The Atlantic Ocean Opens
When the seas subsided, Scotland emerged to face yet another ordeal, this time by fire. Scotland was still bound to the eastern flank of North America around 65 million years ago, but yet another substantial re-arrangement of the continents created the North Atlantic Ocean. It developed as a tear in the Earth's crust, with volcanic eruptions adding new rocks along the central spine of the nascent ocean.
The associated stretching and thinning of the Earth's crust near the margin of the emergent ocean allowed molten rock to break through the crust to form a line of volcanoes running from St Kilda to Ailsa Craig. Many individual volcanoes were active over a period of about 5 million years (Ben More on Mull, Ardnamurchan, Rum, Skye, St Kilda and Rockall), throwing ash clouds high into the air and spewing out vast thicknesses of lava.
The island of Skye is perhaps the finest example of an ancient volcano whose bowels and inner plumbing have been laid bare by the elements. The scarp face and gently sloping topography of the Trotternish Peninsula and northwest Skye were fashioned from the lava fields which developed around the volcano. The boiling magma chamber, which contained molten rock by the cubic kilometre, lay underneath the lava field. It later solidified to form a rock known as gabbro. This is the Black Cuillin. The Red Cuillin was formed as the molten mass of gabbro came into contact with the lower crust causing it to melt and form a red granite. Erosion by wind, water and ice removed more than 2km of the volcano's super-structure and, as a result, the relationship between its various component parts can be studied. Indeed, the Skye volcano has been the subject of detailed geological investigations for the best part of a century and a half, and many of the ideas developed have been used to unravel the complexities of similar features around the world.