DESCRIPTION:
Nevado del Ruiz Eruption and Lahar, 1985

[Image,36K,JPG]
Armero, Colombia, destroyed by lahar on November 13, 1985.More than 23,000 people were killed in Armero when lahars (volcanic debris flows) swept down from the erupting Nevado del Ruiz volcano. When the volcano became restless in 1984, no team of volcanologists existed that could rush to the scene of such an emergency. However, less than a year later, the U.S. Geological Survey organized a team and a portable volcano observatory that could be quickly dispatched to an awakening volcano anywhere in the world.
-- USGS Photo by R. J. Janda, 1985

When the seismograph began to record the violent earth-shaking caused by yet another eruption of the Nevado del Ruiz volcano in Colombia, no one thought that a few hours later more than 23,000 people would be dead, killed by lahars (volcanic debris flows) in towns and villages several tens of kilometers away from the volcano. Before the fatal eruption the volcano was being monitored by scientists at a seismic station located 9 kilometers from the summit, and information about the volcano's activity was being sent to Colombian emergency-response coordinators who were charged with alerting the public of the danger from the active volcano. Furthermore, areas known to be in the pathways of lahars had already been identified on maps, and communities at risk had been told of their precarious locations.

Unfortunately, a storm on November 13, 1985, obscured the glacier-clad summit of Nevado del Ruiz. On that night an explosive eruption tore through the summit and spewed approximately 20 million cubic meters of hot ash and rocks across the snow-covered glacier. These materials were transported across the snow pack by 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, and created large volumes of water that swept down canyons leading away from the summit. As these floods of water descended the volcano, they picked up loose debris and soil from the canyon floors and walls, growing both in volume and density, to form hot lahars. In the river valleys farther down the volcano's flanks, the lahars were as much as 40 meters thick and traveled at velocities as fast as 50 kilometers per hour. Two and a half hours after the start of the eruption one of the lahars reached Armero, 74 kilometers from the explosion crater. In a few short minutes most of the town was swept away or buried in a torrent of mud and boulders, and three quarters of the townspeople perished.

... the eruption at Nevado del Ruiz in November 1985 ejected a very small amount of magma -- only about 3 percent of that erupted at Mount St. Helens. Yet, this tiny eruption generaged high-volume debris flows that killed more than 23,000 people. ...

The 1985 Ruiz eruption offers another, much more tragic example of the need to understand the entire history of a volcano in assessing hazards. The town of Armero, Colombia -- buried by mudflows triggered by the 1985 eruption at Nevado del Ruiz -- was located on a debris fan that was overrun by destructive mudflows in the year 1595, shortly after the arrival of the Spanish colonists, and again in 1845, killing hundreds of people in each instance. During the ensuing 140-year period of inactivity, people forgot and the town was rebuilt at the same site and grew in population. Although a preliminary hazard-zone map for Ruiz, completed one month before the November 1985 eruption, clearly delineated Armero as being especially vulnerable to mudflows, emergency-response measures taken during the eruption were entirely inadequate to save the more than 23,000 lives lost when the mudflows struck.

Map [35K,InlineGIF]
Map showing hazards expected from an eruption of Nevado del Ruiz, Colombia. Such a map was prepared by INGEOMINAS (Colombian Institute of Geology and Mines) and circulated one month prior to the November 13, 1985, eruption of Nevado del Ruiz. Map shows danger from mudflows in the valley occupied by the town of Armero, Colombia, as well as areas affected by the hazards that resulted from this eruption. Circle denotes 20-kilometer limit.
-- Modified from: Wright and Pierson, 1992, USGS Circular 1073

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