Volcanism on the Arabo-Ethiopian swell has accompanied the development of the three traversing spreading zones conjoining at Afar: the Red Sea, Gulf of Aden, and African rift systems. The Red Sea and Gulf of Aden floors are formed by oceanic tholeiites, but Afar and the main Ethiopian rift show a wider range of more alkaline volcanics. related to slower crustal spreading rates. The high plateaus between the rifts have had a strongly alkaline volcanism. The plateau basalts and especially the intermediate lavas are much more undersaturated than their rift equivalents, and are enriched in potash and Al/Fe. The silicic volcanics of the rift are soda-iron enriched relative to the plateau silicics. Magmatic composition can be equated with depth of melting, but the voluminous silicic volcanics are not the products of fractionation, direct partial melting, or anatexis of continental crust: the accretion hypothesis of Battey is supported and developed.