El Rojo Sur is a monogenetic volcano in the Andes, in the form of a cone. It is similar to El Rojo Norte farther north. A mafic andesite lava flow was erupted from the high cone that also features red scoria. Ages obtained by potassium-argon dating are 2.93±0.13, 3.4±0.4 and 3.23±0.12 mya. Rojo Sur erupted undifferentiated mafic magmas. Its lead isotope composition is different from the composition of that segment of the Andes, being non-radiogenic. This isotope signature may be the effect of assimilation of deeper basement rocks during magma formation. In addition, the magmas are rich in incompatible elements. References Pliocene volcanism Monogenetic volcanoes Volcanoes of Arica y Parinacota Region Cinder cones of Chile