Glacial ‘armoring’ helps mountains grow taller

Research in the Andes challenges some of the conventional wisdom about glaciers and mountains.

In cold climates, glacial ice protects mountains from erosion as they’re lifted up by tectonic activity

By Summit Voice

SUMMIT COUNTY — It’s long been known that glaciers help tear down mountains by scouring out rocks and soil. But now, geologists have discovered that in cold climates at low latitudes, glaciers also help protect mountains from erosion, allowing them to grow taller as movements of the Earth’s crust push them up.

That’s why the Andes in the far south are taller than the peaks in the same range farther north, according to researchers from the University of Arizona who studied the range extensively to understand the role of glaciation and climate in mountain-building.

The University of Arizona researchers were surprised by what they found in Patagonia;s Andes Mountains, said Stuart N. Thomson, a research scientist in the University of Arizona department of geosciences.

This is the south flank of glaciated Cordillera Darwin and Bahia Pia (Pia Bay), highest point on Tierra del Fuego, Chile, taken from the Beagle Channel. These peaks and fjords were named after exploration undertaken during Charles Darwin's Voyage of the Beagle to this region in the 1830s. PHOTO BY STUART N. THOMPSON.

“What we’re seeing is that below certain latitudes, glacial buzzsaws clearly and efficiently operate, but south of about 45 degrees, it not only doesn’t work – it has the opposite effect,” said Peter Reiners, one of the co-authors of the study. “The glaciers actually protect the surface and allow the mountains to grow higher.”

Reiners said their research contrasts with the belief that glaciers limit the height of mountains worldwide.

The key is climate. Glaciers atop mountains in temperate latitudes flow downhill, scouring away the surface of the mountain. Over millennia, such erosion can reduce the height and width of a mountain range by miles.

However in very cold climates such as the Patagonian Andes, rather than scraping away the surface of the mountain, the team found that glaciers protect the mountain top and sides from erosion.

The team dubs the action of the cold-climate glaciers “glacial armoring.”

“Climate, especially through glaciers, has a really big impact on how big mountains get,” said Reiners.

“What we’re seeing is that below certain latitudes, glacial buzzsaws clearly and efficiently operate, but south of about 45 degrees, it not only doesn’t work – it has the opposite effect,” he said. “The glaciers actually protect the surface and allow the mountains to grow higher.”

He and his colleagues anticipate that glacial armoring also occurs on cold-climate mountains very far north, such as those in Alaska.

The paper based on the research is scheduled to be published Sept. 16 in the journal Nature.

The Andes are the textbook example of actively growing mountains that are limited in height and size by glaciers, Thomson said. The Andes are actively being pushed higher by movements of the Earth’s crust. However, if the glacial buzzsaw is active, the mountains also are ground down.

“We’re trying to understand how mountains are built and destroyed,” Thomson said. “Why are mountains high?”

In actively growing mountains, hot rocks from deep in the Earth are being thrust up. At the same time, erosion sands away the tops and sides of the mountains, bringing those once-hot rocks closer to surface. The speed at which the rocks cool indicates how rapidly the surface material above the rocks was removed by erosion.

To figure out how fast the glaciers had scoured the Andes, Thomson and his colleagues needed to analyze rocks now exposed on the mountains. The scientists sailed up glacially-cut fjords to the foot of remote glaciers and collected soccer-ball-sized rocks. The team collected rocks from latitude 38 degrees south to 56 degrees south, for a total of 146 samples.

They analyzed the rocks in laboratories at the UA and at Yale University to determine what geologists call the “cooling age” of the rocks. The cooling age tells how fast the rock was exposed by erosion.

The researchers used two independent dating methods, apatite uranium-thorium-helium and fission-track dating, to determine cooling ages. Both methods showed the same result — that the rocks cooled faster in the north and slower in the south. The slower the cooling, the more slowly the mountains are eroding.

Reiners said, “What corroborates this is that the mountains are higher in the south than in the north. Uplift is winning in the south, and the glacial buzzsaw is winning in the north.”

The importance of climate in the formation of mountains is currently a matter of scientific debate, Thomson said.

The new finding indicates that climate plays a key role.

Said Thomson: “Climate determines the size of a mountain range – whether there is a glacial buzzsaw or glacial armoring.”


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