Cerro Galan is the first major caldera to be described from South America, and is one of the largest such structures in the world. Formation of the caldera was preceded by rifting and construction of andesitic composite volcanoes, and followed by further rifting and minor basaltic volcanism. Ignimbrite sheets erupted from the caldera cover over more than 3,500 km2 and have apparently climbed about 300 m up opposing slopes. The caldera is strikingly similar in morphology, geological evolution and tectonic setting to the classic Valles caldera of New Mexico. There is a clear relationship between block faulting (rifting), contruction of ignimbrite shields, caldera formation and alkalic volcanism in both North and South America, which may be related to extensional tectonics in a back arc environment.