Weight lifting: Storm waves have the power to move huge boulders

© Gareth Mccormack/Photodisc/Getty

Detailed modelling of geological deposits in Ireland and New Zealand confirms that storm waves can transport massive boulders 

While tsunamis are recognized for their sudden, destructive power and their ability to reshape landscapes by shifting tonnes of rock, the power of huge waves that batter coastlines during storms is perhaps less well recognized. 

John Dewey at University College, Oxford, and Paul Ryan at the National University of Ireland in Galway, have shown that the storm waves that hit the Atlantic coast of Ireland every year have the capacity to lift enormous boulders from nearby cliffs.

Their study focuses on two boulder, or ‘megaclast’, deposits — one at Annagh Head on the present-day west coast of Ireland, thought to be deposited within recent decades, and the other at Matheson Bay, New Zealand, which dates to between 15 and 23 million years ago. Each location contains individual boulders more than 50 tonnes and 4.1 metres. While the New Zealand location is confirmed as a tsunami deposit, experts were undecided as to the origins of the Irish boulders. 

“There has been considerable debate in our field as to what extent storm waves can create megaclastic deposits; often, huge boulders on cliff tops or storm beaches are automatically attributed to tsunamis,” says Ryan. “We integrated geological, geomorphological, oceanographic and historical data with numerical modelling to justify our conclusion that the Irish boulder fields were created and deposited by storm waves.”

The researchers had a wealth of oceanographic data from the Atlantic to draw on. Their numerical models prove that storm waves, which have been recorded at heights of over 60 metres off western Ireland and can move boulders as heavy as those moved by tsunamis. 

“The key to distinguishing storm boulders from tsunami boulders is to look closely at the deposit within or upon which they lie,” says Ryan. “Storm boulders are commonly localized on low-lying headlands exposed to oceanic swells.  They are well sorted and require a local source of slab-like blocks, such as cliffs — indeed, we identified pluck-sites on the cliffs near Annagh Head. Tsunami deposits, on the other hand, are extensive, chaotic, affect entire coastlines and may contain non-local material.”

The team also highlights the extensive amounts of grit and shells within the New Zealand deposits, indicative of rapid deposition from a soup-like wave carrying vast amounts of suspended sediments. 

“Developing realistic models for storm wave damage is essential, particularly as society continues to exploit shorelines for tourism, communications and energy production,” says Ryan. “We will develop tools for remote sensing of megaclastic deposits to assess storm wave transport capacity and damage potential.”

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  1. PNAS, E10639-E10647 (2017). doi: 10.1073/pnas.1713233114