'Middle Earth' Mountains: Steep and Strong

It's the composition of the rocks, not elfin magic, that makes New Zealand's mountains some of the steepest on Earth -- without being particularly prone to landslides.

A new survey of the mountain ranges that form the spine of New Zealand confirms the steepest are made almost entirely of tough but otherwise unexciting rocks called greywackes and schists.

The lack of variety, paired with a great range of climates and mountain growth rates throughout the ranges, makes the region an ideal test of the importance of rock types in determining how often the slopes fail and generate killer landslides.

"They're pretty boring rocks," confirmed avalanche and landslide researcher Oliver Korup of the Swiss Federal Research Institutes in Davos, Switzerland. They are simply petrified deep sea sediments that have been pushed up to form the mountains, he said. "They don't even have fossils."

Korup is the author a paper on the New Zealand mountains in the July issue of Geophysical Research Letters.

In general, there are three things that control the steepness of mountain slopes: 1) the rate the mountains are being uplifted, or growing, 2) the rate that erosion is wearing the mountains away, and 3) the strength of the rocks, as measured by their ability to pile up without collapsing as a landslide.

By measuring slopes and landslide frequencies in New Zealand, Korup found that regardless of the growth rates or rainfall in a particular mountain range, the slopes tend to be steeper than mountains elsewhere in the world. That suggests the only other variable -- rock type -- is especially important.

That is not, however, the way many geomorphologists have seen it in recent years, particularly when using computers to model landscape changes, explained geologist David Montgomery of the University of Washington.

"The funny thing is," agreed Korup, "a lot of these sophisticated models know a lot about the physics of erosion, but not rock types."

His New Zealand study suggests that's a big omission. In fact, it turns the erosion-based approach to interpreting landscapes almost on its head.

"You might be able to look at a mountain range and explain the slopes by the different rock types," said Montgomery.

Korup hopes that someone will look at another fairly homogeneous mountain belt somewhere else to see how well this idea holds up. The Olympic Range in Washington, he suggested, might be a good candidate.