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Scientists Discover Tsunami Not the Cause of Boulderite Deposits in the Northwest of Ireland
NUI Galway and University of Oxford study proves centuries-old giant boulder deposits in the Northwest of Ireland were caused by high Atlantic storm waves

Professor Paul Ryan from NUI Galway and Professor John Dewey from University of Oxford have carried out research that proves the spectacular boulder deposits of Annagh Head in County Mayo were caused not by an unknown tsunami but by Atlantic storm waves of up to 30 metres breaking against the shore for hundreds of years. The findings were published this week in the leading journal, Proceedings of the National Academy of Sciences.

4th Dec em ber 2017

Enormous boulders, some over 50 tonnes, piled on the foreshore or at the top of a small cliff in a deposit called a boulderite are evidence of the power of extreme waves. Tsunamis are known to have massive power and most such deposits, including those along the Wild Atlantic Way, are believed to be 'tsunamites'. However, in 2004, the late Professor D Michael Williams of NUI Galway argued that the boulders on the Atlantic cliffs of the Arran Islands were due to storm waves, which caused considerable international debate at the time.

View of Annagh Head, Belmullet, Co Mayo. (Below): Lower Miocene tsunami boulderite with clasts up to 140 tonnes at Matheson Bay, North Island, New Zealand. Photos: Professor John Dewey, University of Oxford

In an attempt to resolve this controversy Professors Dewey and Ryan compared two deposits: a tsunamite from the Miocene of New Zealand and a present-day boulderite at Annagh Head in County Mayo. Field data shows that in the North Island of New Zealand a 10 million year boulderite which contains boulders in excess of 140 tonnes, the Matheson Formation Bay, was produced by a 12-13 metre-high tsunami within a period of about one hour. The origin of the boulders at Annagh Head, which exceed 50 tonnes, is disputed.

The researchers combined oceanographic, historical, and field data to argue that this is a cliff-top storm deposit. A computer simulation of a cliff-top storm deposit was developed, which shows that boulder shape in addition to density and dimensions should be taken into account when applying hydrodynamic equations to such deposits. The model also predicted that Northeast Atlantic storms, which historically have produced waves of over 60 metres, are capable of producing boulderites that cannot be distinguished from tsunamites when size alone is considered. Comparing and contrasting these two deposits helps indentify the origins of boulderites. Climate change means our shorelines are becoming more vulnerable and the ability to read these piles of boulders will help us understand how much more vulnerable.

Co-author of the study, Professor Paul Ryan from Earth and Ocean Sciences in the School of Natural Sciences at NUI Galway, said: "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."

Professor John Dewey from University of Oxford and co-author of the study, said: "The triple junction between land, sea and air is perhaps the least well understood in the Earth Sciences. We should pay greater attention to our shores."

December 2017 Issue - Vol 30 No.07

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