The impact of volcanic hazards depends on their predictability
...through to loss of life. For example in cases where the volcanic activity is unpredictable such in Nevado del Ruiz, an eruption on November 13, 1985, generated pyroclastic flows melting ice and snow at the summit. This melt formed lahars that rushed down several river valleys traveling more than 100km, destroying many towns and houses such as the town of Armero which was completely covered by debris. Approximately 21,000 people (out of 28,700) died in Armero. The eruption caused an estimated 25,000 deaths, 5,000 injuries and destroyed more than 5,000 homes. In short the people of Nevado del Ruiz were not ready for this kind of hazard. Their lack of adequate technology to predict the eruption and speed of eruption meant that the impacts were greatly increased, and even if they did have some indication that an eruption was likely, it was not deemed certain enough to require evacuation of people or put into action possible responses to the hazard. The opposite is true of the eruption at Pinatubo in 1991, in which many volcanic predictions systems had b...