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Alaji Basalt

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2.5.1 Alaji Basalt

Alaji Basalt group are volcanic rocks (Alaji rhyolite and Basalt) and parts of the escarpment were out poured from the end of Oligocene until middle Miocene. This unit is composed of basalts, which show variation in texture from highly pophyric to aphyric. Within this unit, there is an intercalation of gray and glassy welded tuff. The outcrop of Alaji basalt extends from the crest of Entoto (ridge bordering the northern parts of Addis Ababa) towards the north (Girmay and Assefa,1989). The unit is underlay in by tuffs and ignimbrites. Its stratigraphic relationship with the Entoto silicics is difficult to determine as they occur in a fault contact. Mohr (1967) as cited in Alemayehu etal. (2003) proved that the Entoto trachyte …show more content…

It is mainly found in the central part of the town and underlain by the Entoto silicics and overlain by Lower welded Tuff of the Nazareth group. The maximum thickness exceeding 130 meter is found at ketchene stream. It is porphyritic in texture, composed of labradorite, bytownite, olivine and augite as phenocrysts. The ground mass is made of andesine, labradorite, olivine, agnetite and pyroxene (Girmay and Assefa, 1989). (NOT ABLE TO READ, WHAT IS IT)

Olivine porphyritic basalts outcrop in the central part of the town that includes Mercato, Teklehaymanote and Sidist Kilo. The distribution of plagioclase porphyritic basalt is almost the same as that of the olivine prophyritic basalt, but only little more north wards. It outcrops in an area, which includes Sidist Kilo, General Winget School and French Embassy. The thickness of the former varies from 1m or less in the foothills of Entoto, Lideta Airfield and Filwoha to greater than 130meters at Ketchane stream (Morton,1974; Varnier etal., 1985 as cited in Alemayehu etal., 2003). The Lower Welded Tuff overlies both types of basalt near by the Building College, the Kolfe Police School, the Kokobe Tseba School and YecaMariam Church. On the other hand, only in the gorge of the Ketchane stream, the olivine pophyric basalt is overlain by the plagioclase porphyritic basalt, while else where the relationship between them is very difficult to determine (Varnieretal.,1985 as cited in Alemayehu etal.,2003). Addis Ababa basalt yield ages clustering around 7my and seems to have no time/composition equivalent (Morton etal., 1974 as cited in Alemayehu etal.,

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