Nevado del Ruiz volcano in central Colombia, 130 km WNW of Bogota, is a broad, glacier covered volcano. A relatively small eruption in 1985 caused a devastating mud flow that killed almost 25,000 people in the town of Armero, marking one of the worst volcanic disasters in history. Sadly, this tragedy could have been easily avoided if clear warnings by volcanologists had been taken seriously.

Nevado del Ruiz covers more 200 sq km and it is composed by 3 major andesitic and dacitic edificed of lavas and tuffs. The present-day cone is composed by lava domes built within the summit caldera of the older Ruiz volcano. Its summit contains the 1-km-wide and 240-m-deep Arenas crater.

The prominent La Olleta flank cone on the SW flank was probably built in historic time. Nevado del Ruiz has repeatedly produced large mud flows (lahars), triggered by melting of the summit glacier during eruptions. Deposits of these mud flows are found on its broad flanks and these events have been known since the 16th century.