The Roots of Volcanoes
The cracks in the earth's surface which fed these volcanic eruptions continued to act as pathways for molten magma long after the surface volcanic activity had ceased. Magma hardened in these fissures as vertical sheets of rock known as 'dykes'. There are many examples of dykes on Skye, particularly on the coast where they cut through the sedimentary rocks and stand proud like walls because they erode less easily than the softer rocks around them.
In northern Skye, the magma also forced its way sideways to form sheets between the layers of sedimentary rocks beneath the lavas. These gently sloping sheets can be up to 90 metres thick and are called 'sills'. Many display well-developed columns throughout most of their thickness. This dramatic effect can be seen in the aptly-named Kilt Rock south of Staffin.
The Cuillin of Skye is acclaimed as the most spectacular mountain range in the British Isles. The arc of jagged peaks, almost 1,000 metres high, which makes up the main Cuillin Ridge, together with Sgurr na Stri and Bla Bheinn are a delight for all visitors, particularly mountaineers. The legendary frictional properties of the rough gabbro and the abundant huge cliffs and slabs, swept clear of debris and polished by glaciation, make this area a playground for rock climbers. Less spectacular but equally attractive in their own way are the Red Hills extending from Glamaig to Beinn na Caillich. The granite of these hills is more uniformly worn by wind and water so that the hills have smooth outlines with few steep rocky faces.
Despite their contrasts, these two groups of hills both represent the deeply eroded roots of large volcanoes. Active volcanoes can be seen in many parts of the world, but if we want to understand how they develop, we must look at long extinct volcanoes such as those of the Inner Hebrides where their roots, exhumed over many millions of years, can be seen. This makes Skye a centre of worldwide recognition and importance.
The mountainous areas that we see now are the solidified remains of the magma chambers which fed the volcanoes. Because they cooled slowly, deep in the earth, they now consist of a wide variety of spectacular and beautiful coarse-grained rocks. In the Cuillin these rocks are almost gabbro - a coarse-grained equivalent of the basalts like those which make up most of the older lava flows which can be seen on Skye. The granites of the Red Hills are the coarse-grained equivalents of other types of lavas which are now largely eroded away. Scattered around both the Cuillin and the Red Hills are rocks which themselves consist of a jumble of rock fragments of various types, ranging from fine dust particles to huge blocks.
These outcrops are the remains of volcanic "vents" - the pipes through which the magma chambers were connected to highly explosive surface eruptions. The vents contain materials brought up from deeper levels as well as the products of the eruption, such as lava and ash, which have collapsed back down the vent. Good examples of this phenomenon can be seen in the Kilchrist - Kilbride area between Broadford and Torrin.
Magma rising in a gentler, less explosive manner commonly solidifies as fine-grained sheets of rock in contrast to their coarse-grained host rocks. Many of these sheets are vertical dykes and some are near-horizontal sills. In the Cuillin, many take the form of 'cone-sheets' - sloping sheets which have the form of huge cones several kilometres in diameter at the surface, dipping inwards towards the deep centre of the volcano. The dykes are responsible for much of the jagged outline of the Cuillin mountains because of their different weathering characteristics compared with the surrounding gabbro. They commonly account for notches in the ridge, but can also form upstanding walls or blades like the Inaccessible Pinnacle of Sgurr Dearg. For the climber the sheets often form convenient ledges, but many have a dangerous tendency to brittleness, with many close spaced joints, and are notoriously slippery when wet.
A distinctive feature of the gabbros in some parts of the Cuillin is a very obvious layering reminiscent of that seen in many sedimentary rocks. On closer inspection this is seen to be due to a repetitive variation in the proportions of the individual minerals which make up the rock, such as feldspar, olivine and pyroxene. This happened during the cooling of the magma, as the individual minerals crystallised in a definite order and either floated or sank according to their density relative to the remaining magma. In general the layering dips inwards towards the centre of the magma chamber which consequently has an internal structure resembling a stack of saucers.