The Concepcion de Ataco caldera in the Ahuachapan-Chipilapa region is part of the southern, Plio-Pleistocene volcanic belt that borders the Central Graben of El Salvador. About 17 km(3) of primitive lavas erupted between 1.7 and 0.77 Ma, covering basaltic-andesitic lavas and older agglomerates forming the Cuyanausul, Apaneca and Empalizada volcanoes. These lavas preceded the evacuation of 63 km(3) of silicic pyroclastics (i.e. breccias, surge deposits, and ignimbrites) associated with the main caldera cycle. The eruption of this pyroclastic material led to the collapse of a 5 km by 3.5 km caldera between the times of the youngest pre-caldera eruptions (0.77 Ma) and the intrusion of the Himalaya dome (0.28 Ma). The caldera collapse was reactivated at its northeastern flank by a new eruption of pyroclastics of limited areal distribution. These distinctive pyroclastics correspond to laminated tuffs (＇＇Cebra tuffs＇＇). Domes of dacitic-to-andesitic composition erupted along the caldera border as well as within it. Their magmatism is calc-alkaline; their ages vary between 0.28 and 0.1 Ma. Three phreatic explosions, whose craters are in the eastern part of the Concepcion de Ataco caldera structure, only deposited debris locally. A hydrothermal system was emplaced during the final stage of the post-magmatic activity, evidenced by areas of hydrothermal alteration and surface manifestations. Four main fault systems, oriented NE-SW, NW-SE, NNW-SSE, and semicircular, have been identified. These faults created relatively high-permeability regions within the caldera and nearby grabens, that are favorable for the development of geothermal systems.