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.
The Koni volcanic centre is dominated by a series of calderas, eight of which have been identified by Cole (1969), although the earlier ones are much obscured by later volcanic products.
The Boseti volcanic complex is about 20 km in diameter and comprises the two adjacent volcanoes of Gudda and Bariccia both of which are over 2000 m high. Activity has been divided into two cycles (Brotzu et al., 1974 and 1980) with initial volcanism apparently centred on the Gudda centre.
This extensive area southeast of Addis Ababa is wholly occupied by volcanic rocks which fall into four main categories (Gasparon et al., 1993). (1) Early basalts, which are found in linear outcrops along river valleys.
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.