Nevado del Ruiz 1985

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. On November 13, 1985, history repeated itself for the third time in 400 years, with another eruption and another deadly lahar racing down the River Lagunillas. This time, over 23,000 people were killed, including most of the residents of Armero. With proper planning, this tragedy could have been averted. Click volcanic hazards map for more information.
Nevado del Ruiz is the northernmost of several Colombian stratovolcanoes in the Andes Volcanic Chain of western South America. The Andean volcanic belt is generated by the eastward subduction of the Nazca oceanic plate beneath the South American continental plate. Typically, such stratovolcanoes generate explosive Plinian eruptions with associated pyroclastic flows that can melt snow and glaciers near the summit, thus producing devastating lahars.

(section was taken from: http://www.geology.sdsu.edu/how_volcanoes_work/Nevado.html)