Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 18-24 October seismicity at Nevado del Ruiz was characterized by a slight decrease in the number and magnitude of earthquakes compared to the previous week. Some seismic signals were associated with gas-and-ash emissions which were confirmed by webcam images, and Parque Nacional Natural los Nevados (PNNN) and SGC officials. A steam, gas, and ash plume rose 2 km above the crater and drifted W on 18 October. Two low-energy thermal anomalies were detected on 18 and 20 October. The Washington VAAC reported that on 19 and 20 October ash plumes rose to altitudes of 7.3 and 6.1 km (24,000 and 20,000 ft) a.s.l. and drifted NW and SW, respectively. 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.

Sources: Servicio Geológico Colombiano (SGC), Washington Volcanic Ash Advisory Center (VAAC)