The Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that eruptive activity at Nevado del Ruiz continued during 3-9 December. The number of seismic events associated with fluid movement decreased in number but increased in magnitude compared to the previous week. Additionally, the number of low-level, long-duration events decreased. Webcam images confirmed that higher-temperature ejecta was sometimes associated with seismic signals indicating pulsating ash emissions that rose as high as 2.5 km above the crater rim. A period of continuous ash emissions began on 29 November and ceased during the morning of 3 December. Seismicity associated with rock fracturing remained similar in number but slightly increased in magnitude compared to the previous week. The earthquakes were located below Arenas Crater, and mainly beneath the NE and SE flanks within a distance of 6 km, and had depths of 1-8 km. The largest earthquake, a M 2.5, was recorded at 0956 on 8 December located 2 km SE of the crater at a depth of 4 km. Low-to-moderate thermal anomalies on the crater floor were identified in satellite data when weather permitted. Gas plumes generally rose less than 1 km above the crater rim and drifted NW, W, and SW. Significant levels of sulfur dioxide emissions were detected in satellite data, similar to previous weeks. 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)