Lying southeast of the Erta Ale Range the Tat'Ali Range is a complex, multiple volcano of four principle units (Barberi et al., 1973b): a lava field fed from fissures, the Borawli rhyolite volcano, and the Tat'Ali and Mat'Ala volcanoes. The earliest activity was submarine and the fissure-fed lavas are all basaltic ranging from olivine-rich picrites to aphyric basalts. The Tat'Ali complex is a shield volcano with an elongate northwesterly-trending sink produced by faulting. Lavas emitted range from basalts through intermediate types to final pantellerites. Mat'Ala is a shield volcano of low profile with a 3.5x2.5 km diameter summit caldera of approximately 300 m depth. It comprises principally lavas of basaltic and intermediate composition. Borawli (Barawli-Franca complex of Tazieff et al., 1970) lies on the western margin of Tat'Ali and is a conically-shaped strato-volcano of silica oversaturated trachytes with to the south domes of pantelleritic obsidian and associated, but scarce, pumice. This is probably the centre from which the pumice found all around Lake Giulietti derives. Fumarolic activity is widespread around the Tat'Ali Range. The chemical variability of the Tat'Ali rocks is illustrated by Barberi et al. (1973b) with a series of variation diagrams for major elements.