Nevado del Ruiz is a broad, icy stratovolcano that covers more than . Three major parts, made of andesitic and dacitic lavas and andesitic pyroclastics, have built up since the beginning of the Pleistocene. The cone consists of a wide cluster of lava domes produced within the summit caldera. The 1-kilometer wide, Arenas crater is the summit. The prominent La Olleta pyroclastic cone is on the volcano's southwest flank and may also have been active in historical times. The mountain's flanks are lined by steep and massive landslides. Melting of its summit icecap during historical eruptions resulted in devastating lahars, including the famous eruption in 1985 that was the world's deadliest. There was an eruption in 2016.