Coastal boulders rock’n’roll

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 620-tonne boulder – equivalent in mass to about 90 large African elephants – moved several metres on the island of Inishmore in the winter of 2013-14 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 more than 1000 boulders that moved along Ireland’s coasts during the storms, Cox said.

The researchers detected six boulders weighing more than 100 tonnes 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 the pieces flung into the boulder ridge at 90 metres 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.

The work by Cox and three colleagues at Williams was recently published in Earth Science Reviews. 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 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.

‘‘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.

The real question raised by the work, though, is what it says about the future of climate change. If seas rise and storms worsen with a changing climate then understanding the damage that can be wrought by battering waves will be important, Cox said.

– The Washington Post