Dykes are vertical or near-vertical intrusions of igneous rock formed when magma is injected along cracks which are opened in the Earth's crust. Dykes formed throughout the three phases of activity of the Ardnamurchan volcanic event, radiating out from the Centres along lines of weakness formed as the magma punched its way upwards, their straight lines cutting across both pre-existing sedimentary and earlier intrusive rocks.
The photo above shows a large dyke cutting through Jurassic sediments on the shore line at the west end of Ormsaigbeg. To right and left are the faces of the original crack into which the magma intruded, formed of bedded limestone and shale. The wall-like structure at centre is the middle of the dyke, a particularly hard rock which has resisted the sea's erosion. Between it and the walls on either side, the dyke was made of softer rock which the sea has eroded away.
Higher up the hill, the dyke forms a ridge which has been used by crofters as the base of a drystone wall; the dyke can be seen again at the top of the picture where it crosses the skyline.
A short distance away along the coast, another dyke snakes up the hillside. This dyke, about a metre wide, is unusual in not running straight. It too shows a harder centre and more easily eroded lateral zones: the dyke was probably injected into the existing rock in a single episode, but cooled more quickly along its sides where the magma was in contact with colder, existing rock, the finer-grained result being more easily eroded.
The final stages of the volcanic events which produced the volcanoes of Skye, Rhum, Ardnamurchan, Mull and Arran was the injection of a mass of dykes, called a Dyke Swarm, which ran northwest and southeast from the Centres.
Some of these cut across Scotland into the rocks of northern England, the largest being the Cleveland dyke which reaches Fylingdales Moor in North Yorkshire (see map above), and more appear in the mountains of North Wales.
A network of small intrusions formed with each Centre. The photo, taken along the coast to the west of Ormsaigbeg, shows dark veins of basalt intruded into a lighter sedimentary rock, which seem to have developed from a larger intrusion of basalt above.
Geological map of North Yorkshire courtesy Harkey Lodger on Wikipedia, here.
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