New K-Ar dates are presented for areas in W and SE Ethiopia. In the west, the dates distinguish the Geba Basalts of 40 to 32 Ma (previously determined by Merla et al. 1979) from the Welega Shield Volcanics which are shown to range from 11.2 ± 2.2 to 7.8 ± 1.6 Ma. In SE Ethiopia, the Lower Stratoid flood basalts range from 30 ± 4.5 to 23.5 ± 4.5 Ma and are unconformably overlain by the Reira-Sanete shield volcanics which range from c. 15 to c. 2 Ma. The unconformity is marked by a palaeosol as are several of the intervals between the major volcanic stages of Ethiopia.
Using new field observations, together with previously published results and unpublished data from the Ethiopian Institute of Geological Surveys, it is suggested that the Ethiopian Flood Basalts were erupted in three major stages. Stage 1, which is mainly older than 40 Ma is separated from Stage 2, 34 to 30 Ma for NW Ethiopia and 40 to 30 Ma for SW Ethiopia, by erosional unconformities. Stage 3 spans 30 to 26 Ma in NW Ethiopia, and 30 to 21 Ma in SW Ethiopia and both are marked by the incoming of silicic volcanism. In W Ethiopia, Stages 1 and 3 are not developed, whilst in SE Ethiopia the Tertiary volcanism commences with Stage 3 flood basalts. The overlying shield volcanics; (25 to 13 Ma in NW Ethiopia and 15 to 7 Ma in W Ethiopia) represent a localized terminal episode built on the Plateau and are considered a fourth stage.
The earliest volcanism is restricted to two areas: in SW Ethiopia, where the Akobo Basalts give ages as old as 49.4 Ma (Davidson & Rex 1980), and in NW Ethiopia where the Ashange Basalts underlie the Aiba Basalts which are dated at 34 to 30 Ma. The Ashange volcanism may have begun in the Palaeocene—Eocene but radiometric dates are ambiguous and inconsistent. These two volcanic centres show lithological and geochemical differences, and can be related to two separate rift zones north of Lake Turkana and further north in the Tana Graben. Volcanism appears to have spread by the expansion of these areas, and to have migrated eastwards with time towards the Main Ethiopian Rift which begun to form about 14 Ma ago. There is a complex pattern of domal uplifts within these two volcanic centres with volcanism either preceding doming, or the two being contemporaneous.