The eruption started at 15:00 o'clock local time November 13, 1985 with smaller explosions in the crater. Ash was carried by the wind in north-eastern direction, however only minor ash fall occurred in the city of Armero (Colombia), located 48 kilometers east of the "Cumanday" - the smoking nose, as the Indians used to call the volcano.
In the evening the intensity of the eruption increased, however it was still considered only a medium sized event for the Nevado del Ruiz. At 23:00 most of the 25.000 inhabitants of Armero were sleeping, despite some preoccupation for the sounds coming from the distant mountain, then suddenly - as an eyewitness describes - "the world screamed."
The underground was trembling and a terrible roar followed. A mixture of water and debris overwhelmed the entire city. In 20 to 30 minutes three or four lahars (mudflows of volcanic material) occurred. Some survivors later reported that the first waves were formed by cold mud, followed by waves of hot mud. Nearly 22.000 people were killed by these mudflows that coming from the summit of the volcano followed the river valley of Lagunillas until reaching Armero.
Video 1. Video produced by the World Health Organization to document the aftermath and rescue operations at Armero (the video shows victims and injured people - viewers discretion is advised).
The volcanic nature of the Nevado del Ruiz (5.389m) was already known in ancient times as the indigenous name reveals. The volcano erupted in historic times in the years 1595 and 1845; and already during this last eruption a mudflow killed 1.000 people in the Lagunillas valley.
Before 1985 the presence of vapour on the mountain summit was noted and a small glacial lake filled the bottom of the crater. In November 1984 an intense earthquake activity started, probably marking the slow rise of magma inside the volcano. In the last year, between December 1984 and September 1985, the volcanic activity steadily increased. The relatively small eruption from November 1985 melted partially the glaciers covering the Nevado del Ruiz (estimated 10% of the ice-area), forming the lahars that had such disastrous effects in the valleys surrounding the volcano.
After the catastrophe an intense debate about responsibility began, the compiled hazard map (one of the first in Colombia) of the entire area got widely ignored and misinterpreted, as were the signals of activity by the volcano. The authorities were informed previously that Armero was located on the deposits of the lahar of 1845 and a similar disaster could occur again even by minor eruption. After the disaster the government of Colombia created a special program to prevent such incidents in the future.
DECKER, R. & DECKER, B. (1991): Mountains of Fire: The Nature of Volcanoes. Cambridge University Press. Cambridge: 243