 Calabozos and the majority of the Andean volcanoes formed from the subduction of the oceanic Nazca Plate under the continental South American continental lithosphere . The caldera is in a transitional region between thick and thin lithosphere , and is probably supplied by a pool of andesitic and rhyolitic magma . It sits on a historic bed of volcanic and plutonic sedimentary rock ( rock formed within the Earth ) that in turn sits on top of a layer of merged sedimentary and metamorphic rock . 
