Tertiary volcanic rocks cover more than one third of the Wollega-Ilubabor area in western Ethiopia. Most of the volcanic rocks are basaltic but there are some occurrences of alkaline rocks. Wollel (Tulu Wollel) is a large volcano with a base of about 40x30 km. The lowermost rocks are aphyric hawaiites which are overlain by trachyte and then a succession of basalts, hawaiites, mugearites and minor trachyte flows (Berhe et al., 1987). The uppermost rocks are trachyte lavas and these are also found in the Sayi area. Merla et al. (1979) indicate that phonolite plugs are also frequent. Mohr (1962a) says that the trachytes of western Wollega and Ilubabor tend to be peralkaline and consist of feldspar, aegirine, aenigmatite and/or riebeckite.At Tulu Jergo, which is 50 km south of Ghimbi, a "summit pyramid" (Mohr, 1962a) consists of rocks referred to as bostonites comprising K-feldspar, nepheline and aegirine. Phonolite also occurs to the east of the Wama valley and in the upper Ghibbie valley (Mohr, 1962a).
BERHE, S.M., DESTA, B., NICOLETTI, M. and TEFERRA, M. 1987. Geology, geochronology and geodynamic implications of the Cenozoic magmatic province in W and SE Ethiopia. Journal of the Geological Society, London, 144: 213-26.MERLA, G., ABBATE, E., AZZAROLI, A., BRUNI, P., CANUTI, P., FAZZUOLI, M., SAGRI, M. and TACCONI, P. 1979. A geological map of Ethiopia and Somalia (1973) 1:2,000,000 and comment with a map of major landforms. Consiglio Nazionale delle Ricerche Italy. Centro Stampa, Firenze. 1-95.MOHR, P.A. 1962a (reprinted 1971). The geology of Ethiopia. University College of Addis Ababa Press, Addis Ababa. 268 pp.