It is a shield volcano with a diameter of or and an estimated volume of . The summit of Emi Koussi is formed by two overlapping calderas, which together form a wide elliptical caldera that extends from northwest to southeast. The northern caldera formed first, then the southern caldera which is about deeper below the northern one. high scarps form the inner margin of the southern caldera, with the floor at an elevation of . Before the formation of the calderas, the volcano may have reached a height of . Emi Koussi has been compared to a Martian volcano, Elysium Mons. The caldera rim is cut by one outlet, the Porte de Modiounga, from which gorges emanate into the caldera. The highest point of Emi Koussi lies on the southern rim of the caldera.