Beneath crumbling walls: how rock glaciers took over the southern rockies
The headwall of Mount Sopris looms above a large rock glacier in the basin below. Photo by Robert Anderson.
20,000 years ago, during the last glacial maximum, hulking formations of flowing ice stretched across the Southern Rocky Mountains in present-day Colorado. Most of those glaciers melted away by around 13,000 years ago, but that doesn’t mean Colorado’s glaciers are gone. They’ve just gone underground. Today, the state is home to thousands of rock glaciers — icy formations protected by layers of debris.
“Anybody who climbs the fourteeners of Colorado, whether they know it or not, is climbing across a rock glacier here or there,” INSTAAR fellow and distinguished professor of geologic sciences Robert Anderson explained.
Last month, Robert Anderson and Suzanne Anderson, also an INSTAAR fellow and professor of geology,. According to their analysis, the main ingredients are snow and a protective layer of rocky debris, usually provided by a tall, erosion-prone cliff overhead. Importantly, there can’t betoo much ice and snow.
“The speed at which the ice moves away from the base of the cliff is governed by how thick the ice is. The thicker it is, the faster it goes,” Robert Anderson explained. “It can’t be too thick. So, avalanche run-outs are sort of the magic zone for the top of a rock glacier.”
A gap in geologic history
The Andersons’ new model of rock glacier formation led to a second insight — one about the past. According to the study, the rock glaciers we see in the Colorado Rockies today likely formed hundreds, or even thousands, of years after the big glaciers disappeared. That’s because rock glaciers actually require more temperate conditions to establish themselves.
“We argue that you lose almost all of the ice out of the alpine valleys. Then, only later, it comes back in a rock glacier mode,” Robert Anderson said.
The model is also backed by physical evidence collected by the Andersons, their students and a number of other researchers over the past three decades. To determine the age of rock glaciers, these researchers tested samples from boulders on the surface of the glaciers for a rare isotope of the element beryllium. The results indicate how long the boulder has been there, and thus how long ago the glacier formed. According to these analyses, Colorado’s rock glaciers arose around 10,000 to 12,000 years ago, which places them at least a thousand years after deglaciation.
Robert Anderson says that understanding how rock glaciers formed gives us a more clear understanding of their role in the landscape today. It might help us conserve them too.
“It’s an important part of climate history,” he said.
Rock glaciers and water
Suzanne Anderson gathers water samples from a mountain creek near the Mount Sopris rock glacier. Photo by Robert Anderson.
Today, rock glaciers are ubiquitous in Colorado’s alpine landscapes, but the role they play in mountain ecosystems is still poorly understood. According to Suzanne Anderson, the key to further understanding may lie in the unique water rock glaciers provide.
“It’s different from other water sources in the landscape for two reasons,” Suzanne Anderson explained. “It’s cold, even late in the summer, because it's coming directly from ice melt, and its chemistry is different — it’s a little more nutrient-rich.”
Some research has already suggested that. The Andersons and their students have also found elevated levels of nitrogen, an important nutrient for both plants and animals, in rock glacier meltwater.
One of Suzanne Anderson’s students, M.S. student Maya McDonough, is furthering these investigations. This summer, McDonogh will sample meltwater from a rock glacier near Imogene Pass, between Telluride and Ouray, Colorado and analyze its chemistry.
“We’re testing the water chemistry to see what ions are in rock glacier discharge, because those nutrients might be really important to certain high-altitude habitats,” McDonough said. “I’ll test as many samples as I possibly can.”