The increased seismicity reported last month was followed by resumption of upwelling in Crater Lake, a sharp increase in its temperature, and a small ash eruption.

Geologists recorded a Crater Lake temperature of only 10.5°C on 8 October, a decline of 3° since 28 August and the lowest since detailed measurements began (although the lake was reportedly frozen in 1886 and 1926). The lake showed no sign of sulfur slicks or steam and only minor upwelling was evident, at the N end. When geologists returned late 12 October, sulfur was upwelling from the center of the lake at 10-20 minute intervals and the temperature had risen slightly, to 11.8°C. Tilt measurements indicated that about 8 µrad of inflationhad occurred since 17 August. The distance across the N part of the crater rim had increased 21 mm between surveys 1 May and 28 August, also indicating inflation, but this increase had been entirely reversed by 13 October.

Float plane pilot K. Newton reported that he saw a patchy layer of ash extending southward from the crater during the week prior to 30 October, probably about 27 October. Geologists inspecting the crater area 4 November observed a thin layer of ash extending 500 m S of the lake. Snow that fell 1 November had covered the ash, but subsequent melting exposed some of the distal end of the deposit. Tilt measurements 4 November indicated that the 8 µrad of inflation noted 13 October had been completely reversed. The lake temperature had increased to 24°C, its color had changed from blue-green to its normal battleship gray since mid-October, and upwelling in the center produced black sulfur slicks.

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.