English: Modal layering in gabbro near Meall Meadhoin in Ardnamurchan in Scotland.
This is one of the localities described in the geological excursion guide to Ardnamurchan. It is made up of obvious layers of gabbro. These are formed in the magma chamber deep in the crust of the Earth. As the magma cools, certain minerals crystallise out of the liquid rock more readily than others, and these tend to be the heavier, darker minerals. They fall to the bottom of the chamber, settling in layers. The preferential crystallisation of some minerals alters the chemical composition of the remaining melt, and as the temperature falls, different minerals crystallise and settle. New injections of fresh magma with chemical composition similar to the original magma re-set the process, with the result that darker and lighter layers form in the cooling rock.