English: Dyke on Beinn a' Ghraig The southern part of the hill has an exposure of pink granophyre. This ends with the summit tower made of a block of dark coloured resistant igneous rock. To the north of the crags is this striking dyke of basalt which has weathered into a red colour, probably due to iron. The Loch Bà area was once the site of a big volcano which produced a swarm of dykes cutting through the preexisting rock.

Loch Bà is a very important geological site. Many of the igneous rocks from the volcanic episode have been eroded away. This is the site of a big volcano formed after the basalt floods that make up much of the island. Once the reservoir of magma under the volcano was exhausted the mountain collapsed into the empty chamber. This collapse preserved rocks that have elsewhere been lost in the last 55 million years.

This process occurs with many volcanos and is responsible for the most catastrophic of eruptions. Examples in the last few thousand years include Krakatoa and Santorini.