The Brissago Islands are located in the northernmost part of the insubric Alps, in the Kinzigitic Zone, so named by the kinzigitic gneiss forming the rocky substrate and emerging in several points, particularly along the shore of the Isola Piccola.
In the remaining part the rock is covered by a thin morainic layer. The Brissago Islands are the two tops of a hill shaped by erosion along the ancient watershed line of the Ticino and Maggia valleys.