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Andes reach for the rain, says geologist

...looks a lot like the Swiss Napf region Thomas Jung

Irregular rainfall distribution over the past ten million years has influenced the shape of the Andes mountain range, according to a Swiss geology professor.

This content was published on December 15, 2011 - 21:27
Susan Vogel-Misicka, swissinfo.ch

The result is that the western side of the central Andes is a flat dry slope, while the eastern side is verdant and hilly – similar to central Switzerland’s Napf region.

Fritz Schlunegger of Bern University teamed up with Kevin Norton of New Zealand to discover that the interaction between tectonic uplift and erosion is very much controlled by the local precipitation rates.

Their study has been published in Nature Communications, an online scientific journal.

Extremes collide

Rainfall distribution in the central Andes is extremely uneven. To the west is the Atacama desert, one of the driest places in the world. To the east is the Bolivian Jungas region, which annually gets up to 3,000 mm of rain fall per square metre.

This contrast leads to different rates of erosion. On the eastern side it is about 1mm per year; on the west side it is one hundredth the amount – less than 0.01 mm annually.

“It’s very little per year, but over the course of a million years it’s a kilometre. What’s important in geology is not so much the amount of material, but rather the balance between the rate of erosion and the rate of the tectonic processes, such as the uplift of rock,” Schlunegger told swissinfo.ch.

He and Norton studied the consequences of this east-west difference in the development of the Andes. They found that the west’s development had pretty much finished somewhere between seven and ten million years ago. Most of the action since then has been on the eastern side.

“It’s really one of the most amazing landscapes I’ve ever seen. For me it was so impressive to feel the differences in climate and how one side has affected the other – and how this has evolved over the past ten million years,” Schlunegger said.

Thick crust

He explained that for mechanical reasons, there is a limit to how much a mountain can thicken – that is, it can only support so much weight.

“Erosion takes material away, and plate tectonics tend to add new material. It can either be in the middle of the mountain belt or on the lateral sides,” he said, noting that the western side of the central Andes had become drier as the eastern side had become more humid.

“That increased humidity also increased the amount of erosion, leaving space for more material on that side,” he said. Conversely, a lack of erosion on the west side made its crust too thick to keep moving on account of plate tectonics.

According to Schlunegger, the situation is not going to change any time soon.

“We don’t anticipate that the conditions will change significantly; the central Andes will probably continue to rise toward the rain over the next million years,” he said.

Swiss little sister

Schlunegger compared the verdant part of the central Andes with the Napf region in central Switzerland.

“The Napf is basically a miniature version of the Andes on the eastern side – it has all these wrinkles and many valleys,” he said.

The Alps, meanwhile, also have an influence on their surroundings.

“The wind comes from the north and northwest, and the clouds release rain in the Jura region and across the plateau, but then they get stuck in the Alps … so everything past that – like the Valais – gets very little rain. Most of the rain falls in the Napf,” Schlunegger said.

Mountains

Mountains are formed over millions of years by volcanism, erosion, and disturbances in the earth's crust. 

The Earth's lithosphere (crust and mantle) is made up of seven main "tectonic" plates, which float on the asthenosphere (a superheated and extremely viscous liquid). 

These plates move at 20-75 millimetres per year, and when they collide, the resulting pressure can cause earthquakes, volcanic activity and mountain-building.

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