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 caldera contains eight pantelleritic and comenditic lava domes a number of which also occur around the caldera. These rocks contain phenocrysts of sanidine, quartz, oligoclase, aegirine-augite and sodic amphibole. There are extensive pantelleritic pumice fall deposits and welded ignimbrites around the caldera with a similar mineralogy to the domes but containing also clasts of basaltic rock.