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Andes' Height Due to Climate, Study Says

John Roach
for National Geographic News
October 22, 2003
 
Not all mountain ranges are created equally. Consider the central Andes of South America: A pair of Earth scientists reports today that the second highest mountains in the world reached their lofty height thanks to frigid ocean waters and parched soils.

Only the Himalaya are taller, which have formed as the Indian and Asian landmasses have slowly plowed into each other over the last 50 million years. The Andes, by contrast, have formed as the floor of the Pacific Ocean slips uneasily beneath South America, ruffling the land along the west coast of the continent.


The Andes stretch the length of South America's west coast, from Colombia to the southern tip of Patagonia in Chile. The mountains' middle range—which stretches from central Peru, across Bolivia, and down to central Chile and Argentina—is nearly twice as tall as the more northern and southern sections of the cordillera on average.

"I began to wonder why part of the mountain belt should be so much higher," said Simon Lamb, an earth scientist at the University of Oxford in England.

Lamb and his colleague Paul Davis, a geologist at the University of California, Los Angeles, describe in tomorrow's issue of the science journal Nature how the fault along this section of the Andes is starved of lubricating sediments, giving it the stickiness required to support mountains that reach heights over 20,000 feet (6,000 meters).

The lack of lubricating sediments, they say, results from a cold ocean current that flows up from Antarctica, keeping the sky free of clouds and the soil parched. No rain means no rivers to carry sediment back into the ocean.

John Dewey, a geologist at the University of California at Davis, said the basic principles described in the paper have been proposed for other mountain ranges, including the Southern Alps of New Zealand.

"This is a well known idea," he said. "What they've done is nicely applied it to the Andes. What they say is basically logical and right."

Fault Lubricant

Like skyscrapers in New York City, the Andes need a foundation to prevent them from falling down. For the Andes, this support comes from the so-called subduction fault where the Pacific Ocean slides beneath South America and into the Earth's interior.

"The rougher this fault—the less lubricated it is—the more push it can exert holding up the Andes," said Lamb, noting that the challenge of a well-lubricated fault holding up a mountain range is like to trying to push a stalled car along an icy road: a difficult task if you can't get a grip on the slippery road.

Lamb and Davis calculated how slippery the fault is along the length of the Andes and found that it is at its roughest, or stickiest, in the area that supports the central Andes.

The researchers also found that the ocean floor is barren of sediment where the Andes have their greatest height. Whereas in northern Peru and southern Chile, where the Andes are half as tall, the fault is much more slippery and the ocean floor is thickly covered in sediment. The pair concluded that ocean floor sediment acts like a lubricant.

"The sediment is sloppy, wet, and weak, it's like adding oil to the subduction zone," said Lamb. "In those lubricated parts you'll never get high mountains because you can't hold them up."

Cold Currents

The amount of sediment washed to the ocean floor is controlled by how much rain and snow falls on the mountains. Wetter regions like southern Chile have large rivers that carry debris down into the ocean. No rivers gush down from the much drier central Andes.

According to Lamb, the reason the west coast of central South America is so dry is because the so-called Peru-Chile current brings up unusually cold water from Antarctica.

"This cold water mass actually plays a major role in keeping the climate along the coast dry," he said. "It suppresses formation of rain clouds, so you get almost no rain forming in this area."

This cold current is related to the general cooling of the global climate over the past 50 million years or so. According to the researchers, the rise of the Andes closely follows this cooling trend.

When the global climate began to cool about 50 million years ago, the ice sheets on Antarctica were small and the west coast of South America was at or below sea level.

By 40 million years ago, the Antarctic ice sheets had begun to grow and the Andes began to rise, said Lamb. When the ice sheets rapidly expanded about 14 million years ago, they caused the Peru-Chile current system to cool sufficiently to dry out central South America, starving the ocean floor of sediment.

"Once starved of sediment, the ocean floor became rough and sticky and suddenly you had forces available to stick up the mountains," said Lamb. "So, climate change can trigger mountain building."
 

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