Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 6-12 December seismicity at Nevado del Ruiz was characterized by a decrease in the number and magnitude of earthquakes compared to the previous week. Significant amounts of water vapor and gas rose from the crater. On 7 December a low-energy thermal anomaly near Arenas Crater was detected by the MIROVA system. On 9 December gas and water vapor plumes sometimes containing ash rose 2 km above the crater rim and drifted between SW and NW directions. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

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)