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.
ALULA, D., BOCCALETTI, M., GETANEH, A., MAZZUOLI, R., TORTORICI, L. and TRUA, T. 1992. Geological map of the Nazret-Dera region (Main Ethiopian Rift). Consiglio Nazionale delle Ricerche, Ministero degli Esteri, Direzione Generale Cooperazione allo Sviluppo.