The Ardnamurchan Volcano - 3

The original 1930 British Geological Survey Memoir on Ardnamurchan described the volcanic event as occurring in three phases, each consisting of a broadly similar sequence of events. The plume of magma rising from the Earth's mantle created a huge basaltic magma chamber into which a 'plug' (1) - roughly the shape of an inverted cork - sank to allow the collapse of a circular, steep-faced Caldera (4). This allowed magma to intrude (2) upward in sheets (3) and across the top of the 'plug'. The magma subsequently cooled to form a coarse igneous rock of a broadly basaltic composition called Eucrite. These massive, circular, concentric sheets are called Ring Dykes.

Where the magma reached the surface it formed volcanic vents (5) which erupted, sometimes producing lava flows and ash clouds, sometimes collapsing upon themselves to form masses of smashed rock which blocked the vents, called Agglomerates.

Stresses within the overlying crust also allowed magma to inject into the existing rock along cracks which were circular (6), with their centres in the centre of the Caldera. These are termed Cone Sheets.
Erosion in the subsequent 60 million years has been intense, not least because the landscape of Scotland has several times been covered by ice sheets. This erosion stripped off the original volcanoes and destroyed the calderas, but erosion works differentially, leaving areas underlain by harder rock - the Ring Dykes and Cone Sheets - protruding. These give Ardnamurchan's volcanic centres their characteristic, present-day landscapes, with the Ring Dykes (1) forming the highest and most rugged hills, and the Cone Sheets (2) forming distinctive ridges around and outside them. Within this ring-structure, and around it, lie the remains of the volcanic events, lava flows, solidified ash and Agglomerates, marked in purple (3).