Since the middle of March 2003 the temperature of Ruapehu's summit Crater Lake had been slowly rising. The lake temperature rose from 30°C on 5 March (BGVN 28:02) to a high of 41.6°C on 15 May (table 11). Similar values were recorded in January 2003 when the lake temperature reached 42°C. This is the fourth time that the temperature of the Crater Lake has risen above 35°C since the start of 2001, and the temperature has been above 30°C since December 2002. It is not unusual for the temperature to cycle over periods of 6-9 months; minor hydrothermal activity can occur in the lake during temperature peaks. Lake temperatures dropped steadily from 41°C after mid-May. However, during the late morning of 26 May a steam plume was observed rising 200-300 m above Crater Lake. No seismicity accompanied this plume, suggesting that it was generated by atmospheric conditions alone (a warm lake and a cold, windless, morning). Steam plumes were also observed on 28 March and 21 April.

Table 11. Lake water temperatures measured at Ruapehu's Crater Lake, 5 March-1 June 2003. Courtesy of IGNS.

Weak intermittent seismic tremor was recorded through early April, then remained at a constant moderate level during 12-17 April. The following week, 18-24 April, there was an increase in tremor accompanied by discrete volcanic earthquakes. By 2 May volcanic tremor levels had declined, but volcanic earthquakes continued to occur. Levels of volcanic tremor fluctuated during the week of 3-9 May, with several periods of enhanced tremor and small volcanic earthquakes. Tremor had declined by 16 May, and seismicity remained very low through the 30th. The level of volcanic tremor began to increase slightly in early June, but the lake temperature was still declining during the week of 7-13 June. Very low levels of activity continued through the 20th. There were no significant changes observed in the lake water chemistry. The hazard status remained unchanged at Alert Level 1.

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: Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).