The 1985 eruption of the Nevado del Ruiz volcano in Colombia caught nearby towns unaware despite warnings, sending pyroclastic flows and lahars down its slopes at 50 km/h that melted glaciers and picked up speed. The lahars engulfed the town of Armero, killing over 20,000 of its almost 29,000 inhabitants in one of the worst volcanic disasters in history.

2.  The Armero tragedy was one of the major
consequences of the eruption of the Nevado del Ruiz
stratovolcano in Tolima, Colombia, on November 13,
1985. After 69 years of dormancy, the volcano's eruption
caught nearby towns unaware, even though the
government had received warnings from multiple
volcanological organizations to evacuate the area when
volcanic activity had been detected in September 1985

3.  As pyroclastic flows erupted from the volcano's crater,
they melted the mountain's glaciers, sending four
enormous lahars (volcanically induced mudslides,
landslides, and debris flows) down its slopes at 50
kilometers per hour (30 miles per hour). The lahars
picked up speed in gullies and coursed into the six
major rivers at the base of the volcano; they engulfed
the town of Armero, killing more than 20,000 of its
almost 29,000 inhabitants.