The Affara Dara peralkaline granite is exposed over 35x25 km but to the west is partially covered by younger basalts that emanated from the Alayta and Boina eruptive centres. In the south highly altered basalts, thought to be of Tertiary age, are intruded; the metamorphic aureole is less than 50 m wide. The most abundant rock type is a coarse-grained riebeckite granite but syenites also occur. Near the margins a finer grain size prevails. Small dykes of peralkaline granite aplite and biotite-bearing pegmatites are found near the periphery, the latter also occurring in the altered basaltic country rocks.