Basalt is a dark-coloured, fine grained, extrusive igneous rock, composed mainly of plagioclase feldspar and pyroxene with or without olivine. Basalt may often contain phenocrysts of plagioclase or olivine. In terms of igneous rocks, basalt is the "daddy", so to speak. Basalt flows cover nearly 70% of the Earth’s surface, and differentiation of basaltic magma can produce a whole suite of cogenetic magmas, from basic to intermediate in composition. Basalt itself is a product of partial melting of mantle peridotite.

Basalt flows may erupt from volcanoes, be extruded from fissures in the crust (mid-ocean ridges), and can also occur as sills or dykes at extremely shallow levels. The majority of basalt found on the Earth occurs as part of the oceanic crust.

During the Carboniferous the western part of the Midland Valley was subject to the thick development of basaltic lavas. The Clyde Plateau Volcanic Formation is the name given these lavas, and they belong to the Strathclyde Group of the period. These flows outcrop on Arran, and they can be seen near the base of the Carboniferous succession at Corrie. Basalt has a very distinctive weathering pattern known as "onion-skin weathering" and this is very useful for identification; see photo.

Careful inspection of the basalt at Corrie shows that it often contain phenocrysts of olivine which have weathered to a red colour. The photo on the right shows a basaltic lava with phenocryst.
Other outcrops of basalt on Arran include the pillow lavas of The North Glen Sannox Beds, and some small fragments of flood basalt from the British Tertiary Province.