Corbetti, Chabbi And Urji

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). The whole complex is some 20 km east-west and is surrounded by welded and unwelded ignimbrites and pumice tuffs that cover much of the floor of the rift in this area. To the north the complex overlaps with the O'a centre (No. 45). The Urji volcano occupies much of the caldera and is formed of pumice flows and fall deposits. Several pumice flows emanated from a large summit crater while obsidian flows, interbedded with pumice, outcrop on the eastern side. The eastern margin of Urji is in contact with the Chabbi volcano which has been described by Mohr (1966b) and Macdonald and Gibson (1969). Chabbi consists of obsidian flows, that are typically 10-20 m thick, with intercalated air-fall tuffs and agglomerates. There are several vents which coalesced to give a dome shape to the volcano. The Chabbi lavas are, according to Macdonald and Gibson (1969), unusual in being aphyric but their chemistry indicates that they are pantellerite. However, Di Paola (1972) has found porphyritic rocks at Chabbi which contain phenocrysts of anorthoclase, quartz, aegirine and fayalite; he also gives an analysis of a pumice. The ignimbrites of the Urji centre contain phenocrysts of anorthoclase, aegirine, fayalite and in some aenigmatite, while rhyolite lavas, which may be spherulitic, contain the same phases together with quartz (Di Paola, 1972).