Quite simply, the Isle of Rùm is beautiful. From the mainland the silhouetted outline of the Rùm Cuillin ridge tugs at you, willing you to look closer. With a little imagination, and a little bit of geology, I’ll help you do just that.
The Isle of Rùm is what remains of a massive volcanic complex, part of the main series of volcanic activity on what is now the West Coast of Scotland around 60 million years ago, during the formation of the Atlantic Ocean. As the Americas and Greenland ‘unzipped’ and pulled away from Scotland and Ireland, the Earth’s crust began to thin and stretch. This thinning of the crust allowed magma to rise and reach the surface in the form of fissure volcanoes, which stretched the length of Scotland, from North Skye to the Antrim coast of Northern Ireland.
At this point, there was no ocean around the West of Scotland, only mile upon mile of basaltic lava flows from the fissure volcanoes. Imagine a vast, flat plain interspersed with cracks through which the lava spurts, rather like parts of Iceland today. As the lava was basaltic there was little in the way of explosive volcanic activity. Some of the early basalts are preserved in the lava fields on the Isle of Eigg, a few miles from Rùm.
As the heat from the fissures slowly melted the Earth’s crust, an altogether different pattern of volcanic activity emerged. The Earth’s crust contains silica-based rocks, the melting of which produces silica-rich magma. Unlike basalt, silica-rich magma is highly explosive. Around 60.5 million years ago, a bulge appeared in the Earth’s crust where Rùm is now, rising above the surrounding land. The pressure inside the newly-emerging volcano was immense. Eventually, a massive explosion underground caused the caldera to collapse. The explosion was perhaps one magnitude higher than Krakatoa, similar in terms of devastating impact on the landscape to the Minoan eruption at Santorini.
Evidence of this first stage of the Rùm volcanic complex is found in the landscape and rocks around the Rum Cuillin. Surrounding the hills is the central ring complex, around which the Torridonian sandstone bedrock is buckled upwards and altered by the heat from the volcano. A short walk from Kinloch up towards Coire Dubh lays the evidence out perfectly. On the rim of Coire Dubh are the remains of pyroclastic flows – fast-flowing, ground-hugging clouds of gas and broken rocks which travel at speeds of up to 450 miles per hour, flattening everything they touch. Clasts which have been thrown high up in the air can be found embedded in the rocks at Coire Dubh. Elsewhere, on the hillside above the main track from Kinloch to Harris, a fantastic example of mega-breccia can be seen – a rock incorporating huge chunks of gabbro (basaltic rock) up to a metre in diameter thrown up from deep underground by the sheer force of the eruption.
Much of the evidence for stage one, however, was ‘shoved out of the way’ by the rise of a massive magma chamber, the remains of which make up the Rùm Cuillin hills we see today (after extensive and very rapid erosion). The main difference between stage one and stage two is the make-up of the magma – stage one was silica-rich, stage two saw a return to basaltic lavas, creating a far less explosive, though no less massive, shield-volcano.
The Rùm Cuillin are famous for their layered, crystalline appearance. The unusual structure comes from the way in which they were formed inside a slowly cooling magma chamber. Over hundreds of years, the 2nd stage volcano pulsed away, pushing up new injections of magma into the chamber every so often as the material from the previous pulse cooled down. It is thought that the rocks in the Cuillin hills crystallised no more than 2 kilometres below the surface of the Earth’s crust.
In the centre of the island, not far from the main track to Harris from Kinloch, lies a slab of troctolite rock from the centre of the magma chamber. Displaced and tilted from its original horizontal settling position, it holds the intriguing imprint of a piece of softer peridotite rock which has dented the crystal layers surrounding it. The likely explanation of this anomaly is that the peridotite fell from the upper part of the magma chamber whilst the troctolite was still a semi-molten ‘crystal slush’ in the bottom of the chamber, disturbing the layering and preserving the indentation from the peridotite falling into it. The layers above are unmarked, suggesting they settled and solidified after the block fell.
Once the 2nd stage volcanic activity had died away, the complex was subjected to one final ‘last gasp’ from the Loch Long fault, which runs South West to North East across the island. The final movements of the fault shunted everything north of the fault northeastwards, splitting the volcanic evidence and disrupting the geology of the island. Other minor faults move elsewhere as a result, so in some places dykes and cone sheets of injected material from the volcanoes are also interrupted.
Two million years after the Rùm complex, the huge basalt volcanoes on Skye erupted in the last stage of volcanism in the West of Scotland. Bloodstone Hill, to the north of Rùm, is a basaltic lava flow from the Skye volcanoes. Lying between the Bloodstone lavas are layers of sedimentary conglomerate, containing boulders of gabbro and peridotite from the Rùm central complex, showing that the magma chamber was already exposed and eroding away rapidly by this point. Bloodstone itself, from which the hill gets its name, is a chemical precipitate flecked with red iron oxide which formed in bubbles in the basaltic lava flows. These beautiful stones appear at Guirdil Bay, beneath Bloodstone Hill, in every shade of green imaginable.
For me, the Isle of Rùm has a beauty, intricacy and power like few other places in Scotland. Once you have its geological story at your fingertips, it only adds to the pull of the place, and every visit leaves you wanting more.
All text and photographs copyright L. Reid 2015