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November 22, 2021

Yellowstone is well-known as one of the largest volcanic systems in the world. Few people know, however, that the largest-known subaerial landslide on Earth is located just next door.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Annie Carlson, Research Coordinator at the Yellowstone Center for Resources, Yellowstone National Park.

Map of the Heart Mountain slide block
Map of the Heart Mountain slide block. From Mitchell et al., 2015 ("Catastrophic emplacement of giant landslides aided by thermal decomposition: Heart Mountain, Wyoming." Earth and Planetary Science Letters 411: 199-207), modified from Anders et al. (2010).

Among the geologic hazards in the Yellowstone region, you may be familiar with the possibility of earthquakes, hydrothermal explosions, and volcanic eruptions. But what about… landslides? We know that small landslides are pretty common in the region. Some are historical, like the Gros Ventre slide east of Jackson Hole. Others are prehistoric, like the travertine slide that formed Silver Gate near Mammoth Hot Springs. With seemly endless geologic wonders around Yellowstone, you may not be aware that the largest known subaerial (above sea level) landslide ever occurred here: the Heart Mountain Detachment. This mind-boggling landslide has often elicited more questions than answers, but over the decades geologists have pieced together the evidence into a compelling theory.

To give the incredible size of the Heart Mountain Detachment (HMD) some perspective, recall images you’ve seen of the 1980 Mount St. Helens eruption. On the morning of May 18, 1980, a magnitude-5.1 earthquake occurred, marking the start of an enormous landslide on the north slope of the mountain that uncorked the magma body that had been growing within the mountain and unleashed a devastating lateral blast. Millions of people around the globe still remember watching the events on the news. It was the largest landslide in recorded human history. But it was tiny compared to the HMD.

Satellite image showing area of Heart Mountain detachment slide block
With the northwestern breakaway point near Silver Gate, MT, the Heart Mountain slide block moved southeast toward Cody, WY (orange arrow). Notable deposits of the debris avalanche include White Mountain, Heart Mountain, and the McCulloch Peaks.

The HMD occurred ~49 million years ago, concurrent with the volcanic eruptions that built the Absaroka range. At least 3,500 km2 of land slid during the HMD, which is an area larger than the size of Rhode Island! This enormous block of land lay east of the present-day Yellowstone National Park boundary. With the northwestern breakaway point near Silver Gate, MT, the block slid southeast toward Cody, WY. The upper plate was composed primarily of Paleozoic (about 250-540 million years old) carbonate rock and younger Absaroka volcanics. Notable deposits of the landslide include White Mountain, namesake Heart Mountain, and the McCulloch Peaks near the southeastern terminus. A scenic drive along the Chief Joseph Highway 296 provides spectacular views of this landscape.

View of White Mountain from the Sunlight Basin Road
A view of White Mountain -- a deposit of the Heart Mountain detachment -- from the Sunlight Basin Road in Wyoming. Much of the evidence supporting the lamprophyre diatreme triggering mechanism theory for the landslide was gathered at White Mountain.

What triggered the massive HMD landslide? Did it happen catastrophically all at once or incrementally over the course of millennia? How could such an enormous block of land move along a slope that was angled at only ~2 degrees, as opposed to the much steeper slopes that are typical for landslides? These questions have prompted rigorous debate among geologists for decades (continuous allochthon vs. tectonic denudation, anyone?).

Recent studies support a theory that the triggering mechanism was the emplacement of a lamprophyre diatreme (a volcanic plug that formed when magma was forced through flat-lying sedimentary rock) associated with Absaroka volcanic activity. The diatreme emplacement triggered a catastrophic landslide, traveling at >100 m/sec. It is also theorized that the landslide was able to quickly move a great distance across a 2-degree slope because CO2 was rapidly generated as the carbonate rocks slid past each other. This gas layer lifted the upper portion of the landslide and allowed it to move with minimal friction, like a hovercraft. Much of the colossal slide movement may have happened within minutes. It is difficult to comprehend the power and scale of such an event!

Granted, larger landslides have occurred under the ocean. And it’s possible that there have been larger subaerial landslides in geologic history that we have not yet found evidence for, or the evidence has been erased by subsequent geologic processes. But as it stands, the Heart Mountain Detachment is the largest known subaerial landslide in the world—another chapter in the astounding geologic story of the Yellowstone region!

Further reading:

Malone, David H., et al. "Volcanic initiation of the Eocene heart mountain slide, Wyoming, USA." The Journal of Geology 125.4 (2017): 439-457.

Mitchell, Thomas M., et al. "Catastrophic emplacement of giant landslides aided by thermal decomposition: Heart Mountain, Wyoming." Earth and Planetary Science Letters 411 (2015): 199-207.

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