The Rocks of Acton’s Beach
The rocks along the coast near Clifden ecoBeach have been visited by generations of geologists because they provide an outstanding opportunity to see what happens deep inside the Earth’s crust when magma rises into it and volcanoes erupt at the surface.
The mountains of Connemara are mostly composed of old metamorphic rocks, including quartzite, schists and marble. These were originally deposited as sediments late in Precambrian time, before the first animals with shells evolved, and were sands, muds and limestones. Around 470 million years ago, during the Ordovician period, these rocks were caught up between two ancient crustal plates as they came together. They were buried under perhaps 15 kilometres of younger rocks, heated to temperatures of around 600 degrees Celsius and became deformed and folded as the plates moved together. The originally muddy rocks grew minerals such as micas and garnet and you can see spectacular examples of folds formed at this time in the rocks at the north end of the beach. At the end of this period of collision between plates, the deformation of the crust ended and the rocks were uplifted to form mountains which began to be eroded.
This final stage in the plate movements triggered renewed melting in the deep crust and around 400 million years ago, granite magmas rose up and were intruded into the metamorphic rocks of Connemara. Omey Island, just across the bay, is made of one such granite body, and the granite also extends onto the mainland at Claddaghduff. The contact between the massive pink granite and the flaggy grey metamorphic rocks lies about 400 metres north of Acton’s beach. 400 million years ago, the rocks of the beach would have been 10 kilometres below the surface and the granite of Omey Island would have been a hot molten magma, not solid rock.
Of course the older metamorphic rocks near the granite were heated up again by the magma, and new minerals grew in them as a result. All the way along the coast line from the Acton’s Beach to the granite contact it is possible to find evidence of this younger metamorphism, and the rocks form part of a metamorphic aureole around the granite. Some slabs of brown rock contain long, narrow crystals (up to a few centimetres in length) of a hard, dark mineral that weathers up from the surrounding rock. This is a mineral called andalusite; it clearly formed long after the folds because the crystals are randomly oriented in the rock. At both ends of the beach, as well as nearer the granite contact to the north, marble layers in the schist have reacted with hot, acid water coming off the granite magma as it solidified. The result is layers of coarse-grained rocks, known as skarn. Most commonly, the skarn layers are pink, formed of garnet, but other minerals can be present including white, fibrous wollastonite, green diopside and dark brown idocrase.
These outcrops are some of the best examples seen anywhere in Europe and are a listed site. They are only preserved for you to see because for the past 50 years visiting geologists have agreed to leave them untouched. If you want to collect specimens for souvenirs, pick up loose blocks from the beach; please never take a hammer to any of the outcrops.
Professor Bruce Yardley
School of Earth and Environment
University of Leeds, UK