The island of Bømlo presents a geological section through a Caledonian convergent plate margin. The Lykling Ophiolite and the overlying ensimatic are sequence of the Geitung Unit were emplaced onto the Baltic Shield, probably in Arenig times. After a period of deep erosion, the Basin and Range type volcanics of the Siggjo Complex were deposited followed by a hiatus in the magmatism and a weak but widespread phase of vertical tectonism, possibly due to the successful subduction of a spreading ridge. At the top of the mainly sedimentary Ashgill/Llandovery Vikafjord Group, the Basin and Range type volcanism resumed. This was probably preceded immediately by the calc-alkaline, subaerial volcanism at the base of the Langevåg Group, which passes stratigraphically upwards into basin-fill lithologies such as submarine volcaniclastics, cherts, turbidites and submarine greenstones of chemistry compatible with the early stages of ensialic back-arc basin development. The stratigraphy and volcanism are discussed in context with recent views on the rest of the Norwegian Caledonides.