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Because you watch SciShow, Brilliant is offering you a 30-day free trial and 20% of an annual premium Brilliant subscription, at Brilliant.org/SciShow. In the late 1800s, a geologist in Wyoming realized that a giant piece of mountain was not where it was supposed to be.

Out in the Rockies, at a place called Heart Mountain, a layer of limestone hundreds of millions of years old was sitting on top of rocks just a fraction of its age. Normally, younger rock sits on top of older rock as layers of sediment pile up over time. So something had to have happened here, and scientists have spent more than a century slowly unraveling the mystery.

Along the way, they uncovered not just the biggest landslide on Earth ever, but one that slid more than 45 kilometers at one-third the speed of sound. The slope of the ground at Heart Mountain was just two degrees, though. So how could this happen?

And also, could it ever happen again? [♪ INTRO] Now, landslides in Wyoming wouldn’t have been especially unusual when this all went down nearly 50 million years ago. There was lots of volcanic activity going on, shaking things loose from time to time. But the landslide that slid Heart Mountain across 45 kilometers of Wyoming was a whopper.

Geologists realized this when they uncovered slabs of ancient limestone scattered across an area bigger than Rhode Island. And along with Heart Mountain, they all seemed to come from one giant sheet of rock. This was the biggest landslide ever discovered on land.

And as evidence piled up that this landslide may have reached one-third the speed of sound, geologists realized they were dealing with an absolutely mind-boggling event. For comparison, your typical fast-moving landslide doesn’t get much faster than a car on a residential street. Now, not everyone agrees on exactly how fast it moved.

But even conservative estimates suggest it was all over in a matter of hours, maybe even minutes. So, putting all this together, geologists had a baffling puzzle in their hands: How did this giant chunk of earth just glide across Wyoming like it was a slip ‘n’ slide? Even though scientists have been debating and researching the history of Heart Mountain for years, no one agrees on a single answer.

But all of the possibilities are pretty wild. The landslide likely started after volcanic explosions split part of the original mountain off from the rest. Maybe there was a weak layer of rock under the limestone made by some build-up of hot liquid pushing up against it from deeper underground.

Or maybe there was a weak spot somewhere caused by all the volcanic activity rattling stuff around. Either way, after volcanic activity triggered the landslide, there was no going back. One hypothesis suggests that the heat at the base of the slide vaporized a bunch of carbonate molecules in the rock’s minerals, creating a cushion of high-pressure carbon dioxide gas for the monstrous slab of rock resting on top of it.

In other words, there was now an almost frictionless, slanted surface that let the limestone just slide… no, levitate downhill. And as unbelievable as that sounds, something like it has happened before. In 2009, an avalanche of rocks broke off a mountain in southwestern China, and traveled around two kilometers in under one minute.

Researchers investigated the landslide to figure out how it had slid so far so fast. And they found that, as the rocks heated up, the carbon had vaporized and formed CO2, and that helped buoy the avalanche as it slid over the ground. But other researchers argue that the Heart Mountain Slide didn’t float entirely on a layer of gas.

Instead, they hypothesize that little grains of rock formed at the base of the future landslide, and began acting like a fluid as they mixed with some of that CO2. Suddenly, most of the friction holding the slab of mountain in place was gone, and it basically surfed downhill on this layer of rocks masquerading as a fluid. Maybe that sounds just as far-fetched as geology’s version of an air hockey table, but we do know that a bunch of grains or larger solids can sometimes act like fluids.

Just think about quicksand. When sand or soil particles get saturated with water, there’s less contact between the actual grains, so those grains have less friction and the whole mass of them starts acting like a liquid. The same thing can happen if the spaces between grains get filled with gas instead of water.

Really any kind of fluid will do the trick. And that’s what some scientists think could have happened here. In the end, no matter how this supersized Heart Mountain Slide came to be, there are multiple ways it could have.

And we know that because it’s not the only one. Over in southwest Utah, which also used to be a volcanic hotspot, researchers recently discovered a trio of smaller, but still massive landslides that happened around 21 million years ago. And we can even see evidence of supersized landslides on Mars, although those are driven by completely different mechanisms.

Altogether, these events show how reducing friction, in any number of ways, can produce unimaginably dramatic landslides. But we still don’t know just how common events like this are on Earth. Because erosion and plant cover are great at hiding geological scars, and they make it hard to see the traces of ancient disasters… …which is how an event like the Heart Mountain Slide hid in plain sight for so long.

Clearly, events like this don’t happen very often compared to human lifespans, or it wouldn’t have taken till the late 19th century to discover the first one. But supersized landslides might not be as rare as we’d like to believe. Under the right conditions, Earth can… and does move mountains.

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