Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that eruptive activity at Nevado del Ruiz continued during 24-30 September. The number of seismic events associated with fluid movement increased in both magnitude and number compared to the previous week but were variable between low and moderate levels throughout the week. Webcam images confirmed ash emissions associated with some of these events along with occasional ejections of hotter material. Ash plumes rose as high as 2 km above the crater rim and drifted NW and WNW during the first half of the week and N, E, and SE during the second half of the week. Ashfall was reported in Cabaña de Brisas (NW) and in areas to the E around the Lagunilla River. Seismicity associated with rock fracturing increased in the number of events, though their magnitudes were comparable to the previous week; these earthquakes, with depths ranging from less than 1 km down to 8 km below the summit, were primarily located within 4 km SSW, S, SE, and SE of Arenas Crater. The largest event was an M 1 which was detected at 2131 on 28 September and located about 2 km SSE and 4 km deep. Sulfur dioxide emissions were variable, and drift directions were not towards measuring stations; satellite measurements indicated increased emissions and a high value on 29 September that had not been recorded since February 2023. Low-to-moderate thermal anomalies on the crater floor were identified in satellite data. 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)