Lascar rests atop the Atana ignimbrite, a rhyodacitic sheet which was erupted by La Pacana caldera 4.5-3.7 million years ago. The Pampa Chamaca and Tuyajto ignimbrites are somewhat younger, 2.6-2.2 million and less than 1 million years respectively. These ignimbrites form a 3° steep slope in the area. Other basement rocks are the sandstone-containing marine Devonian-Carboniferous Lila formation, the red-orange Permian Cas formation containing volcanic rocks and granites, as well as the volcanic Permian-Triassic Peine formation and Cerro Negro strata, which also contain intruded rocks and lake sediments. These formations are not visible in the Lascar area, but they crop out close to the Salar de Atacama. Tertiary sediment and volcanic rocks can also be found. The presence of Mesozoic limestone is indicated by xenoliths in Lascar's lavas; the only place they crop out farther east is in Argentina. This limestone formation has been identified as the Yacoraite formation. Later deposits include the Cenozoic sedimentary Quepe strata. Landforms over this basement include ignimbrites, lava domes, and stratovolcanoes. Exposures of the basement are often delimited by faults.