Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that the eruption at Nevado del Ruiz continued at low-to-moderate levels during 26 September-2 October. Seismicity indicating the movement of fluids was similar to the week before. The number of signals indicating rock fracturing significantly increased during 28-29 September. These events were located in areas 1-4 km S and SSE of Arenas Crater at depths of 2-5 km. Earthquakes recorded at 0512 (M 3.5) and 0614 (M 3.8) on 28 September were felt by residents. Rock-fracturing earthquakes were at low levels on the other days. Seismicity was generally low. Several thermal anomalies in the crater were identified in satellite images. Ash-and-gas emissions continued during the week, with the highest plumes rising to 2 km above the crater rim on 30 September. Plumes drifted NW, W, and SW; ashfall was reported in Manizales (27 km NW) on 30 September. The Alert Level remained at Yellow, Level III (the second level on a four-level 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)