INGEOMINAS reported a gradual increase in seismicity from Nevado del Ruiz on 30 September. Earthquakes were located beneath the Arenas crater at depths of 0.5-2 km. The largest earthquake was M 1.9. A plume of white gas rose 700 m above the caldera and a sulfur odor around the volcano was reported. The Alert Level was raised to III (Yellow; "changes in the behavior of volcanic activity") on 1 October. The report also noted changes in deformation and geochemistry during the previous few months. Seismic levels fluctuated during 2-3 October.

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)