The Colombian volcano Nevado del Ruiz is an active stratovolcano with a history of generating deadly volcanic mudflows (lahars) from relatively small-volume eruptions.
In 1595, a lahar swept down the valleys of the River Guali and the River Lagunillas, killing 636 people.
In 1845, an immense lahar flooded the upper valley of the River Lagunillas, killing over 1000 people. It continued for 70 kilometers downstream before spreading across a plain in the lower valley floor. The young village of Armero was built directly on top of the 1845 mudflow deposit. Over the ensuing years, Armero grew into a vibrant town with over 27,000 residents.
The eruption was not very alarming, but the lava flow combined with melting snow took everything resulting in a high speed Lahar on his way down.
This time, over 23,000 people were killed, including most of the residents of Armero.
More resources :
an Diego State University about the Nevado el Ruis >>>>
BBC "on this day">>>
A video footage about Armero>>
The volcaono that changed the course of risk management >>>