The eruption caused the glaciers on the mountain to melt, sending massive lahars (volcanically induced mud and landslides) rushing into the valley below. The slide reached speeds of around 40 kilometers an hour. It crashed into the town of Chinchiná and Armero, killing around 23,000 people and burying the landscape in about 30 feet of mud and ash.
The composition of the mud, made it impossible to use heavy machinery to help dig out survivors.
Failure by the Colombia government to order the evacuation suggested by vulcanologists was cited as the cause for most of the deaths. Information was poorly distributed and the common man was not aware of the danger.
The Nevado del Ruiz is still erupting on occasion. In fact, while we were in Colombia this summer, our flight to Cali took extra time as we needed to take a different route because the volcano was erupting. There is still a threat of lahars and there are still people in the path of possible distruction.
Here is a video about the event, but be warned, it was a tragedy and some of the footage might be disturbing for some.