For years, geologists have been intrigued by landslide deposits in the the Marysvale volcanic field in southwest Utah. It was assumed that these formations were produced by multiple events. But a new study suggests it's the result of a single massive landslide that happened 22 million years ago, the likes of which boggle the imagination.

Top image: A Google Earth image of the area covered by the Markagunt landslide.

If confirmed, the Markagunt gravity slide would be the second largest landslide known to have occurred on Earth. As far as geologists know, the biggest was the Heart Mountain Slide that took place some 50 million years ago in northwest Wyoming. But Markagunt could prove to be much larger than the Heart Mountain Slide once it's mapped in greater detail.

Inconceivably Huge

So how big was it? According to the new study authored by Robert Biek of the Utah Geological Survey and Peter Rowley of Geologic Mapping Inc. of New Harmony, Utah, the Markagunt gravity slide was 90 kilometers (56 miles) long and it enveloped an area somewhere between 1,700 to 2,000 cubic kilometers in volume (408 to 480 mi3). It was up to 200 meters (656 feet) thick in some places.

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It took place about 22 million years ago in an area between what is now Bryce Canyon National Park and the town of Beaver, Utah. It covered about 3,370 square kilometers (1,300 square miles). To put that into perspective, that's a space bigger than three Ohio counties — Cuyahoga, Portage and Summit — combined.

Lee Siebert/Smithsonia/CC

Despite this, evidence of the slide is not immediately apparent at the site today.

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"Looking at it, you wouldn't even recognize it as a landslide," noted Hacker in a release.

The researchers showed that the single landslide consists of a large sheet of volcanic rock broken up by faults. It was divided into three distinct sections: a 58 x 42 km (36 x 26 miles) bedding plane segment, a narrow ramp segment, and a 32 km (20 miles) long land surface segment.

Hot Rocks

The sight of this event as it happened would have defied comprehension. The length of the landslide shows that it was as fast as it was massive. Evidence reveals that the slide was catastrophic — occurring within minutes — by virtue of the presence of pseudotachylytes — rocks that were melted into glass by the immense friction. Any animals and plants living in its path would have been completely annihilated.

Related: The Most Deadly American Landslide Was Strangely Normal | How To Recognize The Signs of an Impending Landslide

According to the paper, which now appears in Geology, the slide originated when a volcanic field consisting of many strato-volcanoes — a type similar to Mount St. Helens — collapsed and produced the massive landslide. The collapse was instigated by the vertical inflation of deeper magma chambers that fed the volcanoes at the site. The researchers made this determination by analyzing data collected at the location over the past several years.

The American Geophysical Union explains more:

The question of course is how such an enormous landslide can form. The base of the Markagunt gravity slide consists of of a clear shear surface with brecciated (i.e. intensely shattered) rocks. However movement has occurred on a shear surface that has an inclination of only a small number of degrees. The authors suggest that as the Maryvale volcanic field developed, it uplifted the Turshar Mountains, generating a slope in the surrounding rocks. At the base of what was to become the landslide is a very weak volcanic deposits known as the Brian Head formation, allowing sliding to develop.

Future Threat?

Though rare, it shows the magnitude of what can happen when a volcanic field collapses. The researchers say the Markagunt gravity slide, along with Heart Mountain, represent "a new class of hazards in volcanic fields."

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Disturbingly, they say a similar thing could still happen, including at modern volcanic fields like the Cascades.

Read the entire study at Geology: "Catastrophic emplacement of the gigantic Markagunt gravity slide, southwest Utah (USA): Implications for hazards associated with sector collapse of volcanic fields".

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