Gedemsa (Gadamsa)
Gedemsa is an 8 km diameter caldera located immediately east of Lake Koka. The geology is clearly depicted on a 1:50,000 geological map of the Nazret-Dera region (Alula et al., 1992).
Complete list of alkaline rocks and carbonatites
Gedemsa is an 8 km diameter caldera located immediately east of Lake Koka. The geology is clearly depicted on a 1:50,000 geological map of the Nazret-Dera region (Alula et al., 1992).
The adjacent volcanoes of Bericcio and Bora are located between the Zwai (Ziway) and Koka lakes on the floor of the rift valley (Di Paola, 1973 and 1977), and immediately northwest of the Chilallo volcano (No. 42).
Chilallo is a large volcanic cone on the eastern shoulder of the Ethiopian rift valley. It is elliptical in plan covering 30x20 km and has a 6 km diameter summit caldera around the inner rim of which are very large, radially disposed dykes.
Badda is a large volcano lying immediately east of, and overlapping, Chilallo (No. 42). Like Chilallo it appears to be formed predominantly of alkaline trachyte lava flows.
The Alutu volcanic centre lies immediately south of Lake Zwai (Ziway), covers some 90 km2 and rises 650 m above the surrounding lake deposits and tuffs.
O'a is the largest of the calderas in the Ethiopian rift valley with a longer axis of 17 km. It lies beneath the eastern half of Lake Shalla (Hora O'a), parts of which are some 250 m deep.
The Baltata volcano is located immediately northwest of Cacca (No. 47). No details of the petrology have been found but Baltata is shown on an unpublished map of Di Paola (1976) as including pantellerites and alkali trachytes.
On the key of an unpublished map of Di Paola (1976) this volcano, covering about 18x12 km, is shown as comprising "..pantelleritic ignimbrites associated with mugearitic-alkali and peralkaline trachytic central volcanoes." It has a radial dyke swarm (Mohr, 1980).
The Corbetti volcanic complex consists of the Corbetti caldera, which has a diameter of 12 km, and the younger volcanoes of Urji, which is located in the centre of the caldera, and Chabbi, which overlaps the eastern caldera margin (Di Paola, 1972).
No details have been traced but this occurrence is shown on an unpublished map of Di Paola (1976) and Di Paola (1973) as being pantelleritic.