When NZGS personnel visited Ruapehu on 15-16 and 22 June, conditions at Crater Lake were very similar to those observed on 15 May. The lake color was battleship gray, and water temperature measured at the outlet remained 16.5°C. Concentrations of both magnesium and chlorine (750 and 7,455 ppm) were also virtually unchanged. Discontinuous sulfur slicks indicated intermittent up-welling over the main vent. Upwelling was also observed over three sites at the N end of the lake. According to the NZGS, the volcano has entered a phase of steady, but low activity.

Gray mud and yellow sulfur covering the outlet suggested higher recent rates of outflow, but snow lay within 30 cm of the water, indicating no recent large surges. Varying pH values measured by the New Zealand Railways Communications Section, Taumaranui, on the Whangaehu River, which drains Crater Lake, marked periods of strong overflow (low pH) alternating with little or no overflow (high pH).

The horizontal deformation survey on 22 June revealed a 20 mm decrease since mid May in the distance between stations on opposite sides of the 600 m-wide crater. Since the stations were installed in 1976, this distance had shortened to its 22 June length on only a few occasions (in early 1976 and several times in 1980 and 1981). The NZGS interpreted the shortening as deflationary contraction indicating low magmatic or gas prassure.

No volcanic (B-type) earthquakes were recorded [8 June through 21 August]. Bursts of possible moderate-frequency, moderate-amplitude tremor were recorded during the first half of June.

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: I. Nairn, NZGS, Rotorua; P. Otway, NZGS, Wairakei.