Ribkwo

Ribkwo is a multi-centred phonolitic trachyte volcano with a maximum thickness of 750 m that forms a low-angle shield of approximately 50x15 km (Webb and Weaver, 1976; Key, 1987b). The volcano exhibits two morphologically contrasting areas: an eastern central generally rugged region which comprises a chain of six volcanic centres of lavas, pyroclastic rocks, lahars and minor intrusions, and a western and northwestern deeply dissected plateau of sub-horizontal lava flows within which three distinct units are recognised. Four of the six volcanic centres of the eastern area include trachytic lavas, including a distinctive fayalite trachyte, breccias, pumice and welded tuffs and phonolitic trachyte lavas. These rocks are overlain unconformably by products of the last two centres, which comprise trachytic lavas and tuffs, including welded varieties, and intrusive trachyte stocks, and these are associated with two periods of caldera collapse. The lavas of the plateau are principally feldspar-phyric trachytes and phonolitic trachytes but in thin section nepheline microphenocrysts are also seen and may be rimmed by aegirine and aenigmatite and altered to cancrinite. The groundmass consists of sanidine laths, aegirine and aegirine-augite, sodic amphibole, and variably katophorite, arfvedsonite or riebeckite, aenigmatite, biotite and magnetite. There are mugearites and olivine basalts which make up about 5% of the volcanic rocks. Rock analyses are given by Weaver et al. (1972).