The Isle of Rum is an important destination for geologists due to the nature of the rock formations and the scale of the processes that occured here.
Rum can be split into three main geological regions: the Torridonian sandstones of the north and east, the western granites and the central igneous zone of the Rum Cuillin range. 60 million years ago, magma began to accumulate under the British Isles welling up through faults, many of which are familiar to us today: the Highland Boundary Fault and the Great Glen Fault, for example. The Camasunary Fault is an area of deep water between Rum and mainland Scotland.
On Rum, magma domed up along the Main Ring Fault, raising the earth’s surface by some 2,000 metres. After the release of the pressure of the magma the huge dome collapsed forming a massive caldera over the southern half of Rum. The magma pushed up Lewisian Gneiss as it rose from beneath the Torridonian sandstone. This gneiss is some of the oldest rock in the world at nearly 3 billion years old. The caldera was slowly filled in by further eruptions and collapses of the surrounding walls, forming the layered structures of Hallival and Askival. When the eruptions eventually stopped the Rum volcano was subjected to erosion and repeated periods of glaciation, sculpting the island into its current shape.
Swedish researchers have revealed that Rum may once have been a supervolcano two or three kilometres high. Sixty million years of erosion by glaciers, wind and rain have reduced the mountain to what remains of Rum’s cuillin ridge today.
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