Seismic activity (high- and low-frequency earthquakes, long-period events, and tremor) significantly decreased in April, continuing a 2-month trend. SO₂ emissions measured by COSPEC varied between 700 and 3,700 t/d with a monthly average of 1,800 t/d (figure 26). No significant changes in deformation were measured.

Figure 26. Rates of SO2 emission measured by COSPEC at Ruiz, July 1986-April 1989. 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.