Nevado del Ruiz, the highest active volcano in the Andes Mountains of Colombia, suffers a mild eruption that generates a series of lava flows and surges over the volcano's broad ice-covered summit. Flowing mixtures of water, ice, pumice, and other rock debris poured off the summit and sides of the volcano, forming "lahars" that flooded into the river valleys surrounding Ruiz. The lahars joined normal river channels, and massive flooding and mudslides was exacerbated by heavy rain. Within four hours of the eruption, the lahars traveled over 60 miles, killing more than 23,000 people, injuring over 5,000, and destroying more than 5,000 homes. Hardest hit was the town of Armero, where three quarters of the 28,700 inhabitants died.
The volcano first began showing signs of an imminent eruption a full year before, and most of the river valley's residents would have survived had they have moved to higher ground.