alert bulletin: Apr 23 2008, 10:00 am - Ruapehu Volcano

Science Alert Bulletin RUA-08/01 - Status report; Alert Level remains at Level 1

GNS scientists have continued to monitor the activity at Ruapehu since the moderate-sized eruption on September 25 2007. No further eruptions have occurred, but an increase in gas output and the internal temperature of the volcano are indications of elevated unrest. It is unclear if this is a sign of further eruptions in the near future. The Alert Level remains at Level 1.

Data from the Crater Lake sampling, seismic activity and airborne gas measurements have continued to show a change since the September 25 eruption.

The airborne gas plume measurements show the SO₂ flux has increased since last year’s eruption. CO₂ flux has varied, but has shown increases recently, while H₂S gas has become measurable since March 2008.

The lake chemistry shows increased Mg/Cl ratios since the September eruption, with variable sulphate and relatively constant chloride concentrations. Chemical equilibrium relationships amongst gases discharging from the volcano suggest that temperatures are increasing in the hydrothermal system at depth.

The Crater Lake temperature rose in response to the eruption by about 20 °C and reached a peak of 37.5 °C by December 21 2007. Since then it has remained elevated and fluctuated between 34 and 36.8 °C. Sustained heat is required to keep the temperature this high for this length of time. It is believed that the source of this heat is the magma that is producing the higher than normal gas flow and chemical changes.

There has been a small increase in the background level of volcanic tremor and some short periods of stronger volcanic tremor. However volcanic tremor and changes in the level of tremor are very common at Ruapehu.

Since September 2007 magma has started to degas more than is normal at Ruapehu and this is carrying with it additional heat which is keeping the temperature of Crater Lake hot. It is not clear if these changes are signs that further eruptions will occur. If further eruptions do occur, they may occur without any warning.

The Alert Level remains at Level 1 (departure from typical background surface activity, signs of unrest) and will continue to be re-assessed regularly.

Brad Scott
Volcano Surveillance Co-ordinator