Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that eruptive activity at Nevado del Ruiz continued during 13-19 August. The number of seismic events associated with fluid movement increased in both magnitude and number compared to the previous week but were variable throughout the week. Through webcams several ash emissions associated with these events were confirmed along with the occasional ejection of hotter material. Ashfall was reported in the municipalities of Villamaría (28 km NW) and Manizales (28 km NW), especially during 17-18 August. Seismicity associated with rock fracturing decreased in both the number of events and their magnitudes compared to the previous week; these earthquakes, with depths ranging from less than 1 km down to 8 km below the summit, were primarily located beneath the NW, ESE, and SE flanks of Arenas Crater and within 8 km N and NE of the crater. The largest event was an M 1 which was detected at 1747 on 13 August and located about 8 km ESE and 5 km deep. Sulfur dioxide emissions were variable, and gas plumes rose as high as 1.2 km above the crater rim and drifted NW, W, and WSW. Thermal anomalies on the crater floor were identified in satellite data, though weather conditions often obscured views. The Alert Level remained at Yellow (the second level on a four-level scale), and the public was warned to stay out of the restricted areas around Arenas Crater, not to spend long periods of time on the Murillo-Cerro Gualí Road, and to avoid drainages in the high-threat zones.

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)