It can be very wet and windy on the Aran Islands, a group of three rocky islands at the mouth of Galway Bay off Ireland’s west coast, so Prof. Rónadh Cox and her team were kitted out in waterproofs and warm fleece. They were on their way to study the islands’ unusual boulders, which occur on coastal platforms and cliffs next to the wild Atlantic ocean.
Rónadh, Professor of Geology and Mineraology at Williams College in Massachusetts, recalled the trip, which she took with a group of her undergraduate students in the summer of 2014:
The Aran Islands are a beautiful place to work. The challenge is the weather: you have to deal with lots of rain and wind. I have many pictures of soggy students trying to manipulate equipment, with wind blowing things everywhere.
Armed with waterproof Rite in the Rain all-weather paper, they were investigating whether huge waves in the winter storms of 2013-14 had shifted any of the boulders.
Waves move rocks all the time, but these are no ordinary rocks. Many weigh hundreds of metric tons –so large that some scientists had argued that storms couldn’t possibly move them. But by photographing boulder locations after the storms, and comparing them to pictures taken in previous years, Rónadh’s team showed that the storm waves had shifted boulders as big as 620 metric tons – about the same weight as four houses – or 100 elephants.
In total, Rónadh and her students documented displacement of more than a thousand boulders, with photographs and measurements showing their dimensions and masses as well as where they started from and ended up. The two biggest– approximately 620 metric tons and 475 metric tons – each skidded about 4 meters during the storms. The waves also moved smaller rocks (but still weighing tons or even tens of tons) at elevations up to 26 meters, and as much as 222 meters inland of high tide.
Watch a video about the boulders
This video was made by one of Prof. Rónadh’s students:
It’s one thing to show movement of these giant boulders but another to understand how it happened – how storm waves shift such huge, heavy objects, some on high cliffs and others at long distances inland. To help solve this problem, geologist Rónadh has enlisted the expertise of theoretical modelers.
Crossing disciplinary boundaries
The movement of the largest boulders was a surprise; until Rónadh’s work, nobody had documented storm-wave transport of such enormous blocks. She took the data to Prof. Frederic Dias, an applied mathematician and modeler at University College Dublin who works on extreme waves.
Rónadh describes her initial connection with Prof. Dias as “serendipitous.” She had read a paper on wave amplification, written by his research team, showing that under certain circumstances, coastal waves could become much bigger than predicted by simple linear physics. She met him in person when visiting UCD to give a talk on her findings. That initial chat set them on the path to a successful multi-year, multi-institution collaboration, with students and colleagues — geologists, mathematicians, physicists and engineer s— working on different aspects of the research.
This project has brought together field observationalists and theoretical modelers, which makes for great synergy. Being exposed to this wide range of expertise helps me understand the boulders, and what's happening with the boulders helps my colleagues think about the complexities and the chaos of what happens as extreme waves come ashore. There are all kinds of really interesting new ways of thinking going on here.
This collaboration culminated in an international conference in late 2018, hosted by the Fondation des Treilles in France. Half the attendees were observational scientists and the other half were modelers. These different types of professionals do not usually interact much, so the gathering generated many interesting conversations and new ideas.
It's not that scientists from different disciplines aren't willing to talk to each other, but we all tend to exist in our own specialty bubbles, so you have to create forums to bring people together and get them talking. We've set up a group called xWaves, which we hope will promote broader thinking about extreme waves and how they interact with the coast.
Learning a new language
The xWaves initiative harnesses the power of intellectual discomfort, exposing its participants to the excitement and challenge of other people’s expertise. It can provide new perspectives and a fresh way of learning. As Rónadh discovered:
Being in a room with modelers, I’m learning a lot – I understand some things I didn't understand before, and I'm learning to adopt more mathematical ways of thinking. What’s fascinating is that it’s really a two-way street, because the modelers sometimes have difficulty with things that field geologists are very comfortable with – uncertainty and weirdness and outliers and irregularity. Everybody learns.
The boulders typify this intersection: a modeler will characterize a boulder as a rectilinear solid with uniform properties and precise dimensions, whereas a geologist is likely to drag a tape measure across its lumpy irregular surface and be happy if her mass estimate is within 10 percent of the boulder’s actual size. Coastlines aren’t straight, and rocks are difficult to measure; the inherent uncertainty arising from that complexity is fundamental to geoscience. Rónadh found it interesting to find that mathematicians can be confounded by the ease with which field scientists deal with what she calls the “squishiness” of the natural world. There are benefits for everyone, as she explained:
The more that modelers confront natural complexity, the more realistic the physical models they build. And it’s super useful for field scientists to think quantitatively about what exactly is going on with, for example, the coefficient of friction, or how different bedrock is going to affect drag. Everybody's learning a different language. Bridging the gaps between disciplines creates real richness, this synthetic understanding of the system. It really is where the land meets the sea, where the geologists are meeting the physicists.
The impact of technology
New technologies help by providing more precise and complete measurement sets. Since 2015, Ronadh’s team has been using drone-based photogrammetry to map the boulders, making the field work easier and the analysis more complete. Each drone photo captures a large section of coast; the team can map a kilometre in half an hour and can capture high-resolution, centimeter-per-pixel imagery of the entire Atlantic coastline of the Aran Islands in three days – if the weather is good, that is:
Instead of going out with rulers and tape measurers, laser rangefinders and clinometers, we're now standing around watching the drone while it does all the work. It's more efficient, it's quicker, and we get better data. But the irony is that the drone is less resilient than a field geologist. It’s picky about weather. Too much wind is a problem, and even a little rain gets water on the lens, so you can't fly it.
It's funny that we have this technology that makes the work much easier, but places a bigger restriction on when we can go out.
The drone takes overlapping photographs — hundreds of them — by traversing back and forth along the coast, with 80 percent overlap between each successive photograph and 60 percent overlap between the traverses. The photos are fed into software that uses their different perspectives to create a three-dimensional model of the terrain — much as our brains use the different perspectives seen by each of our eyes to generate depth perception.
Three-D photogrammetry is awesome. It means that we can make digital elevation models, we can look at the deposits in detail, and then – this where it gets ridiculously cool – we can also use software to overlay models from different years, and it will quantitatively compare them and detect whether individual boulders have moved around. We can make precise volumetric measurements of the effects of storms in successive years. That’s just amazing.
Drone data are revolutionizing boulder mapping, but understanding how waves move the boulders requires a different approach. As part of the multi-institutional collaboration, Rónadh’s students have worked with civil engineers at Queen’s University in Belfast to create a scale model of wave-boulder interactions in a wave tank. High-tech computer-controlled paddles allow researchers to reproduce realistic ocean waves, including authentic storm conditions. Precise scaling means that the physics in the tank will match the physics in the real world. (This is not easily done; if you’ve ever seen a really old movie with ships tossing in a storm, you probably noticed how unrealistic those scenes look. That’s because the forces aren’t properly scaled when you put toy boats in a bathtub – you can’t just make things smaller and expect the physics to be the same.)
Their results show that only a few waves in a storm will have the power to move the biggest rocks. And, surprisingly, it is not the biggest waves that do the most work. Rónadh’s team are beginning to quantify the “goldilocks conditions” that dictate when a wave will come over a cliff edge with just the right approach to unleash a flow that can move a 600-metric-ton boulder.
The project continues to evolve, and our knowledge of storm waves and their impact on the coastline grows with it, Rónadh said:
We’re really trying to get at the hydrodynamics: the relationships between the boulders and the flows that the waves produce, and how the waves get to be the size and shape that they are. We’re constantly learning, across disciplines and with new technologies. I’m thrilled about the collaborations that have developed, and I’m excited about what we’ll discover in the future by working together.
All life paths have unexpected twists. Prof. Rónadh Cox went to university planning to be a biologist. She took a geology course as an elective in her first year and three weeks in, “biology was out the window, I was hooked.” She read about the Aran Islands boulders originally in a newspaper article. She found the original research paper and shared it with students in her classes. One student, Danielle Zentner (in the main photograph) was fascinated by the paper. She wanted to research the rocks herself and asked Rónadh to supervise her project. Rónadh agreed and once again was hooked.
Danielle became the first of many students to work on the Aran Islands with Rónadh. “It’s a great place to take students, and the measurements were easy to make, so I just kept doing it and doing it and doing it,” Rónadh said. “We hit the jackpot with those storms in 2013, and that's when everything took off.”
Read her research article
Rónadh and her team’s paper on the boulder movements was published open access in Elsevier’s journal Earth-Science Reviews: “Extraordinary boulder transport by storm waves (west of Ireland, winter 2013–2014), and criteria for analysing coastal boulder deposits.”
More about Rónadh Cox
Prof. Rónadh Cox has been a geologist for about 40 years. She grew up in Ireland and got her BSc from University College Dublin. She has a PhD from Stanford University and spent time first as a post-doctoral fellow at Rand Afrikaans University in Johannesburg, and subsequently as a Visiting Assistant Professor at the University of Illinois, before taking a position at Williams College in Massachusetts in 1996. She now holds an endowed chair in the Williams Geosciences Department as the Edward Brust Professor of Geology and Mineralogy.
Rónadh is grateful that the world is made of rocks and that you can study them anywhere. Tweet her @ronadh_cox.
Peter Cox of Peter Cox Photography launches his drone to map the boulder deposits. (Photo by Rónadh Cox)