Lavas of the African Rifts, their chemistry and tectonic setting

The state of knowledge and opinion as to the African Rift valleys and their lavas was, a decade ago, such as to suggest that a definite correlation might be made between the tectonics and the chemistry of the lavas. The Western Rift Valley seemed to have been formed as a result of compressive stresses in the crust and to have associated with it lavas of potash‐rich character. The Eastern Rift Valley seemed to have been formed as a result of tensional stresses and to have associated with it lavas of soda‐rich character. The picture now presented is less simple. As a result of further investigation there has arisen much diversity of opinion as to the tectonic forces controlling the formation of both Rift valleys and there seems therefore no adequate basis, for an attempt to correlate lava‐chemistry with contrasted tectonics. In addition, recent studies of the lavas have brought to light many exceptions to the general tendency towards preponderance of potash in the Western Rift and preponderance of soda in the Eastern Rift Some new analyses are given which illustrate these exceptions and emphasize the fact that, as in the case of the tectonic relations, no simple picture of the chemistry of the lavas can be outlined at the present time. Indeed, when the question of the origin of the dominant lavas of the Western Rift is reviewed in the light of latest knowledge, their potash‐rich character seems to be a relatively accidental circumstance. This question is discussed and the lack of any convincing relation between lava chemistry and tectony in the Rift valleys is emphasized.