The largest of the Very Large Boulders. Photo Courtesy Peter Cox

On the rocky shores of a windswept island just west of Ireland, the 620-ton boulder looks almost at home. But careful analysis of its position over the last few years has revealed something odd: between the summers of 2013 and 2014, the boulder shifted a couple meters toward the sea. That discovery is causing scientists to rethink what they know about the impacts of powerful storms.

In fact, the rock is one of more than a thousand boulders—including a handful of Very Large Boulders (VLBs and yes, that’s a technical term) weighing over 50 metric tons—shuffled around by the powerful storms that pounded Ireland’s west coast during the winter of 2013-2014, the stormiest in decades. Described in a new paper in the journal Earth Science Reviews, these boulders offer some of the first concrete evidence that storm waves, not just tsunami waves, can pack enough punch to hurl giant chunks of Earth around. (For comparison, 100 metric tons is about half the weight of a Boeing 747.)

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In a warming world where more energy in the oceans and atmosphere could mean more powerful storms, that’s an important insight.

“Ten years ago, it was possible to say storms can’t move 50 ton boulders,” lead study author Rónadh Cox, a professor of geosciences at Williams College, told Earther. Cox was referring to an argument some researchers had previously made that, since nobody had shown direct photographic evidence of storm waves moving boulders, tsunamis were the more likely culprit. “If you were building a model of storm intensity or thinking about risks posed by severe storms, then your upper level for storm energy were to some extent informed by that understanding.”

“What our work is showing is in fact yes, storm waves can be tremendously strong,” she continued.

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Photo Courtesy Ronadh Cox

It was a discovery that came about through years of boulder surveys on the Aran Islands and along the west coast of Ireland; work led by Cox and involving a slew of undergraduates. In particular, Cox and her students were interested in how the thousands of boulders scattered across these rocky shorelines got where they are today. Could storm waves be responsible for the positions of the boulders, some of which are perched hundreds of meters inland? Or are only tsunamis—long, powerful sea waves generated by earthquakes and volcanoes—strong enough to push VLBs around like it’s NBD?

Back in 2012, the team published a historical analysis comparing the position of boulders on the Aran Islands with survey maps produced by 19th century geographers. The data revealed that many boulders had been moved by storm waves over the course of the last century, “but we still didn’t have any before and after pics of the rocks moving,” Cox said.

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Cox finally got her chance to catch the boulders in action during the 2014 summer field season, which came on the heels of an incredibly stormy winter. Along with a half-dozen undergrads, she visited roughly 100 locations on the west coast of Ireland, from County Mayo to County Clare to the Aran Islands. The team navigated to the sites using GPS, and compared images of each stretch of coastline with geo-tagged reference images from years past.

The changes weren’t subtle.

“On the plane over I was thinking, this could be a real bust, what am I going to do if nothing has changed?” Cox said. “It was an extraordinary amount of change.”

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Photo Courtesy Ronadh Cox

In total, the team cataloged 1153 boulders that had been displaced the previous winter, ranging in size from a tenth of a metric ton to more than 500. Eighteen of the boulders were VLBs, while six of these Large Boys exceeded 100 metric tons in weight. The biggest? A 475-ton and a 620-ton boulder, both located on the island of Inishmore.

Boulders weighing hundreds of tons were generally restricted to a few meters of movement, and tended to huddle around the shoreline. Smaller boulders, however, could be transported further, and some were seen at up to 222 meters (730 feet) inland, and nearly 30 meters (100 feet) uphill.

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Taken together, the authors write, these findings show that yep, storm waves can move massive rocks. That’s an important insight for a few reasons.

For one, it tells us something about total potential power of storm-generated waves. That has obvious implications for forecasters and urban planners hoping to minimize the impact of storms on coastal infrastructure today.

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Secondly, we live on a warming planet, and one of the expected consequences of climate change is more intense storms that can dump more water and pack fiercer winds. Understanding fully the impacts of the most powerful storms will only become more relevant as we prepare for the future—and try to mitigate the worst effects of climate change.

Adam Switzer, a coastal hazards researcher at the Earth Observatory of Singapore who was not involved with the research, praised the new study for containing “a very detailed review and a great dataset that will now allow engineers and modellers new insights into offshore wave power and rocky shoreline dynamics.”

“We know that storm waves are capable of moving very large boulders but when it comes to individual storms we have very little data on how many, how far, and how they move?” he said. “This study really adds a great body of evidence to what storm waves can do at the coast.”

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Robert Weiss, a coastal hazards researcher at Virginia Tech who also wasn’t involved, called the study’s dataset “impressive” adding that “it will give us work for the next decade.”

“The way the data was collected should serve as role model for studying the impacts of storms in the future,” Weiss said.

Photo Courtesy Ronadh Cox

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There’s still plenty of work ahead. Cox wants to know how exactly boulders are transported by storm waves—and how waves are amplified along coastlines to move boulders that were already perched over 20 meters uphill. They must be getting amplified, she said, because the largest wave heights recorded during the winter of 2013-2014 were in the range of 15 to 20 meters. She’s working with wave physicists at the University College Dublin to address this question.

And while the 620-ton boulder sets a new record for what storm waves can toss around, Cox doubts it’ll keep the title for long. The storms that hit Ireland four winters ago were strong, but stronger storms have hit these coastlines before.

She has her eyes set on an even bigger boulder on the northwestern shore of Inishmore—a 780 ton beast that’s teetering on a bedrock step. It was clearly dragged to its present location by waves at some point in the past.

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She’s hoping to catch it in the act of moving again.

“I anticipate that if there’s a more intense storm in the future, that boulder will move,” Cox said. “And when it moves, I want to see it.”

[h/t Washington Post]

Update 1/26/18: Watts Up With That ran a blog post yesterday taking Dr. Cox’s quote “Ten years ago, it was possible to say storms can’t move 50 ton boulders” out of context in an apparent attempt to discredit her. A line has been added to the text clarifying her meaning that prior to this study, some researchers had argued only tsunamis, rather than storms, were capable of moving VLBs.

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