Mar 19 2007, 1:00 pm - Ruapehu Volcano

As a result of the rapid lake level drop caused by the dam failure, there has been an increase in volcanic seismic activity. Minor hydrothermal eruptive activity may now occur at the Crater Lake. A visit to Ruapehu Crater Lake this morning has confirmed a 6m drop in the water level of the Crater Lake following Sunday's lahar.

The rapid removal of water from the top of a hydrothermal system may introduce instability in the system, resulting in small scale eruptions. In the case of a warm crater lake, rapid lowering of the lake may lead to increased heating. This may produce steam driven eruptions through the lake, similar to that observed in October 2006. If these events are large enough they will generate waves in the lake and may induce overflow from the Crater Lake, and present a danger below the outlet in the Whangaehu Valley and around the lake.

Since the dam failure yesterday we have recorded an increased number of earthquakes. These include small volcanic earthquakes, up to magnitude 1, high frequency and hybrid volcanic earthquakes. These volcanic earthquakes are taken to be an indication of changes to the hydrothermal system in the volcano, since the water level fell.

Following the small volcanic eruption in the Crater Lake on 4 October 2006 the temperature of the Crater Lake has been rising. The most recent temperatures show the lake had heated to 28ºC (an increase of 12ºC since October), which is warm for the Crater Lake.

Based on these observations we conclude there may be increased activity at the Crater Lake over the next few days to weeks as the hydrothermal system readjusts to the new lake level.

The Alert Level for Ruapehu remains at Alert Level 1; there is no significant change in volcanic activity at Ruapehu.

Brad Scott
Volcano Surveillance Co-ordinator