Teachers Guide to Stratovolcanoes of the World
Eruption Feature

Nevado del Ruiz, Colombia:
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Lahars

Lahars are debris flows and/or mud flows produced by loose soil and rock flowing down the sides of a volcano. The name comes from the Indonesian term where lahars are common volcanic hazards. Nevado del Ruiz generated killer mud flows in 1595 shortly after the arrival of Spanish colonists and again in 1845. Hundreds died in settlements located at the site of Armero. However, after the 1845 eruption Nevado del Ruiz was quiet for 140 years. People forgot about the destruction and devastation and Armero was built on the same site, growing to a town of 30,000 by 1980. The 1985 lahar began when twenty million cubic meters of hot ash and rocks erupted from the vent of the volcano and rained down or flowed across the ice and snow-covered summit. The hot ash and gases moved across the snow pack in avalanches of hot volcanic debris (pyroclastic flows) and fast-moving, hot, turbulent clouds of gas and ash (pyroclastic surges). The hot pyroclastic flows and surges caused rapid melting of the snow and ice. This released a lot of water and debris that swept down canyons from the summit. The mud flows gained volume and density until they were as much as 40 m (132 ft) thick, traveling at speeds of up to 50 km/hr (30 mi/hr).

Scientists were monitoring Nevado del Ruiz at the time of the eruption. Likely lahar pathways were identified on maps, and nearby communities were warned of the danger. The November 13, 1985 eruption took place during night. Although Armero was 74 km (44.5 mi) from the crater of Nevado del Ruiz, it took the lahar only two and a half hours to reach the village. A warning did not reach the people in time. The tremendous death toll at Nevado del Ruiz inspired a new respect for volcanic sites around the world where fire and ice meet in a deadly combination.