Why Do Rocks the Size of Cars Suddenly Break From Cliffs?

Why Do Rocks the Size of Cars Suddenly Break From Cliffs?


– Holy crap, holy crap. In Yosemite Valley, rockfalls like this one
can happen every 4-5 days, where boulders that can be larger than your average car or apartment building thunder down steep mountainsides. Greg stock is a geomorphologist who works alongside USGS engineer Brian Collins. Using a combination of high-res photography, LiDAR, aerial surveillance, and 3D modeling, it’s their job to monitor rockfalls in the park, which is critical, especially when five million visitors pass through Yosemite each year. GREG: I do think that Yosemite is tailor-made for rockfalls, because the cliffs are gigantic. One example would be a rockfall in 1996 where a big chunk of rock free fell most of the 3,000 feet down to the valley floor, and when it hit as an in-tact block, it generated
category five hurricane strength winds and that blew over a thousand trees
in a matter of seconds. So what happened in those final moments to cause the rock to leave the cliff? GREG: We know that there are probably decades,
or even centuries of preparing that rock to fall but the trigger would be that last thing.
That’s the straw that breaks the camel’s back. Roots can grow into these cracks and the roots will expand and push the rocks off, when water freezes in the cracks,
that can also push them off. However, roughly a third of the rockfalls in Yosemite happen at times when Greg and Brian aren’t able to identify a trigger. Historically, rockfalls were only recorded
if they had been witnessed by humans. So to be able to track and learn from events that may happen when no one is around, Greg and Brian use a host of other tools. BRIAN: One of those tools is ground-based laser scanning, or it’s also called terrestrial LiDAR. This basically gives us just millions and millions of points that we can create a three-dimensional model of the cliff. So we use high-resolution photography a lot of times. You can use these photos to zoom in very carefully on different aspects
of the wall, for trying to identify what part of the rockfall was active recently. In addition, Greg and Brian identify hazard areas
where rockfalls have happened before. Based on their computer models, they know that these are areas where another rockfall event is likely to occur. – Look at that piece! *thundering* GREG: This is really neat, the fact that this new boulder from six months ago, BRIAN: Mm-hmm. Right next to this one. GREG: It’s adjacent to this one, that’s probably a thousand years old or more. BRIAN: Right. You got the same pattern happening. GREG: The past is the key to the present. BRIAN: Yup. Though the team can flag unstable areas, it is difficult to predict when a rockfall will happen. Varying weather conditions, quakes, and even wildlife can be the difference between a crack remaining stable for a hundred years, and a major rockfall event. BRIAN: We put in specialized gear called crackmeters behind the cracks, and they measured how much the crack would open over the course of a day. And we tracked this over three and a half years; what we found was that during periods of heating the crack would move in and out every day. Greg and Brian’s research confirmed something rock climbers and geologists have hypothesized, but never proved: that rockfalls could be triggered
from hot weather. GREG: Even though we’ve identified heat now as a probable trigger, is the most important thing how hot it was that day? Or is it the fact that maybe it was really cold the day before, and then hot that day? We don’t really know that. Despite the unknowns, Greg and Brian’s research is highly valued by scientists that study mountains in Switzerland and Brazil. That’s because the team
have a unique opportunity to amass a huge volume of data based on the number of rockfalls in Yosemite. There are rockfall deposits here in Yosemite that are way bigger than anything that we’ve seen happen in the last 150 years. Which means that there’s potential for rockfalls to happen in the future that are much, much bigger than what what we have seen historically. Which is why Greg and Brian’s work is so important. It’s their data that helps to keep millions of Yosemite visitors safe each year. GREG: People come here and they look at these cliffs and my sense is that they think that you know, they’ve looked this way for thousands of years,
but that’s not true at all. The cliffs are changing constantly because of rockfalls. This episode was presented by the US Air Force.
Learn more at airforce.com. For more episodes of Science in the Extremes,
check out this one right here. Don’t forget to subscribe, and come back to Seeker for more episodes. Thanks for watching

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