Careful mapping, anecdotal evidence and radiocarbon-dated clams tell the true story
By Summit Voice
SUMMIT COUNTY — Seemingly made of sheer rock, the Aran Islands, off the coast of Ireland, look like they were built to withstand the stormy waters of the North Atlantic forever.
But new research suggests the sea has enough force to push ridges of giant boulders — some weighing as much as 78 tons — far inland from the edge of the imposing cliffs.
Many geologists had long speculated that only a tsunami could have enough force to rearrange the massive slabs of limestone. Armed with equations that model the forces generated by waves, some researchers concluded that no ordinary ocean waves could muster the force necessary to move the largest of the boulders this high above the ocean surface and so far inland. The math suggests the rocks in the ridges could only have been put there by a tsunami.
But the new study finds that plain old ocean waves, with the help of some strong storms, did the job.
The mathematical equations tell one story. The islands’ residents tell another. According to some locals, enormous rocks have moved in their lifetimes, despite the fact that there hasn’t been a tsunami to hit the islands since 1755.
“Unless you have little green men from mars doing this on the quiet, it must be storm waves,” said Dr. Rónadh Cox, who led the research with her Williams College students.
While the anecdotes from residents are interesting, Cox and her team went in search of more concrete evidence. The clincher came when the team compared modern high-altitude photos of the coastline to set of meticulous maps surveyed in 1839.
The 19th century surveyors, who Cox describes as “possibly the most anal men on the planet,” carefully mapped not only the boulder ridges, but all of the criss-crossing stone walls that farmers built between fields. The researchers digitized the maps and overlaid them on the modern images, using the walls to line the two up accurately.
“Not only did they map every wall, they did it right. The maps aren’t even off by even a meter.”
The overlay of the new photos with the old maps shows definitively that sections of the ridges have moved substantially since 1839—nearly 100 years after the most recent tsunami. Some sections moved inland at an average rate of nearly 10 feet per decade. In some places, the ridge had run over and demolished field walls noted on the old maps.
Other lines of evidence corroborate residents’ accounts of recent movement. When the boulders were ripped from the bedrock, tiny clams that live in cracks and crevices sometimes came along for the ride. Using radiocarbon dating, Cox and her team found that some of the rocks have been pulled from the coastline within the last 60 years. What’s more, the researchers have been photographing sections of the ridge during each field season since 2006, and they’ve documented movement from year to year.
So what of the equations that point to tsunami as the only possible earth mover?
“We’ve eliminated tsunami and I think we can rule out little green men,” Cox said. “What that says is our equations aren’t good enough.”
Cox thinks the characteristics of the Aran Island shoreline are throwing off the calculations. The Aran cliffs rise nearly vertically out of the Atlantic, leaving very deep water close to the shore. As waves slam into the sheer cliff, that water is abruptly deflected back out toward the oncoming waves. This backflow may amplify subsequent waves. The result is an occasional storm wave that is much larger than one would expect.
“In this kind of environment these would be less rare,” Cox said. “You only need a couple of them to move these rocks around. The radiocarbon data show that not only are some boulders moving in recent years, but also that some of them have been in the ridges for hundreds and even a couple of thousand years. Accumulated activity of rare large-wave events over that time could certainly build these structures”
Cox plans to add a physicist to her research team in the near future to try to shed some light on the wave dynamics on the islands, but it’s clear from the evidence the team has already gathered that storm waves can do more than some researchers thought.
Following the devastating Indonesian tsunami in 2004, there has been renewed interest in learning about how a tsunami can change the landscape. Cox’s findings have important implications for that research.
“There’s a tendency to attribute the movement of large objects to tsunami,” she said. “We’re saying hold the phone. Big boulders are getting moved by storm waves.”