Ash emission began 11 September at about 1300 and lasted up to 7 hours, accompanied by a persistent roaring noise and electrical discharges. A few millimeters of ash fell on the cities of Manizales and Chinchiná (about 30 km NW and WNW of the volcano) and ashfall was reported to have locally reached 2 cm. A mudslide blocked a road on the E flank of the volcano. Before the eruption, tremors were recorded at about 1.5-hour intervals by seismographs about 3 km N, S, E, and W of the volcano's summit. Tremor was continuous during the ashfall, punctuated by discrete earthquakes.

On 13 September, morphological changes in the glaciers on the W side of Ruiz were reported by ground and air observers. New fissures were also reported on the icecap N of and below the active crater near a small growing pond on the glacier surface.

Nevado del Ruiz is a broad shield-shaped volcano covering more than 200 km², composed of an extensive sequence of hypersthene-augite andesite, hornblende andesite, basaltic andesite, and dacite. Its 1595 eruption produced ash and lapilli falls and lahars that spilled down the Río Gualí and Río Lagunillas Valleys N and E of the volcano. Eruptions were reported in 1828 and 1829, but no descriptions exist. Apparently these events produced no tephra. Ruiz is also reported to have been "smoking" in 1831 and 1833 (Herd, 1982).

Reference. Herd, D.G., 1982, Glacial and volcanic geology of the Ruiz-Tolima Volcanic Complex, Cordillera Central, Colombia: Publicaciones Geológicas Especiales del INGEOMINAS, no. 8, 48 p.

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: B. Salazar A., Central Hidroeléctrica de Caldas; Germán Mejía, Univ. Nacional, Manizales.