Apr 22 1996, 10:00 am - Ruapehu Volcano

Situation Summary

At about 13:10 on Sunday April 21 personnel working at Tukino Skifield became aware of a large flood in the Whangaehu River. They were initially alerted by the noise which was described as a roar, and was later described as a flood of mud and boulders in the river valley. At that time the Department of Conservation was notified, which in turn informed GNS.

Examination of the seismic records at Wairakei Research Centre confirmed a seismic event starting at 12:57, lasting for several minutes. Although there was considerable energy release recorded at the Dome there was very little at Chateau and Ngauruhoe. The dominant frequency of the records were also higher than the typical eruption events at Crater Lake. This suggested a very shallow source, possibly a landslide. A GNS field party got away about 14:00 with the intention of visiting the Whangaehu valley. Upon arrival at Turangi to meet with DoC it was decided to fly as the weather was rapidly improving. An observation flight was made between 16:00 and 17:00.

Observations on the flight confirmed a portion of the north east rim of the lake basin had collapsed into the Whangaehu Glacier and flowed down into the Whangaehu River valley. The collapse had also deposited debris into the lake basin and lake. There was also evidence of head wall erosion back up the Whangaehu Glacier towards the Dome. The water level of the lake appeared to be higher and the lava dome is no longer visible. The failed area is an region thought to have been weakened during the 1995 eruptions and the resultant debris will most likely consist of a mixture of 1995 eruption and weakened crater wall material. This type of failure has been a possibility in this area since September 1995 and further failures could be expected, especially during periods of high rainfall.

We advise that the Alert Level remains at Level 2.

B.J. Scott (for C.J.N. Wilson)
Volcanology Programme Leader