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Geologic tick How a prehistoric landslide in San Juans can help communities deal with climate change by Jonathan Romeo
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newly documented landslide in the San Juan Mountains that dates back to the last Ice Age might help answer one of climate change’s most pressing questions: do retreating glaciers increase the frequency and intensity of landslides? Around 2018, Jonathan Harvey, Chair & Associate Professor of Geosciences at Fort Lewis College, was perusing recently released topography maps – as geologists are wont to do – enhanced by Lidar technology, which uses radar to map out the Earth. While scanning through the Weminuche Wilderness, Harvey noticed some strange happenings in the upper half of Elk Creek, a tributary of the Animas River a few miles south of Silverton (which also runs alongside the Colorado/Continental Divide trails). “I started getting curious about it,” Harvey said. “It became clear there had been an enormous landslide in the past that dramatically changed the landscape. But it appeared no one had mapped it. It was an unexplored geological feature.” So, like all good scientists dealing with the universe’s unknowns, Harvey hiked into the basin to see it in person along with an FLC student. Now, three years later, the “Elk Creek Landslide Project” is showing promising returns. For one, Harvey was happy enough to unlock a geological mystery in our own back yard. But also, Harvey’s research may help inform communities around the world that are also dealing with increased landslides as glaciers melt and retreat due to climate change. And it couldn’t have come at a better time. “This is a hazard that has not been given much attention, but with these fast-receding glaciers, it’s a hazard increasing in potential,” Anna Liljedahl, an Alaska-based hydrologist with the Woods Hole Research Center, said. A stark contrast Elk Creek is a relatively remote area. The only access to the trail is hiking down from Molas Pass, across the Animas River and up into the basin. Or, maybe, if you ask really, really nicely, the Durango & Silverton Narrow Gauge Railroad will drop you off.
A 14,000-year-old landslide deposited massive boulders in Elk Creek./ Photo by Jonathan Harvey But that’s not to say the place isn’t frequently visited. Hikers use the route as a leg of the Colorado Trail as well as the Continental Divide Trail. And, many people take the Elk Creek Trail to get into Vestal Basin. For the first part of the trail, from the confluence with the Animas, you are in a steep, tight canyon. Then, about 3 miles in, the canyon abruptly opens up into a wide valley bottom. Just as eye-catching, however, is what’s on the valley floor: thousands of large boulders, some the size of houses. “It’s a stark contrast,” Harvey said. “It’s this huge sea of boulders for about a mile, unlike anything else in the San Juans that I’ve seen.” During the first field trips in the summer of 2019, Harvey and a student spent a couple days mapping the slide, measuring its scale and taking samples so they could pinpoint the exact time the event happened. Continued study in 2020 sought to reconstruct the valley before the landslide came crashing down. “Boulder fields are obviously common,” Harvey said. “But to have an entire valley bottom filled with boulders struck me as unique, and I wanted to know why and how it happened.”
Glacial retreat Through modeling, Harvey was able to estimate the size of the landslide at about 50 million cubic meters of rock (a washing machine, for example, is one cubic meter, “so imagine 50 million washing machines tumbling down all at once,” Harvey said). The sheer size of the event, he said, likely would have blocked Elk Creek and created a dammed lake in the valley. To pin down the timeframe, Harvey sampled five boulders to see how long they’d been sitting there, through cosmogenic nuclide dating, which basically measures how long a rock has been exposed to the sun (think of it as “cosmic sunburn,” Harvey joked). All five rock samples came back with the same results: about 14,000 years. Which makes sense, Harvey said. At the peak of the last Ice Age in North America, about 21,500 years ago, the San Juan Mountains were covered in glaciers all the way down to Durango (oh, the ski terrain that once was). But, around 17,000 years ago, those glaciers started to melt and retreat. According to a 2017 study published by The Geological Society of America, upper Elk Creek was ice-free by about 15,000 years ago. Silverton was clear by around
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n June 30, 2022
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