Approximately circular and 6 km in diameter, Gamawa consists of a broad ring-dyke of amphibole-aegirine granite which was succeeded by a boss of pyroxene-fayalite-quartz syenite. An early volcanic phase is represented by a narrow zone of breccia along the inner contact of the ring-dyke in the southeast, which is made up almost entirely of fragments of basement granite, and two diatremes within areas of basement granite enclosed within the ring-dyke. The granite of the ring-dyke consists of quartz, perthite, albite, arfvedsonite, aegirine, aenigmatite and accessory zircon and fluorite. The rock of the syenite boss is variable between localities and consists of perthite, a little quartz and aegirine-augite, while fayalite, aenigmatite, amphibole and biotite may or may not be present. A small intrusion of aegirine-riebeckite granite cuts the syenite and late felsite dykes, in which needles of blue sodic amphibole and aegirine are abundant, cut all rock types.