A 4,000-foot-high mountainside inside Glacier Bay National Park in Alaska collapsed last week, unleashing a river of rocky debris that stretches for miles and is roughly 12 feet deep.
The event, which generated shock waves measuring 5.2 in magnitude on the scale applied to earthquakes, started at 8.21 a.m on June 28. It was first spotted on seismic monitoring equipment by scientists from Columbia University's Lamont-Doherty Earth Observatory, according to a university press release.
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The scientists, led by geomorphologist Colin Stark and seismologist Göran Ekström, found that the landslide was triggered by the collapse of a rock face on a high, steep slope. The debris accelerated, covering a mile and a quarter in about a minute, and then collided with Lamplugh Gracier, pushing up snow and ice as it moved.
The landslide stretched about 6 miles, and an airplane pilot who landed near the edge estimated that the debris was more than twice as thick as the typical height of a man, according to the press release.
University of Alaska-Fairbanks seismologist Michael West told KTOO Public Media that the signals from the landslide "shook a decent amount of our planet."
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"Mountains are eroding constantly, and this is just one of the forms of erosion," Haines, Alaska-based geologist Russ White, who subsequently flew over the landslide area, told KTOO. "It's a fairly spectacular form of erosion when a 4,000-foot face of a mountain falls off and shoots itself 6 miles out across the glacier."
Here's a video of the landslide aftermath, posted on YouTube by Paul Swanstrom of Mountain Flying Service.