Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that a volcanic tremor signal at 0214 on 18 December may have been associated with an ash emission from Nevado del Ruiz, although dark conditions prevented visual confirmation. Volcanic tremor which began at 1247 on 20 December was associated with an ash emission observed by numerous people both near the volcano and in Manizales (30 km NW). Many also reported a strong sulfur odor. The permanent DOAS (Differential Optical Absorption Spectrometer) station detected the highest levels of sulfur dioxide measured since the current activity began. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

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)