The first phreatic eruptions since July 1989 began in January, with episodes on the 7th (1215-1230), 12th (1100 and 1355), and 14th (1000, 1200, and 1232). The eruption at 1232 on the 14th sent water to 3-5 m and a nearby rockfall suggested that it may have been preceded by a small earthquake.

Intermittent 2-Hz tremor was recorded 30 December 1989-12 January 1990, and 5 hours of 1-Hz tremor, believed to reflect a deeper source, began on 10 January at 2300. Tremor amplitude increased ~3 hours before the 7 January eruption, but no changes accompanied the eruption, and geologists did not believe that the tremor increase was significant. A new episode of lake heating began around 5 January as Crater Lake's temperature rose from ~15°C on the 5th to 27° on the 11th.

When B. Scott flew over Ruapehu on 7 January at about 0915, a black slick, 100 m in diameter, covered the center of Crater Lake. The remainder of the lake was uniformly gray except for an area (50-100 m wide) of blue-green water extending from the NW through the NE quadrant. Later that day between 1215 and 1230, the first phreatic eruption ejected steam and water to 50-60 m, with a steam cloud 200 m in diameter. The activity turned the lake gray and formed a sulfur strand on the shore.

During 11 January fieldwork, sulfur strands up to 5 m from the lake's edge (~0.75 m above the current lake level) gave further evidence of the 7 January eruption. Lake temperatures of 25.3 and 26.8°C were measured at two locations. The lake was battleship gray and minor upwelling at the N vents formed a yellow slick. Upwelling over the main vent that produced a dark area strengthened by midafternoon, and at 1611 a 10-20-second audible burst of upwelling occurred.

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, DSIR Wairakei.