After a series of magnitude-4 earthquakes on 22 December [1984] marked the onset of stronger seismicity, 25-30 events were felt each month until an episode of strong phreatic activity on 11 September. Seismologists have located about 100 earthquakes, centered 1-2 km NW of the active crater at average depths of 0-2 km below sea level. Increased thermal and phreatic activity began in early 1985 and continued after the 11 September episode into early October. No juvenile material has been recognized in the 1985 tephra.

The vigorous 11 September phreatic activity began at 1330 from the summit crater (Arenas) and had ended by the next morning. Ashfall was 1 cm or less near the volcano and a trace of ash reached Manizales and Chinchiná. Lithic blocks were deposited on snowfields as much as 2 km from the crater. Thunderous detonations and summit-area lightning accompanied the activity. A small- to moderate-sized lahar began at 1830, advancing 27 km down the valley of the NE flank's Río Azufrado, from 4,700 m to 3,000 m altitude. As it traveled at an estimated 10-30 km per hour, the lahar left the river channel at various locations, particularly along curves, and rose as high as 10-20 m up canyon walls. Valley residents were placed on alert but have not been evacuated. A hazard map has been prepared by the international team studying the volcano and distributed to Red Cross and Civil Defense officials.

For 5 days preceding the 11 September activity, seismographs registered a very regular pattern consisting of 15 minutes of strong high-frequency tremor every hour. Although similar seismicity was recorded during three other periods of a day or less in August and September, none of these episodes was as intense or long-lasting.

Phreatic activity was continuous for the rest of September, emitting variable amounts of ash, typically darkening the snow to a few kilometers from the crater. Heavier emissions occurred on 23, 24, and 29 September, producing dense dark plumes that deposited trace ashfalls more than 10 km from the crater and lithic blocks on nearby snowfields. Activity had declined by early October. A steam plume 1-3 km high was visible daily but contained no obvious ash. Seismographs recorded 5-10 microseismic events per day and an irregular harmonic tremor that may have been related to the steam plume. Geologists visited the crater at the beginning of October, noting a slight decline in fumarolic activity and little ash emission. Glaciers seemed unchanged since September.

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: L. Jaramillo, INGEOMINAS, Bogotá; A. Rivera, Univ. de Caldas, Manizales; G. Duque, Univ. Nacional, Manizales; A. César, Central Hidroeléctrica de Caldas; A. Solano, Univ. Nacional de Colombia, Bogotá; M. Hall, Escuela Politécnica, Ecuador; D. Herd, USGS, Reston, VA.