New Zealand's GeoNet, a combination of the country's Earthquake Commission and GNS Science, reported that at 1830 on 13 July 2009, there was a small (M 2) volcanic earthquake beneath Ruapehu's crater lake. As a result of a new research project measuring the temperature and level of the lake, instruments documented a sudden 15-cm jump in lake level following the earthquake. The lake temperature remained unchanged at 20°C.

The lake was examined from a helicopter on 14 July 2009. Viewing conditions were very poor, but no obvious changes had occurred since the last visit on 2 July 2009. No eruption had occurred and the lake was overflowing. The preliminary interpretation was that the volcanic earthquake was followed by about 20 x 10⁶ liters of extra water moving into the lake from the hydrothermal system beneath it. A much larger rise in lake level had followed a very small eruption in October 2006, so lake-height adjustments were not unknown at Ruapehu. However, this was the first time that scientists had been able to correlate such a small rise with a single volcanic earthquake. The Volcanic Alert Level remained at Level 1 (a designation signifying a departure from typical background surface activity and signs of unrest).

In October 2010, GeoNet reported that the lake had began a heating cycle, the eighth since the lake was re-established in 2002 after the 1995-1996 eruptions (BGVN 20:09 and 20:10). Later, on 7 March 2011, GeoNet reported the lake temperature at 40°C, the third highest temperature recorded since the re-establishment of the lake (table 14).

Table 14. Summary of reported temperatures in Ruapehu's Crater Lake. Courtesy of GeoNet.

Other monitored indicators had shown variable trends in parts of March 2011. Those indicators included gas output, seismicity, lake chemistry, and ground deformation. Such variable trends were like those previously seen during lake heating cycles. GeoNet reported on 5 April 2011 that Ruapehu had undergone a sustained period of high Crater Lake water temperatures. In recent weeks changes also occurred in volcanic gas output, seismic activity and lake water chemistry. These changes suggested unrest above known background levels, hence authorities elevated the Aviation Color Code to Yellow but kept the Volcanic Alert Level at 1. After 4 April there was a general decrease in activity, with lower CO₂ gas flux, less seismicity, little change in lake-water chemistry, and cessation of lake overflow accompanying the start of the cooling trend. On 18 April 2011 GeoNet reported decreased lake temperature; other monitored indicators in recent weeks also suggest a slow decrease of activity. On 2 May 2011 authorities lowered the Aviation Color Code to Green, the lowest hazard status. This followed a continued decrease in lake-water temperature and several weeks of slow decreases in other available indicators.

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: GeoNet (URL: http://www.geonet.org.nz/).