“We had boulders that were north of 100 tons, sitting tens of meters above sea level and tens of meters inland of the high tide mark, that got moved several meters, or several tens of meters,” said Cox. “There were boulders that were created from bedrock that were ripped up and disaggregated and the pieces flung into the boulder ridge at 90 meters inland and 15 meters above sea level.”
The discovery could reset our assumptions about how to explain the mysterious boulders found littered along shorelines around the world, which scientists have traditionally linked to powerful tsunami waves. Most prominent among those are a pair of boulders on the Bahamian island of Eleuthera. The 925-ton and 383-ton rocks sit on a coastal ridge well above sea level. They’ve been cited as evidence of extreme superstorms occurring more than 100,000 years ago, the last time the planet was as warm as where we’re currently headed, and thus interpreted as a lesson about the dangers of climate change. But scientific counterarguments suggest the rocks could have instead been moved by some type of nearby tsunami.
The work by Cox and three colleagues at Williams that was recently published in Earth Science Reviews is more certain. That’s because Cox’s team knows precisely where these rocks were before they moved and what happened in the area during the brief period between then and now — there were no tsunamis. The scientists had surveyed and photographed rocks up and down the coast of the Aran Islands and other parts of the Irish west coast. So when something moved, they could prove it.
What emerges from the research is a landscape that is constantly being transformed and rearranged by waves. The Atlantic-facing coast of the Aran Islands lies up against very deep ocean waters. The islands feature steep cliffs, as well as gradually sloping rocky shores. Major rock movement was detected in both areas, although clearly it was easier for the ocean to move rocks at lower altitudes.
Further inland on the island of Inishmore there are also extensive rock landscapes where boulders and smaller stones pile up in what appears to be a field of debris created by the ocean. “The waves are just surging up these coastal platforms and doing enormous amounts of work at a quarter of a kilometer inland, basically,” Cox said.
Before now, the biggest boulder documented as having been displaced by a storm, rather than by a tsunami, was a 180-ton rock moved by the super intense Typhoon Haiyan on the Philippine island of Samar. But the Aran Islands boulders greatly outweigh that — and Ireland doesn’t get hurricanes. The rocks in the Aran Islands were moved by winter cyclones, which don’t feature wind speeds as strong as hurricanes but can generate very large storm waves.
The real question raised by the work is what it says about the future of climate change. If seas rise and storms worsen with a changing climate — as some predict — then understanding the damage that can be wrought by battering waves will be important, Cox said.
“The more intense the storms, the higher the wave energies, so the likelihood of these stronger wave energies hitting in other places becomes greater, as well,” she said. “So I think understanding the dynamics of these deposits in these remote areas is going to matter more as energy levels on coastlines around the world tend to increase.”
“Their study demonstrates that the wave energy developed during storms against cliffs is very large, probably larger than we commonly think,” said geoscientist Alessio Rovere of the University of Bremen, who has studied the rocks in the Bahamas but was not part of the current study. “Studying how storm wave energy will change on different types of cliffs and under different scenarios of sea level rise is a very intriguing question that will surely keep coastal scientists busy for years to come.”
More fundamentally, the work also gives us a picture of the raw power of the ocean, which scientists are only now beginning to document. “We’re still trying to really measure the upper limits of what’s possible,” said Cox.
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