The massive boulders deposited at Annagh Head in Mayo, and possibly some other locations along the northwest coastline of Ireland were not caused by a tsunami, but by storm waves breaking on the foreshore for hundreds of years.

That’s according to research by Professor Paul Ryan from Earth and Ocean Sciences in the School of Natural Sciences at NUI Galway and the University of Oxford published in the Proceedings of the National Academy of Sciences.

“This study shows the enormous power of storm waves battering the foreshore over centuries, ripping boulders of over 50 tonnes from the cliff face, piling them 100 metres or more inland,” said Professor Ryan.

The researchers found that the huge boulders, some over 50 tonnes, which are piled at the top of a small cliff, got there due to battering from storm waves.

It had been speculated that many of the larger boulders along the west coast of Ireland had been uplifted by tsunamis, but in 2004, the late Professor Michael Williams argued that the boulders on the Atlantic cliffs of the Aran Islands were due to storm waves, not tsunamis. This thesis caused caused considerable international debate at the time.

The researchers here set out to resolve the debate as to whether the large boulders had been moved by tsunamis or storm waves.  They used computer simulations, hydrodynamic equations, as well as oceanographic, historical, and field data. These found that the boulders are a cliff-top storm deposit.

Northeast Atlantic storms can produce waves of over 60 metres, which are capable of lifting massive boulders. This knowledge is important in the context of climate change, the researchers said.

Shorelines are becoming more vulnerable and the ability to understand these piles of boulders along the west coasts will help us understand how much more vulnerable we actually are to storms, the researchers said.