Definition of Lahars of Nevado del Ruiz, Colombia, 1985

Lahars are volcanic mudflows that form on the slopes of many

active volcanoes and can rush downhill at up to 40 miles per hour

(64 km/hr) causing widespread death and destruction. Lahars may

form after explosive eruptions deposit thick accumulations of volcanic

ash and debris on a volcano’s slopes. When the ash mixes

with heavy rain, or water derived from melted ice and snow, the

ash and water mixture forms a material with a consistency of wet

concrete that begins to flow downhill. The mixture of ash and water

is typically about 40 percent ash and rock fragments, and 60 percent

water. This mixture can flow with speeds greater than natural

streams, up to 40 miles per hour (64 km/hr), and travel large distances,

typically about 30–60 miles (50–100 km). Since these flows

are denser and faster than water, they are much more destructive

than normal floods. Additionally, when the lahar event is over it

leaves behind a thick mud and debris layer that hardens and

entombs all that was unfortunate enough to be caught in the flow.

Lahars have formed on many volcanoes. Some of the most

devastating lahars in recent history have been associated with

the Nevado del Ruiz volcano from the Andes. The Nevado del Ruiz

volcano in Colombia entered an active phase in November 1984

and began to show harmonic earthquake tremors on November

10, 1985. At 9:37 P.M. that night, a large eruption sent an ash cloud

several miles into the atmosphere, and this ash settled onto the

ice cap on top of the mountain. The warm ash, together with volcanic

steam, quickly melted large amounts of the ice, which

mixed with the ash and formed giant lahars (mudflows) crashing

down the east side of the mountain into the village of Chinchina,

killing 1,800 people. The eruption continued and melted more ice

that mixed with more ash and sent additional larger lahars westward.

Some of these lahars moved nearly 30 miles at almost 30

miles per hour (48 km at 48 km/hr), and under a thunderous roar

buried the town of Armero under 26 feet (8 m) of mud. Twentythree

thousand people died in Armero that night. Other volcanoes

also have produced disastrous lahars. For instance, since the

1991 eruption of Mount Pinatubo, the homes of more than 100,000

people have been destroyed by lahars. Lahar deposits filled the

channels of the Toutle River in Washington State after the 1980

eruption of Mount Saint Helens, causing much of the death and

destruction from that eruption.

These lahars could have been predicted, and the huge loss of

life prevented, if a program had been installed to map the deposits

of previous lahars, and map the topography showing where future

lahars might flow. With such warnings perhaps the towns of

Armero and Chinchina could have been moved to slightly higher

ground, saving tens of thousands of lives. However, even with such

warnings, many communities would choose to live with the risks

instead of accepting the costs and inconvenience of moving to a

higher location.

extra weight of the building adds extra stresses to the slope

that may be enough to initiate the collapse of the slope.

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