Altered Rocks
Molten magma of the type which crystallises as basalt or gabbro can be seen as hot as 1200° C. Unlike lava flows, which can cool at the surface within a few days or weeks, large magma chambers at depth stay hot for millions of years, so the effect on the surrounding rocks can be considerable. On Skye, there are a wide variety of older rocks which were very close to the Cuillin and Red Hills magma chambers, and in these areas varying degrees of baking and alteration can be seen in Torridonian sandstones, Cambrian to Ordovician age limestones, Jurassic age sandstones, limestones and shales, and early Tertiary age lavas. It has even been suggested that some of the granite magmas were formed by the melting of Lewisian and Torridonian rocks which lay close to the gabbro magma chambers.
The rocks are not only affected by direct heat. The circulation of heated groundwater and hot fluids expelled from the magma chambers also has played a part in the changes in the rocks.
This alteration can be seen in spectacular form in the Strath area to the south of the Red Hills. Here limestones of Cambrian to Ordovician age have been transformed into the famous Skye marble, quarried near Torrin. The limestone is rich in magnesium and contains nodules of silica - impurities which combine to produce the decorative green and yellow streaks. In places, there are also rare fluorine and boron-bearing minerals. Some rocks, richer in iron, have developed 'magnetite' - the magnetic oxide of iron which was investigated as a potential ore when iron was in great demand during the Second World War.