Based on an ASHTAM (a special Notice to Airmen for airborne ash), the Washington VAAC reported a possible emission from Nevado del Ruiz on 28 March. Ash was not identified in satellite images, though a thermal anomaly was detected. A period of increased seismicity was detected the next day, and observers reported another possible ash emission.

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: Washington Volcanic Ash Advisory Center (VAAC)