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 caldera walls are exposed in places, particularly along the southern shores of the lake, and this, together with the rocks outcropping around the lake, have enabled Mohr et al. (1980) to erect a general stratigraphy. The oldest rocks are rhyolitic flows and domes that were followed by thick columnar trachytic ignimbrites. There followed deposition of up to 200 m of pyroclastic flow and air fall pumices, some of which are welded, that were the immediate precursors of caldera formation; a little olivine basalt also occurs within this unit. The two following units are variably welded ignimbrites and although Mohr et al. (1980) give few mineralogical details these ignimbrites are described as containing feldspar and aegirine-augite phenocrysts. There followed a rhyolitic air fall pumice and the building north of the caldera of a pyroclastic cone (Tulu Fike) which has a 500 m diameter breached crater. Activity closed with eruption of basalts from fissures and a solitary scoria cone. Analyses of a pantelleritic ignimbrite and an ash-flow tuff are given by Mohr and O'Donnell (1983).