Storm Waves With the Power to Heave Massive Boulders Over Cliffs

Storm Waves With the Power to Heave Massive Boulders Over Cliffs


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A new study suggests that powerful storm surges are responsible for pushing giant boulders to the tops of cliffs in western Ireland.

Credit
Paul D. Ryan

On a flat peninsula in western Ireland bordered by shallow cliffs that rise from the Atlantic Ocean sits a field of boulders. Some weigh nearly four times more than a school bus. Now scientists have figured out how these boulders reached their high perches.

A study published Monday in the Proceedings of the National Academy of Sciences suggests that powerful storm surges swept the boulders inward. The findings might help scientists better understand the dangers of coastal storms, which climate scientists predict are bound to increase in our warming world.

Scientists have long debated whether these mighty boulders were deposited on the Irish coast by violent storm waves or powerful tsunamis.

To settle the question, John Dewey, a retired professor of geology at Oxford University, and Paul Ryan, a retired geologist at the National University of Ireland, dug into historic records, oceanographic data and field measurements. Both lighthouse records and measurements from offshore buoys point toward a landscape that is commonly ravaged by large storm waves.

And some of those waves can reach enormous heights. Take a storm in 1861, which sent waves crashing over an approximately 220-feet-tall lighthouse near the boulder site. The waves broke the glass and flooded the lighthouse. Its keepers, unable to push the door open against the water, had to drill a hole in the door to release the buckets of water before they could assess the damage.

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A panoramic view of boulders at Annagh Head, on Ireland’s west coast.

Credit
Paul D. Ryan

“Think of the power of a wave crashing on the shore that high,” Dr. Dewey says. A single cubic meter of water weighs roughly a metric ton, or about 2,200 pounds, which is roughly the weight of a giraffe. “If you’re throwing a wall of water, say 30 or 40 meters high over a large area, the volume of water is enormous and the crushing force is tremendous,” Dr. Dewey says.

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