"After peaking in the first half of September, most indicators of activity have been at lower levels. The seismic event count (120 on 9 September) did not exceed 25 events/day. Long-period events, however, continued to occur at about the same rate as before. The most recent large ones were recorded on 5 October (duration ~35 seconds) and 11 October (~25 seconds). Harmonic tremor continued at intermediate levels, but was less stable in amplitude. Long-period and shallow seismic events tended to occur more frequently during periods of low tremor amplitude. Deformation, as measured by dry and electronic tilt, has diminished. September deformation rates were the highest since 1985, but, similar to dry-tilt data from October-November 1985, recent changes were fluctuating rather than uniform in character. The highest SO₂ emission rate during the report period was 7,200 t/d, measured on 18 September. As of 11 October, the rate was 1,000-2,000 t/d. Ashfall has been observed on many days. None of the analyzed samples were of juvenile origin.

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: H. Meyer, INGEOMINAS, Manizales.