According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 10-13 August low levels of tremor were detected at Nevado del Ruiz, possibly associated with continuing gas and ash emissions. On 12 August a total of 140 low-magnitude earthquakes (M < 1.8) were detected in a seismic swarm that began at 0956 and ended at 1800. The earthquakes were located about 4 km WSW of Arenas Crater at depths of less than 5 km. A gas-and-ash plume observed with a web camera rose 1 km above the crater and drifted W. Ashfall was reported in Brisas (50 km SW). Satellite images showed continuing sulfur dioxide emissions. On 13 August a seismic swarm was characterized by events less than M 1, and located NE of Arenas Crater at depths between 3 and 5 km. A thin layer of ash was deposited at the observatory in Manizales. Weather conditions prevented observations of the volcano. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

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)