Explosive Eruptions and Pyroclastic Flows
Volcanic activity on Rum began with molten rock, or magma, rising up through cracks in the Earth's crust. This molten rock collected in a magma chamber, a few kilometres below the surface of the Earth. As the amount of magma in the chamber increased, the rocks above were pushed upwards, forming a dome over a kilometre high and a few kilometres across. Evidence for the existence of this dome can still be seen on the slopes of the Rum Cuillin, where the layers in the Torridonian rocks are inclined steeply away from the adjacent igneous rocks.
Eventually, the pressure on the domed rocks became too great and they cracked, producing a series of fractures around the dome. The rocks of the dome collapsed downwards, forming a massive roughly circular crater known as a caldera. The walls of this caldera were unstable, and so the floor of the caldera gradually became covered with the debris of rockfalls and landslides. This debris, which consisted largely of blocks and pebbles of Torridonian sandstone and Lewisian gneiss, was gradually compressed to form rocks known as breccias that can be seen in Coire Dubh.
Magma continued to rise up into the magma chamber beneath the caldera, and was eventually erupted onto the Earth's surface. The erupting magma was silica-rich, thick and sticky, and so it did not flow out easily from the volcano; instead it was ejected in explosive eruptions, throwing out hot ash and fragments of volcanic rock that spread out across the caldera floor in searing-hot gas clouds known as pyroclastic flows. Several episodes of explosive volcanic activity occurred, each separated by further collapse of the caldera. The rocks that formed from these pyroclastic flows are called rhyodacites. They can be seen in various places around the margins of the Rum Cuillin, and they are particularly well exposed on the ridge between the summits of Ainshval and Sgurr nan Gillean.