Several ash emissions in July deposited 2 mm of lithic ash as far as 5 km from the crater. COSPEC measurements indicated an increase in SO₂ emission during the second half of the month (figure 15) with highest contents recorded on 18 July (5,560 t/d) and on 26 July (4,850 t/d).

Figure 15. July 1988 SO2 emissions from Ruiz, as measured by COSPEC.

Seismic activity also increased in July. The highest energy earthquake swarm in July occurred at the beginning of the month near Nevado Santa Isabel, ~5 km SW of the volcano. At month's end, most activity occurred 2 km SW of Arenas crater at depths of 0.5-5 km. Short-duration tremor was also measured. Only minor deformation changes were detected.

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: M. Calvache, J. Patiño, and C. Carvajal, INGEOMINAS, Manizales.