The landscape on Arran is dominated by the remnants of ancient volcanoes produced in the Tertiary when the North American and Eurasian continents rifted apart to create the north Atlantic Ocean. Continental rifting is intimately associated with magmatic activity, and features such as the exposed roots of volcanoes, sills, and lava flows can all be seen on Arran.

The Northern Granite on Arran, which forms the highest peak on the island, is in fact the roots of a volcano which was active about 60 Ma years ago. The characteristic volcano shape no longer exists; it has been removed by glaciation. What remains is the solidified magma that existed beneath the volcano which had time to cool at depth and become granite. Due to the slow cooling the resulting rock is coarsely crystalline and very resistant to weathering. It is very easy to identify the constituent minerals with the naked eye: quartz, feldspar, mica. Goat Fell is the highest peak on Arran and is composed of entirely of granite. The shrinking and cracking pattern produced upon cooling and exhumation of granite is called tor.

When a volcano erupts it can either be explosive (pyroclastic) or effusive (lava flow). There are lava flows associated with the Clyde Plateau Volcanic Formation, erupted during the Carboniferous, which can be seen exposed on the shore at Corrie. The lava flows are composed of basalt, a fine grained basic igneous rock, which often contains phenocrysts of olivine and plagioclase.

The oldest volcanic rocks on Arran are of suspected Ordovician age, and are submarine in origin. Erupted under water, the magma solidifies in a particular lumpy fashion, hence the name pillow lava. Pillow lavas and volcanic agglomerate can be seen in the North Glen Sannox burn (NR996469).

The magma associated with volcanoes often does not reach the surface and instead exploites weaknesses in the country rock to form intrusive igneous rocks like dykes and sills. Excellent composite intrusions known as Judd’s Dykes, which display a suite of cogenetic igneous rocks, can be seen at the shore near An Cumhann just north of the Kings Caves on the west coast.