mountain and the existence of cleared channels through which future lahars could move quickly indicated that the risk imposed by additional lahars could be high. Geological and seismological data on the activities of the volcano gathered in late 1985 and early 1986 at the Manizales Observatory also indicated that the volcano had a very high probability of another eruption in the then not-too-distant future, although the magnitude and timing of such an eruption were clearly unknown.

The possibility of further eruption of the volcano created the potential for a second major disaster with an even higher death toll than the 1985 incident. Using population and other data from township and risk maps, the study team estimated in 1986 that 50,000-80,000 lives could conceivably be at risk should the volcano erupt again. For example, segments of the population in the towns of Honda, Mariquita, Ambalema, Chinchina, Herveo, Villa Hermosa, Salgar, and La Dorada were all at risk. Some of these towns were close enough to potential lahar tracks to be struck within about one-half hour after an eruption; others would not be reached by the lahars for three and one-half hours or so; still others lie in between these two estimates.

The danger posed by a post-1985 eruption of Nevado del Ruiz resulted in a major effort in the state of Tolima to define future risk, to generate useful public information about this risk, and to undertake emergency preparedness and mitigation measures. The potential zone of ashfall, for example, had been estimated to be 10 km in radius. Since no real warning would be possible for those in this impact zone, the government endeavored to relocate the 5,000 or so people who resided there. In addition, INGEOMINAS has sought to research and document the last major eruption of the volcano (Espinosa, 1986), and a simplified map of volcano/lahar risk ( Figure 4.1 ), prepared before the November 13, 1985 eruption, was widely disseminated in towns at risk. This was an attempt to illustrate danger zones and to serve as a basis for enhancing public awareness and for encouraging preparedness and emergency planning for the many areas at risk that are not slated for relocation.

In the wake of the Armero disaster, Colombia established a plan for the warning and evacuation of threatened areas in the event of another eruption of Nevado del Ruiz. The basis for this planning effort was a 1985 United Nations guidance manual entitled Volcanic Emergency Management (UNDRO, 1985). This text outlined topical planning areas, including time-phased response, identification of hazard zones, population and property census, identification of safe transit points and refuge zones, evacuation route identification, means of transport, refugee accommodation, rescue and medical