On 1 November Servicio Geológico Colombiano’s (SGC) reported that during the previous week seismicity at Nevado del Ruiz was variable and indicated continuing growth of the lava dome in Arenas Crater. The number and size of seismic signals indicating fluid movement had decreased compared to the week before. Some of the signals were associated with gas-and-ash emissions. Although seismic signals indicating rock fracturing decreased in number, magnitudes were higher previous week. The hypocenters were located 0.2-7 km deep, mainly beneath the NE and SW parts of Arenas Crater, and beneath the E, SE, and NW flanks. The largest event was a local M 1.8 that was recorded at 0518 on 28 October at a depth of 1.8 km below the crater. Several low-to-moderate thermal anomalies in Arenas Crater were identified in satellite images and minor changes to the dome and crater floor were visible. Plumes of steam and gas (mainly sulfur dioxide) continued to be emitted, rising as high as 1.8 km above the summit on 31 October and drifting SE, ESE, and ENE. The Alert Level remained at 3 (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.

Source: Servicio Geológico Colombiano (SGC)