alert bulletin: Sept 27 2007, 12:30 pm - Ruapehu Volcano

Science Alert Bulletin RUA-07/04 - Eruption from Ruapehu: Science Alert Level remains at Level 2, updated observations

Scientists from GNS Science and the Department of Conservation made a brief visit to the Dome at Mt Ruapehu today to collect rock samples from Tuesday night’s eruption. They were unable to collect a water sample from the crater lake due to strong winds (40 - 50 knots).

Visual observations of the lake level indicate that it is 2 - 3m below overflow, this indicates that approx 500,000 cubic metres of water was ejected from the lake during the eruption.

There is a strong up-welling from the northern vent under the lake and some sulphur slicks on the lake surface. These are fairly typical phenomena observed at the crater lake following hydrothermal activity.

There is a well-defined and fairly narrow apron of ballistic fall (rocks) to the north of the lake. Most of the ejecta comprised lake floor sediments, some crystalline rocks (altered) and abundant sulphur. So far there is no evidence of fresh magmatic material in the ejecta.

A volcanic gas measuring flight planned for today was postponed due to the strong winds, however data obtained from a gas detection satellite (AURA-OMI) shows no gas signature following the eruption.

These observations suggest that the eruption was most likely hydrothermal in nature, confirming the initial interpretation of the event.

Evidence gathered yesterday indicates that the lahar down the Whangaehu was larger than initially thought, but still much smaller than the March 2007 dam-break lahar.

Monitoring equipment at Dome Shelter was also not as badly damaged as initially thought and seismic data from one of the two systems is continuing to flow, although the data are rather noisy. We are currently relying on nearby monitoring stations to provide us with full seismic monitoring of Ruapehu.

The Alert Level remains at 2 and will be re-assessed on a daily basis.

Craig Miller
Duty Volcanologist