Quick Dips

Moving Mountains

On December 14, 1991, Mt Cook relinquished over 14 million cubic metres of matter in a spectacular avalanche of rock and snow. The avalanche reached speeds of over 300 kilo-metres per hour and registered equivalent to a magnitude 3.9 earthquake on a seismograph at Twizel, 75 kilometres away.

Forty kilometres away and six months later, Mt Fletcher, at the head of the Godley Valley, produced another rock avalanche which sent a 20-metre high wall of water 1.5 km across the lake at the foot of the mountain. The fall reached speeds of 200-300 kph and produced an impressive dust cloud which was still visible from 55 km away three days later.

DSIR geomorphologist Dr Mauri McSaveney says avalanches occur regularly throughout the Southern Alps, although they're not usually as big as the Mt Cook and Mt Fletcher ones. Mt Cook, Mt Fletcher and Falling Mountain in 1929 are the three biggest avalanches New Zealand has had in recorded history, said McSaveney. "Two of the three biggest in the last six months."

Falling Mountain fell during the 1929 Arthur's Pass earthquake, which measured 6.9 on the Richter scale. The cause of the Mt Cook and Mt Fletcher avalanches, however, is not as simple.

Mt Cook was ready to fall, says McSaveney. The cause will remain unknown, but glacial movement provides one possibility. As a glacier moves across a mountain, it dislodges underlying rock and, when part of the glacier falls, it can take pieces of the mountain with it.

If the glacier dislodges a critical piece of rock which supports an unstable buttress, an avalanche can result. If this was the case on Mt Cook, the evidence disappeared in the first moments of the fall.

Why it fell when it did will also remain unknown. Mt Cook fell on a night when there was no earthquake or storm which might trigger an avalanche.

"It could've fallen a hundred years ago or it could've waited another hundred years," says McSaveney. "It was just ready, which is a sobering thought when anyone is climbing around on the mountain."

The cause of the Mt Fletcher fall is more straightforward. The Maud Glacier has been cutting away at Mt Fletcher for thousands of years, and the ice at the foot of the mountain has dropped 250 metres in the last hundred years. That has removed support from the mountain, making an avalanche inevitable.

A large fall in 1988-1989 and smaller ones in the six months before the latest avalanche suggest that the whole slope of Mt Fletcher has been unstable at least since 1988 and possibly before then, said McSaveney.

Rock avalanches are a part of the natural erosion of the Southern Alps. In the valleys of the alps, glaciers and rivers carry away material, and streams and passing rockfalls remove material from the mountain slopes. Along the ridge crests this type of erosion doesn't occur, material accumulates and, with the help of gravity, avalanches are the natural result.

"The only way they can get eroded is by dropping off," says McSaveney. "They don't do it very often, but it's spectacular when they do."

Janine Griffin

Janine Griffin is a freelance journalist specialising in science issues.