When NZGS personnel returned to Ruapehu 10-11 February, they found the lake turbid. It had been clear on their previous visit, 24 January. Upwelling was slight over the central vent with a trace of dark sulfur, and minor from two or three cells at the N end of the lake.

Thick strands of gray sulfur spheroids and some yellow teardrop shapes floated near the outlet. Fine-grained glass-foam fragments also were present in the floating material. Glass-foam appeared in May 1973; was produced in abundance during the April 1975 eruption; was found during the October-November 1977 eruptive period; and appeared in sulfur slicks on 21 February 1978. No glass-foam was found during the February 1980 or November 1981-January 1982 eruptive periods.

Lake water temperature at the outlet was 19°C, 1° cooler than on 24 January. The water's magnesium concentration had remained stable, but chlorine concentration had risen by 250 ppm, indicating to the NZGS a resumption of fumarolic activity. The Mg/Cl ratio was 0.106.

The horizontal deformation survey showed a 12-mm extension of the 600-m-wide crater as measured between 2 stations on opposite sides of the rim. After a period of rapid inflation, then deflation, the distance across the crater had returned to that of 19 October.

When NZGS personnel flew over Ruapehu 6 days later, the lake was relatively clear and a pale blue-green. Upwelling was absent over the central vent, but moderate at the N end of the lake, where 3 brownish cells were visible. The NZGS attributed the lake's rapid clearing (by sediment settlement) to cessation of heat flow from the main vent.

On 22 February the NZGS found the slightly steaming, calm lake a bright blue-green, with no upwelling over the main vent. Yellow and gray sulfur strands were drifting S from moderate upwelling over at least three locations at the lake's N end. Water temperature measured at the outlet was 23.5°C, up 4.5° from 10 February. The horizontal deformation survey showed shortening of 5 mm across the crater. Only 2 µrad of tilt had occurred since the last measurements on 3 January.

The level of volcanic tremor and B-type earthquakes was moderately high throughout January and [tremor peaked on 2 February]. Activity rapidly declined to a very low level 10-15 February. It remained low until 0845 on 23 February, when a B-type earthquake sequence with events of ML 3.0-3.1 was triggered by a magnitude 2.1 roof rock earthquake. On 24 February, the NZGS noted that "The increased seismicity and the recent changes in the appearance of the lake indicate that Ruapehu has entered a phase where the probability of eruption is now at a relatively high level. Visitors to the crater are being advised not to approach the lake too closely." A similar sequence of B-type earthquakes occurred 26 February at 2356. The series of magnitude 3.0-3.1 events was again triggered by a high-frequency roof rock earthquake, of magnitude 2.0. On 1 March at 0757, a third sequence of B-type events reached M 2.9. Depths for the 1 March events were estimated at 300-600 m beneath Crater Lake, somewhat shallower than usual. Weak volcanic tremor began 2 March at about 0500, at perhaps 300 m below Crater Lake. J. H. Latter noted that this probably represented gas moving toward the surface.

The Chief Ranger, Tongariro National Park, and pilot K. Newton both reported that the lake was gray early 24 February, but there were no signs of ash deposits or upwelling from the main vent. The NZGS interpreted the color change "as being due to a sudden, strong upwelling, possibly in the form of a hydrothermal eruption, following the shallow seismic events at 0845 on 23 February." By the 28th, the lake temperature had risen a further 3.5° in 6 days, to 27°C, and an additional 9 mm of shortening (deflation) was measured across the 600 m-wide crater. Moderate upwelling was noted from the N end of the lake but the water became noticeably clearer during 4 hours of observations. The lake was still milky gray when observed from the air 2 March, but only slight upwelling was occurring and there were no signs of recent eruptions.

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, NZGS, Wairakei; J. Latter, DSIR, Wellington.