The eruption was small, but it melted part of the volcano's ice cap. Mudflows called lahars raced down the river valleys and reached Armero, about 50 kilometres away, a few hours later.
Scientists had warned of the danger for weeks, and a hazard map showed the town at risk, but no evacuation was ordered. Photos of Omayra Sánchez, a 13-year-old girl trapped in the debris for three days, made the disaster known worldwide.
Source: Wikipedia