The Colombian volcano Nevado del Ruiz is an active stratovolcano with a history of generating deadly volcanic mud flows (lahars) from relatively small-volume eruptions. In the past in 1595, a lahar swept down the valleys of the River Guali and the River Lagunillas, killing 636 people, before the eruption November 13th 1985.
The Nevado del Ruiz volcano is the highest of the Colombian volcanoes With a summit elevation of 5,389 m (over 17,500 ft), Nevado del Ruiz is the highest of the Colombian volcanoes. Even though it is located only ~500 km from the Earth's equator, its high summit is covered with ~25 square kilometers of snow and ice. Its name, Nevado, means "snow-capped". During the volcanic outbursts of 1595, 1845, and 1985, large volumes of melt water were derived from melting of the ice pack by hot pyroclastic flows erupting at the summit.
The location of the nevado del ruiz volcano in the northernmost of several Colombian stratovolcanoes in the Andes Volcanic Chain of western South America. The Andean volcanic belt is generated by the eastward sub-duction zone of the Nazca oceanic plate beneath the South American continental plate.
After nearly a year of minor earthquakes and steam explosions from Nevado del Ruiz, the volcano exploded violently on November 13, 1985. at 3:06. after 2 hours the ash started to spread around the city. the citizens of Amero remained calm. They were placed by reassuring messages from the mayor over radio, and from a local priest over the church public address system. next the Red Cross ordered an evacuation of the town at 7:00 p.m. However, shortly after the evacuation order the ash stopped falling and the evacuation was called off.
At 9:08 p.m., just as calm was being restored, molten rock began to erupt from the summit crater for the first time (all previous eruptions were steam explosions). The violent ejection of this molten rock generated hot pyroclastic flows and airfall tephra that began to melt the summit ice cap. Unfortunately, a storm obscured the summit area so that most citizens were unaware of the pyroclastic eruption. Traveling at 50 kilometers per hour, the largest of these burst through an upstream damn on the River Lagunillas and reached Armero two hours after the eruption began. Most of the town was swept away or buried in only a few short minutes, killing three quarters of the townspeople.
After the aftermath of the volcanothe government of Colombia created a special program to prevent such incidents in the future.