Like many other Andean volcanoes, Nevado del Ruiz is a stratovolcano: a voluminous, roughly conical volcano consisting of many strata of hardened lava and tephra including volcanic ash. Its lavas are andesitic–dacitic in composition. The modern volcanic cone comprises five lava domes, all constructed within the caldera of an ancestral Ruiz volcano: Nevado El Cisne, Alto de la Laguna, La Olleta, Alto la Pirana, and Alto de Santano. It covers an area of more than , stretching from east to west. The mountain's broad summit includes the Arenas crater, which is in diameter and deep. Nevado del Ruiz, as its neighbours to the southwest Nevado El Cisne and Nevado de Santa Isabel is located over the Palestina Fault that crosscuts the underlying El Bosque Batholith, dated at 49.1 ± 1.7 Ma.