From Molten Rock to Manx Shearwaters – The Rum Cuillin
It is immediately apparent to anyone looking at the peaks of Askival and Hallival that these are formed of an unusual type of igneous rock, because the mountainsides are characterised by a distinct layering structure. Pale coloured layers of very hard rock, standing out as distinct cliffs a few metres high, are separated by layers of brown, much more crumbly rock on which grassy slopes have formed. This layering formed as the magmas cooled and crystallised in the magma chamber.
As magma cools, crystals gradually form within it – like ice crystals beginning to form in water as it freezes – until the magma has solidified into rock. In the Rum magma chamber, the first crystals that formed were composed of a type of magnesium and iron-rich mineral called olivine. These crystals were denser than the hot magma and so they sank towards the base of the magma chamber, accumulating as a distinct layer. These are the brown crumbly layers, which are made of a rock called peridotite.
As the magma continued to cool, different minerals began to crystallise, and this led to the formation of the paler, harder layers. These layers are made of gabbro, a rock that here consists chiefly of olivine together with white crystals of feldspar. Then, a new batch of hot magma would be introduced into the magma chamber, and the next set of layers would begin to form.
The presence of this igneous layering has led to the presence of one of Rum’s most well-known features – the breeding colony of Manx shearwaters, one of the largest such colonies in the world. The shearwaters burrow into the soft, crumbly peridotite layers on the peaks of Askival, Hallival and Trollaval.
As well as the large-scale layers, small-scale layering can be seen in the rocks when they are examined closely. In places thin dark layers of a mineral called chromite can be found within the rocks, and some of these layers contain minerals rich in the platinum group of elements.
On Askival and Hallival, the igneous layering is extremely regular, and has been studied by many geologists as a type example of this phenomenon. However, in some parts of Rum, particularly around Long Loch, the structures in the gabbros and peridotites are much more chaotic. In some places, randomly orientated blocks of layered gabbro are enclosed within structureless peridotite, whilst in other places the layering in the rocks is fantastically contorted. These structures formed in the magma chamber, when the piles of crystals built up until they became unstable. Blocks of solidified rock, together with loose crystals, mixed with still-molten magma, avalanched down the sides of the magma chamber to create a chaotic mixture that eventually solidified to form the rocks we see today.