The mountain is largely composed of Alaji Basalts, the youngest succession of the Ethiopian flood basalts. The most recent flows are only 15 million years old. The geological formation was called after Imba Alaje by geologist William Thomas Blanford, who accompanied the British Expedition to Abyssinia in 1868. In roadcuts, there are conspicuous outcrops of whitish interbedded lacustrine sedimentary deposits.