This is a complex rhyolitic volcano overlying older basaltic flows. The rhyolites form a thick sequence of short flows towards the centre. No petrographical or chemical data have been traced but there may be alkaline rocks in the sequence.
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Main Ethiopian Rift
The 'Main Ethiopian Rift' extends for some 600 km from the Afar Depression south-southwestwards to the Kenya border. Centred on the rift and the adjacent rift shoulders are numerous volcanoes that are mainly alkaline, many being composed predominantly of peralkaline silicic rocks. Descriptions of these centres follow (Nos 054-00-033-054-00-053). Much of the rift floor is covered by welded to unwelded ash-flow and pumice deposits of peralkaline rhyolitic composition which were probably erupted from fissures (Merla et al., 1979), although some may have emanated from the central volcanoes. There are few detailed descriptions of these deposits but information on the central part of the rift will be found in Di Paola (1973) and on the evolution of the northern part of the rift in Kazmin et al. (1980). A general account of the relationships between tectonics and volcanism is that of Mohr (1983) who has collected together chemical data (Mohr, 1962a). Some 77 K-Ar dates for volcanic rocks within the rift and for volcanoes of the rift shoulders will be found in Woldegabriel et al. (1990).
This 15 km diameter volcano with a double-rimmed summit caldera consists of basaltic and intermediate lavas culminating in rhyolite flows and breccias. No petrographical or chemical data have been traced but there may be alkaline rocks in the sequence.
This is a large (45 km east-west) twin volcano the older and higher Ayelu centre lying west of Aabida. Ayelu is built of thick rhyolitic flows with the eastern flank covered by ignimbrites that were emitted from Aabida.
Fantale is a strato-volcano 600 m high extending over about 100 km2 and composed predominantly of pantelleritic trachytes and obsidians in which tuffs are subordinate (Gibson, 1970). The summit area is dominated by a 3.5 km diameter caldera with a depth of 300 m.
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).
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.
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).