The volcanic rocks of which Skomer is comprised date from the Silurian period around 440 million years ago. A series of basalts, rhyolites, felsites, keratophyres, mugearite and associated sedimentary rocks (quartzites, etc.) are grouped together as the 'Skomer Volcanic Series'. The series which is up to 1000m thick also includes trachyte, dolerite and skomerite which is an altered andesite. Basalt is the most common component of this sequence; some of it appears as pillow lava indicating that it was erupted under water. Other basalt flows show signs of contemporary subaerial weathering.