Small hydrothermal eruptions from Crater Lake, accompanied by increased seismicity, continued through early June. Geologists returned to the volcano on 4 June and found activity similar to that observed during the previous inspection on 28 May. However, the frequent minor hydrothermal eruptions that began on or before 21 May appear to have concluded, perhaps as early as 9 June.

Eight small hydrothermal eruptions from the central vent were noted during 8.5 hours of observations on 4 June. Water spurted to as much as 15 m above the vent, and waves surged onto the lake shore and through the outlet. One eruption generated a 100-m steam plume. Convective upwelling between eruptions was observed over an area approximately 50 m in diameter in the center of the main vent area, but was often obscured by steam. No activity was observed from the N vent. Lake temperature remained at 44-45°C but the eruptive activity was accompanied by a major drop in outflow rate to about 5 l/sec, from 200 l/s on 28 May. The lake surface remained battleship-gray in color, and new snow was trimmed back to about 0.5-1 m above lake level.

Geologists returned on 28 June and observed neither upwelling nor evidence of recent eruptions. Dark green sulfur slicks appeared from time to time in the central area but were constantly visible in the N vent area, accompanying faint upwelling there. Steam columns were formed by the large temperature differential (40°C) between the lake surface and the air. Lake temperature had declined 7° since 4 June. The water had dropped to 0.2 m below overflow level, and the channel was full of clean snow. Water samples were taken during both the June inspections.

The start date of this eruptive episode remain uncertain. A major overflow from Crater Lake began 16 May. From Whakapapa (about 9 km NNW of Crater Lake) at noon the next day (± 50 minutes), Quentin Forman of Auckland University heard a noise and saw steam rising from the direction of Ruapehu, but a ridge blocked his direct view of Crater Lake. The first observed eruptive activity was on 21 May. Uncertainty also surrounds the end of the eruption. After the 4 June eruptions reported above, P. M. Otway had a clear view of the volcano from Taupo (roughly 75 km from Ruapehu) on 9 and 11 June, but saw no plume. Steam clouds were seen over Ruapehu by geologists on 25 June (the day before they arrived at the crater) but were thought to have most likely been caused by atmospheric effects.

The eruptive activity was preceded and accompanied by weak seismicity in a wide vertical range beneath Crater Lake. Volcanic tremor associated with heating of Crater Lake began fairly strongly on 5 May at 0452 in the roof rock zone below the lake. The tremor had dominant frequencies of 1.5-2.5 Hz, usual at Ruapehu. A number of small volcano-tectonic earthquakes occurred around that time. The largest, magnitude 2.15 on 6 May, was at 2 km depth. Their amplitudes at the summit station were anomalously high [relative to those measured at the volcano's foot, indicating that they were unusually shallow or near the station]. Other similar events were associated with high-frequency volcanic tremor, suggesting to seismologists that magma was intruding the wall-rock north of Crater Lake conduit. Volcanic earthquakes (multiple events showing characteristic features attributed to an origin in magma or pockets of hot gas), which had begun to occur at shallower foci in September 1983, reached maximum magnitudes (2.25) on 11 May.

High-frequency volcanic tremor (superimposed on the 2 Hz tremor that started 5 May) first became conspicuous on 19 May at 2209, immediately after a M 2.9 earthquake ~10.5 km ESE of Crater Lake. This was the smallest of four earthquakes at the same focus: two others occurred on 19 May (both M 3.0), and one M 3.25 event on 20 May. The earthquakes were shallow, probably less than 2 km deep, on or very close to the prominent fault bounding Ruapehu on the SE side. A composite focal mechanism determination suggested right-lateral strike-slip on this fault, with a compressional component in the direction of the summit. Low-frequency volcanic earthquakes began on 25 May at 1519, culminating in a magnitude 2.4 roof-rock event at 1725, and were continuing 9 June. J. H. Latter noted that the episode is typical of an open-vent period of energy release at Ruapehu. He also noted that the data suggest that the present state of increased activity is similar to, but less intense than, the 1981-1982 period when similar events continued for 3-4 months without any major eruptions.

Preliminary results of deformation surveys indicate that no significant crater deformation has occurred in the last few months. NZGS geologists noted that this appears to be in accordance with the seismic interpretation of open-vent conditions during this episode.

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: A. Cody, I. Nairn, B. Scott, NZGS, Rotorua; J. Latter, S. Sherburn, DSIR, Wairakei; P. Otway, NZGS, Wairakei.