Glaciers, known for erosion, can help mountains grow
Glaciers' 'buzz-saw effect' can level mountains. But at high altitudes, glaciers can freeze to mountains and shield against erosion, new research suggests.
Mountain glaciers have long been recognized as powerful agents grinding down the craggy landscapes they occupy.
But new research suggests that under the right conditions, long-lived mountain glaciers and ice caps also can facilitate the growth of mountains – highlighting the role global climate change on multi-million-year time scales can have on the tectonic processes that stretch, knead, and fracture Earth's fragile crust.
In short, if conditions are cold enough, large-scale glaciers freeze to the land beneath them, significantly slowing the glaciers' movement downslope and substantially reducing the amount of erosion the mountain might otherwise experience. They protect the mountain as it grows, rather than scrape away at it.
That leaves the forces driving the growth of a mountain range free to build the peaks at a faster rate than glaciers whittle away at the summits.
Scientists have observed frozen-base glaciers in mountains in Antarctica and northern Canada. But these mountains are no longer are rising. The new study, published in the latest issue of the journal Nature, represents the first time scientists have been able to document the influence such glaciers have on mountains that are still rising at a geologically speedy clip.
The results "were quite surprising," says Stuart Thomson, a geologist at the University of Arizona in Tucson who led the team of US and Chilean scientists exploring the role of the effect of long-lived mountain glaciers on the evolution of mountain ranges.
The team was gathering data to test an idea dubbed the glacial buzz saw. In effect, the notion holds that glaciers limit the height mountain ranges can achieve. As the crust buckles under the inexorable force of crustal plates colliding, mountains build, finally reaching altitudes where they can sustain a long-lived snow and ice pack. Over time, those frigid caps thicken and form glaciers that flow down the mountainsides carrying large amounts of crushed rock and boulders with them.
The overall effect is to lower the mountain range's average height and narrow its base relative to the surrounding landscape.
The team selected as its laboratory the Andes in Patagonia, a region that arguably helped give rise to modern geology and geophysics through Charles Darwin's first studies of the region in the mid 1830s.
The Andes are undergoing rapid growth, thanks to the collision between the Antarctic and Nazca plates with the South American plate. And the region is home to the third-largest ice cap on the planet, after Antarctica and Greenland.
The widespread growth of glaciers in the region began some 5 million to 7 million years ago, Dr. Thomson explains, as the global climate began a cooling trend still underway today.
But the southernmost portion of the Patagonian Andes seemed to defy the buzz-saw concept. There, the average height of the range was more than 3,000 feet higher than the average height of the range farther north.
To try to solve the riddle, the team estimated erosion rates along the Andes stretching from 38 degrees south latitude to about 56 degrees south – a distance of nearly 1,250 miles. They gathered rocks from various altitudes and used sophisticated lab techniques to determine the ages at which the rocks crystallized and cooled after serving time as magma deep under the crust.
The results clearly indicated that erosion rates along the southernmost portion of the range they studied were far slower than rates for its more northern counterpart. And the southernmost portion of the mountain range the team studied was wider, in addition to being taller, a condition one would expect from reduced erosion rates.
Because the higher-latitude stretch would have been subjected to some of the coldest temperatures as the climate cooled, it would have built the thickest glaciers with their based frozen to the ground. At lower latitudes, melt water from warmer temperatures would have lubricated the glaciers' bases, allowing them to more readily bulldoze their way down the mountainsides.
The team's work represents a significant contribution to the story of how long-term climate change, glaciers, and tectonic forces can work in concert to sculpt the planet's surface, says Michael Kaplan, a geochemist at the Lamont-Doherty Earth Observatory in Palisades, N.Y., who was not part of the research team.
Until now, effort to explore these processes have relied largely on models., he says. The new results "provide real observations on what's happening on the ground."