NZGS personnel returned to Ruapehu 5 February and observed an apparent decline from the fairly vigorous January activity. During 3.5 hours of field work the geologists saw only one explosion; a geysering of muddy black water from near Crater Lake's center that lasted at least 25 seconds. Sounds that may have been produced by two additional explosions were heard during cloudy periods that obscured Crater Lake for most of the last 2 hours of the NZGS visit. In contrast, 2-3 explosions per hour were noted 12 and 21 January. The amplitude of volcanic tremor, measured for 2 hours by a portable seismograph on 5 February, had decreased considerably since 21 January [but J. Latter notes that such fluctuations are common]. Tremor frequency was about 3 Hz. Crater Lake temperature dropped from 57°C on 12 and 21 January to 49°C on 5 February. However, preliminary analyses indicated that both Mg and Cl concentrations and the Mg/Cl ratio continued to increase, consistent with increasing interaction between lake water and magma or rock not previously exposed to lake water. Depth soundings in the central area of Crater Lake indicated that no major lava dome growth had occurred on that portion of the lake bottom.

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.