No hydrothermal eruptions from Crater Lake were observed on 5 or 26 July, when NZGS personnel worked at the volcano. The lake, colored its usual battleship gray, was steaming moderately on the 5th; on the 26th there was upwelling from the center of the lake but only a little steam was rising.

On 5 July the water temperature in the Outlet area was 33°C, 6° higher than on 29 May. Snow within 1-2 m of the water's edge showed no signs of recent large surges. Considerable sulfur coated the lake margin in the outlet area. A preliminary water analysis showed no change in magnesium concentration but a small increase in chloride: Mg/Cl = 0.119. A triangulation survey showed that the length of a precisely measured 600-m line across the crater had decreased 7 mm since 29 May. NZGS interpreted the 1981 crater width measurements as showing virtually no deformation following the 20 mm expansion recorded during the onset of the eruptive period in January.

On 26 July the water temperature had dropped to 24.5°C. Snow lay within 0.5 m of the edge and showed no signs of recent ash deposits or surging. Magnesium concentration in the lake water was down slightly (by 4 ppm), but was only 1 ppm less than the mean January-April value.

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 and B. Scott, NZGS, Rotorua; P. Otway, NZGS, Wairakei.