Shorelines in Ireland and New Zealand host great boulders whose origins have puzzled geologists for some time. A study of these boulders has concluded they come from the sea, with the ones in New Zealand likely put there by a powerful tsunami, while the Irish equivalents could have a similar source or represent the product of frighteningly destructive storms.
Some of the boulders of Annagh Head, Ireland, weigh more than 50 tonnes (55 tons). These sit well inland from the oceans, yet their composition indicate they have been moved there, rather than forming locally. On the other side of the world, New Zealand’s Lower Miocene Matheson Formation contains even more spectacular specimens, some exceeding 140 tonnes (154 tons), and similarly of marine origin. What forces could shift heavy objects so far out of the water?
Geologist Professor John Dewey of Oxford University and Professor Paul Ryan of the National University of Ireland noted that the Annagh Head boulders lie 2-5 meters (6-16 feet) above the high tide line in a shallow trough between rocky headlands. The heaviest of these sits 160 meters (525 feet) from the shore, with smaller rocks deposited further inland.
Dewey and Ryan modeled the rock-moving capacities of 30-meter-high (100-foot-high) storm waves and 7.5-meters (25-foot) tsunamis in a shoreline shaped like Annagh. While a storm wave of this size would struggle to deposit the heaviest boulders quite so far inland, a storm wave in 1861 reached above the top of a 67-meter (220-foot) lighthouse a little north of Annagh Head. Consequently, they conclude in Proceedings of the National Academy of Sciences, it is not necessary to resort to tsunamis off the seismically inactive Irish coast to explain the spread of rocks.
Dewey and Ryan note that even smaller storms are responsible for some of the reshaping of the coastline. Returning to Annagh after big storms in 2013, 2014, and September this year, the researchers found newly deposited boulders, although these weighed just a tenth of the largest ones.
New Zealand, on the other hand, has an abundance of off-shore earthquakes, so it is not surprising a series of tsunamis an hour apart are a likely explanation for the boulders seen along the eastern shore north of Aukland. The forces were so powerful, they not only deposited large rocks far inland of the shoreline of the era, but crushed them against each other, leaving debris. Dewey and Ryan think the tsunami pushed its way 5 kilometers (3 miles) inland, scattering rocks across an area greater than 200 square kilometers (80 square miles). The consequences of something similar today are terrifying to imagine, but there is some comfort in the event having been up to 20 million years ago.