A press release on 31 March noted that small long-period earthquakes had been detected at Ruiz throughout the month, although some may have been related to glacier movement; one long-period event on 24 March saturated the seismic stations near the crater. After several months of low seismicity, a moderate swarm of 80 volcanic-tectonic earthquakes within an hour was recorded on 15 April. The largest had a magnitude of 1.3. Small long-period earthquakes were present during the entire month of April, centered on the SW flank near the crater. Seismicity was still at low levels as of 25 May.

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: INGEOMINAS, Volcanological and Seismological Observatory of Manizales, Avenida 12 de Octubre No. 15 - 47, Manizales, Colombia (URL: http://www.umanizales.edu.co/~uom/).