The caldera Concepcion de Ataco in the region of Ahuachapan- Chipilapa represents part of the southern, Plio-Quaternary volcanic belt of Salvador, which one of the borders of the Central Graben of alkaline lavas were emitted above a basalticandesitic basement and older agglomerates, forming the volcanos Cuyanausul, Apaneca and Empalizada, which preceeded the evacuation of 70 of silicic pyroclastics The pyroclastic sequence is formed by breccias, surges, and ignimbrites Flows occupy the major part with a lateral distribution of 27 I The most distal pumice and ash fall deposits are distributed over an area of 484 The release of this material caused a caldera collapse with a dimension of km x kin. The river carves its bed into this depression The caldera collapse was reactivated at its NE-tlank by a new evacuation, emitting pyroclastics with limited distribution. The pyroclastics correspond to laminated tuff (.Zebra or brown tuff Dome injections of dacitic to andesitic composition partly border the collapses and the internal parts of the caldera The magmatism shows characteristics of a calcalkaline series, by an active continental margin The age varies between Middle Pleistocene and Holocene Three craters of phreatic explosion type. which are mapped in the east of the caldera structure, released debris locally This activity corresponds to a postmagmatic hydrothermal phase. representing the first superficial occurrence of the emplacement of an active hydrothermal system in this region

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