The mystery of the moving boulders

Aran Island boulders

Ridges of giant boulders along the Aran Islands’ wild Atlantic coastlines have puzzled geologists for years. All are far from the waterline, either on cliff tops or at the back of wide coastal platforms.  Waves quarried these rocks and drove them inland, but how this happens is not well known.  Answering questions about how storm waves are amplified along steep coasts to do this kind of work requires collaborations among marine and terrestrial scientists. The results will be important for a fuller understanding of the damage that can be done by coastal storm. 
As sea level rises, so does the threat to coastlines, populations, and infrastructure. Rocky coasts are especially understudied. But recent studies show they are surprisingly sensitive to high-energy events, and need to be better integrated into coastal management and environmental response planning. Predicting the forces likely to act on infrastructure requires understanding extreme wave behaviour in the coastal zone. Surprisingly, we lack data on how big waves can get at the coast, and the masses they can move.  For this reason, it has not yet been possible to fully model or predict effects of storm waves on coasts.

Using a baseline dataset documenting >1000 boulders in western Ireland that moved during the 2013-2014 storms—including nineteen in the range 50-500t—research led by Professor Rónadh Cox is working towards a unified model, integrating field, numerical, and experimental methods, directly linking waves of known magnitude to specific physical work. This international project is sponsored by the United States National Science Foundation, Invest NI/DEL, and Science Foundation Ireland.  

boulders and slabs offshore Aran Island
Boulders and slabs offshore Aran Island, County Galway


Bathymetric mapping carried out by the INFOMAR programme is critical to this work. First, the high resolution offshore data are being used to produce three-dimensional terrestrial-to-marine topography for the Aran Islands. This allows the researchers to link boulder movements to offshore topography as well as coastal geometry. Secondly, the data forms a basis for numerical modelling of non-linear wave dynamics over variable bathymetry, which shows, in a realistic and repeatable way, the specifics of how waves are amplified as they approach the coast over irregular bottom topography.  Finally, wave-tank experiments that  model the Aran Islands, using the bathymetric data to create the representation of offshore depth patterns, test the relationships between topography, wave amplification, and boulder masses moved. The project aims to produce a quantified, multivariable model combining wave forces, geomorphology, and work done onshore (represented by boulder movements). Research outputs will inform coastal engineers, marine spatial planners, and other groups developing renewable marine energy installations.

High resolution rendering of cracks and carsic erosion
High resolution rendering of cracks and carsic erosion


You can read more about the research outputs of this project here.