According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that on 22 May a seismic signal possibly indicated an ash emission from Nevado del Ruiz, though it was not confirmed due to poor weather conditions. On 29 May activity significantly increased; at 0307 seismic signals indicated ash emissions that were confirmed by officials and residents near the volcano as well as with a web camera. The Alert Level was raised to II (Orange; "eruption likely within days or weeks"). A gas-and-ash plume rose 1 km above the crater and ashfall was reported in Anserma (65 km NW), Aranzazu (45 km NNW), Chinchiná (30 km WNW), Dosquebradas (40 km W), Filadelfia, La Merced (60 km NNW), Manizales (30 km NW), Marmato (70 km NNW), Neira (37 km NW), Palestina (40 km WNW), Pereira (40 km WSW), Risaralda (78 km WNW), Salamina (60 km NNW), San José (56 km NW), Santagueda (40 km NW), Santa Rosa de Cabal (33 km W), Supia (72 km NNW), Villamaria (28 km NW), and Viterbo (65 km WNW). Ash also fell in all municipalities in the department of Risaralda (76 km WNW) and El Aguila (85 km W, N of Valle del Cauca). Sulfur dioxide plumes were detected by satellite and a sulfur dioxide odor was reported in multiple towns. Later that day ash emissions rose 600 m above the crater.

Geological Summary. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Source: Servicio Geológico Colombiano (SGC)