Andesite is a fine grained, intermediate volcanic rock named after the Andes mountains in South America. It is characterized by the presence of plagioclase feldspars (oligoclase-andesine) with some combination of pyroxene and amphibole. Andesite is often associated with subduction magmatism, although, may result from the mixing of acidic and basic magmas which may coexist within a stratified magma chamber.

Andesite exposures are relatively small on Arran; elsewhere in Scotland they can be much more extensive and are quarried to produce material for road building (e.g. image on right).

Andesite occurs on Arran in the Central Ring Complex and was associated with an intense period of magmatism which coincided with the opening of the North Atlantic some 60Ma years ago. Volcanic activity persisted for about one million years, with several events of caldera collapse. The variety of igneous rocks found in the complex are a result of the mixing, differentiation and evolution of the magmas of the volcano. Exposures of andesite can be found just around and south of Ard Bheinn. These rocks are often intimitely associated with breccias or are brecciated themselves because they contain volatiles and can be pyroclastic.