The Terceira Rift is a subsidiary zone of crustal generation in the Azores to the E of the Mid-Atlantic Ridge. The geology of the island of Terceira, which straddles the rift, is thought to give an indication of the nature of the volcanic activity along this zone of crustal separation. Terceira is composed of 4 large strato-volcanoes associated with trachytic or trachybasaltic activity which have since been intruded by basaltic magmas to form monogenic cinder cones and lava flows on the flanks of these large volcanoes. There is little direct evidence of eruptions along fissures although lines of cinder cones may indicate zones of weakness in the crust.