The exposed part of the Um Risha complex measures 12 km east-west but an unknown area is concealed beneath Nubian sandstone. The intrusive rocks are peralkaline syenites and granites of varying grain size. The principal mafic minerals are aegirine-augite and a range of amphiboles including riebeckitic types. What are taken to be remnants of a volcanic cone include pyroclastic rocks, trachyte and trachybasalt, the trachyte containing aegirine-augite, amphibole and sanidine. There are a few bostonite dykes within the complex.