively clear surface water and the warmer sediment-laden water 10 m below. The outflow rate 23 March was estimated to be 30 l/s, much less than the 140 l/s on 2 March..

Apparent tilt since the last leveling survey, in March 1991, was insignificant (0.4-1.2 µrad), and did not exceed the normal survey error. EDM measurements were also insignificant (generally <10 mm) since the last survey on 17 February. Shortening of two lines by 14-18 mm is consistent with an 18 mm eastward shift of station C, located ~225 m WNW of the lake. This is a reversal of the early westward movement of station C, which is typical of the changes recorded since about 1987, and interpreted as a seasonal effect related to snow-loading or frost-heave. All other movements recorded by recent surveys can likewise be attributed to station or site instability of various forms. None of the movement recorded in March or April has been interpreted as short-term volcanic deformation. Long-term volcanic deformation over the past 2 years has also been insignificant, including tilt measured at sites 0.5 and 2.1 km N of the crater lake.

The Mg/Cl ratio remained constant from 1 March through 20 April at 0.044. Results have been interpreted as indicating essentially stable chemistry over the last few months. Seismicity has been generally low since February, although the seismograph ceased transmission on 4 April.

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, I. Nairn, and D. Johnston, IGNS Wairakei.