Plateau Hut reopens despite ongoing rockfall risk

ROCK TALK: Geologist Royden Thomson, left, and Department of Conservation ranger Jim Spencer discuss the Mt Dixon rockfall after inspecting the site yesterday.
ROCK TALK: Geologist Royden Thomson, left, and Department of Conservation ranger Jim Spencer discuss the Mt Dixon rockfall after inspecting the site yesterday.

More rockfalls are expected off Mt Dixon in the Aoraki Mt Cook National Park.

However, a geologist hopes that any big incident will have the material going over an icefall rather than into a nearby alpine hut.

Geologist Royden Thomson and Department of Conservation staff visited the site of Tuesday's massive three-kilometre-long rockfall yesterday.

Based on Thomson's findings, the 28-bed Plateau Hut has reopened.

The hut was closed on Tuesday afternoon after debris came as close as 150-to-200 metres from the hut. Later that night, the 15 climbers who were to have stayed there were flown out as a precaution.

While still to look at previous photos to calculate how much material has fallen off Mt Dixon, Thomson said it would be millions of cubic metres.

The debris had split off into two sections, the larger path going down the middle of the Grand Plateau towards the icefall at the edge of the plateau and the smaller flow almost going up hill towards the hut.

"My gut feeling is it would take a major additional rock failure to build up on anything that is there, and in theory the majority would continue out and over the Hochstetter [icefall]," Mr Thomson said.

The rockfall picked up a lot of ice and snow as it moved down over the Grand Plateau, turning it into a type of avalanche.

Thomson suspected the debris would be about five metres deep. He believed if there had been anyone on the plateau when the slide came down, they would not have survived.

He predicts a large overhang on Mt Dixon adjacent to the slide will come down, but suspects that will happen piecemeal. He also identified other areas where material was likely to come away.

"The failure appears to have occurred in relatively weak crusty jointed [closely fractured] material. There has probably been a fault zone which has gone through there," he said, adding that the same fault zone was probably responsible for the slumping that occurred on Ball Pass last week.

Thomson said the heavy rainfall experienced in the region in recent weeks could be a factor exacerbating the stability.

"It [Mt Dixon] is still fretting. There is a bit of dust still coming off. If there is wind or rain it will keep coming off for a while," he said. Because Mt Dixon was "rubbly", it might have been eroded by the rain, causing material to fall, and taking out key blocks.


Other huge rockfalls in the Aoraki Mt Cook National Park include:

About 500,000 cubic metres of rock fell off Vampire Peak in January 2008. The slip took out the left "fang" of Vampire, producing a scar 120 metres high and 70m wide and forming a rockfall path close to two kilometres long. It was the second rockfall on Vampire, with a similar-sized event occurring between April 2002 and February 2004. The earlier event was not noticed until older photos of the area were being studied following the 2008 incident.

The park's largest known rockfall was in December 1991 when an estimated 12 million cu m of rock and ice fell more than 2700m down the eastern side of Mt Cook. The debris path was 500m wide and 700m high. The rock fall took 10m off the height of New Zealand's highest mountain.

A section of the Ball Pass Ridge slumped last week. It was still possible to cross the pass, but the exercise was now being described as "interesting" by those who have crossed the pass in recent days.

The Timaru Herald