From Yosemite's iconic Half Dome mountain to the world's tallest Himalayan peaks, many of the planet's most dramatic landscapes have been carved by glaciers. These mammoth sheets of permanent ice, found on every continent except Australia, slide down mountainsides, eroding the rock beneath them and slicing deep valleys and canyons in their path.
Until now, it was generally thought that glaciers could only inhibit mountain growth because of their crushing weight — an idea known as the "glacial buzzsaw theory." But a new study by Yale geologist Mark Brandon, which made the cover of this week's issue of the prestigious journal Nature, shows that in some parts of the world, at least, glaciers can actually boost mountain growth.
Brandon and his team collected rock samples from a vast track of the Patagonian Andes at the southern tip of South America and found that the rock was much older than expected, meaning it hadn't been eroded by the surrounding glaciers. Instead, they found evidence that the glaciers acted as a kind of protective shield, actually encouraging the 25-million-year-old mountain range to continue rising.
"The glaciers act like armor to protect the uplifting mountains from erosion, allowing them to reach heights well above those predicted by the glacial buzzsaw theory," Brandon said.
The team was surprised by their discovery because, at the southern tip of the Andes, the elevations are lower than farther north. Lower elevations correspond to warmer temperatures, which means that melting, sliding glaciers should have even more of an effect. Instead, the team found that the glaciers in this part of the Andes, which stretches as far as from Washington to San Diego, were cold enough to remain permanently frozen at their base and stuck to the mountainside, providing an icy shield that holds the mountains in place.
"This is the first evidence to contradict the glacial buzzsaw theory," Brandon said. "Next, we'll try to calibrate a global erosion model to understand how climate affects the mountain building process."
Brandon plans to use this latest discovery to try to study how the size of mountains, like the Andes, have been influenced by the cooling of the Earth's climate over the last 15 million years.
"Geologic studies provide an important reminder that the Earth is larger and much older than us," he said. "The variations in climate over time are far beyond what we experience in our lifetimes."
— By Suzanne Muzzin Taylor