Powerful storm waves moved this boulder, researchers say

A boulder - equivalent in mass to about 90 African elephants - moved several metres on Inishmore in the winter of 2013-2014 after being slammed by storm waves. Photo / Washington Post video

Along the rocky coasts of Ireland's Aran Islands, scientists have found a new demonstration of the immense power of ocean waves - and a possible hint of what we should fear if climate change makes coastal storms more intense.

A 630-tonne boulder - equivalent in mass to about 90 large African elephants - moved several metres on the island of Inishmore in the winter of 2013-2014 after being slammed by powerful coastal storm waves, according to the research led by Ronadh Cox, a geoscientist at Williams College.

It was just one of over 1000 boulders that moved along Ireland's coasts during the storms, many of them quite large, Cox said. The researchers detected six boulders weighing more than 100 tons and 18 weighing more than 50 tonnes that had been displaced by waves.

"We had boulders that were north of 100 tonnes, sitting tens of metres above sea level and tens of metres inland of the high tide mark, that got moved several metres, or several tens of metres," 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 90m inland and 50m above sea level."

The discovery could help reset our assumptions about how to interpret a variety of mysterious rocks 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 Eleuthrea. The 940-tonne and 390-tonne 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, and it's hard to know how this debate over ancient events can be fully resolved.

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 in 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 thus emerges 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 some 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.

We're still trying to really measure the upper limits of what's possible

Ronadh Cox

Further inland on the island of Inishmore there are 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 kilometre 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-tonne 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 even get hurricanes. The rocks in the Aran Islands were moved by intense winter cyclones, which don't feature as strong wind speeds as hurricanes but can generate very large storm waves.

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."

More fundamentally, the work also gives us a picture of the raw power of the ocean, which scientists are only now beginning to measure. "We're still trying to really measure the upper limits of what's possible," said Cox.