Since a strong seismic crisis in July, seismicity has remained at elevated levels, . . . . Minor ash emission has occasionally been seen since August. During November, high- and low-frequency earthquake activity increased. The number of high-frequency events rose slightly, to 330 . . ., and low-frequency events increased to 933 . . ., but energy release was relatively low. Shallow (explosion) seismicity declined to 111 recorded shocks. . . . No ash was emitted and deformation measurements showed low to moderate changes. The average rate of SO₂ emission was ~1,500 t/d. Occasional minor ash emission has occurred since . . .ash emission in June.

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.

Information Contacts: E. Parra, INGEOMINAS, Manizales.