How did a 78-ton boulder travel 17 miles above high water, 130 meters inland? Ronadh Cox, a beloved and energetic professor from the Williams Geosciences department, hit off the second lecture of the Williams Oceans Symposium with a discussion of her “Boulder Ideas.” Since 2008, Cox and her students have been studying how storm waves can move “megagravel,” on cliff tops of the Aran Islands in Ireland.
There are two theories about what could emplace these boulders, known as “clasts.” The first states that only tsunamis could cause this movement, because, according to wave models, storm waves do not have the height or energy necessary to move boulders of the sizes found on the cliff tops of the Aran Islands. However, Cox’s research contests this argument based on research of the direction of clast imbrication (overlapping), trapped debris, and local memory. According to Cox, the movement could only be caused by storm waves.
“Why bother with this study?” Cox knew members of the audience might be asking. If these deposits are formed by storms, then we can better understand storm dynamics and coastal processes. This information will be important as global climate changes cause storms to become more frequent and intense. The research could also help to constrain tsunami models elsewhere. “But from my perspective, it’s just cool. It’s just fun,” added Cox.
How was the research instigated? It actually began with the curiosity of a Williams student who applied for a fellowship to visit and study the geology of the Aran Islands. In 2008, she came back wanting to do a thesis and Cox agreed to advise. Since then, the research has continued and Cox will be taking two more students to the islands this summer.
Cox studies 3 islands with limestone platforms, covered with clasts that are structured and well organized: “these are not random piles of unsorted debris.” Some rocks with mussels or the carbonate structure of coral still attached to them are clearly recent arrivals.
With GPS and Brunton compasses, Cox and her students have constructed topographic profiles of the islands with relation to sea level. With this information, they could show that the rocks are imbricated, or overlapped, in a direction aligned with the mean wave direction. Taking transects and measuring the rocks, they could also prove that they are sorted by size. By studying the age of the rocks, they gained further proof that these rocks have been moved by storm waves in recent years. The most recent tsunami to affect the Aran Islands occurred in 1755, but some of the rocks studied were moved within the last 50 years. Cox noted one rock that moved 3.5 meters vertically and 69 meters horizontally in one year. It was a local resident on his bike who told them that the 78-ton rock mentioned above appeared in 1991 after a large storm event.
But what about those wave equations, which demonstrated that only a wave of tsunami strength could cause rock movement of this degree? Cox argues that wave equations do not model interference or reflections off steep surfaces. Cox’s research demonstrates that storm waves gain short-term intensity at these steep cliff edges. The existing equations do not model wave behavior in steep coastal settings, and are “simply not doing the work that they are being asked to do.” Therefore Cox and her student, Christopher Elliot, will work on improving these storm wave models this summer and the following year for his thesis.
Written by Claire Lafave ’12, CES Research Assistant