Small, brief ash emissions similar to those of July continued during August. Seismic energy release and the number of low-frequency earthquakes increased slightly (figure 16). Some clusters of high-frequency events occurred, especially ~2 km NE and SW and 4 km SE of the crater. No significant deformation was recorded. SO₂ emissions (figure 17) averaged 2,139 t/d.

Figure 16. Number of seismic events (high- and low-frequency)/day, June 1985-mid-September 1988, courtesy of the Observatorio Vulcanológico de Colombia. The 11 September and 13 November 1985 eruptions are indicated by arrows.

Figure 17. Daily SO2 flux as measured by COSPEC, mid 1986-early September 1988. Courtesy of the Observatorio Vulcanológico de Colombia.

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: C. Carvajal, INGEOMINAS, Manizales.