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 km/h. 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.
After the fatal eruption, volcanologists of the U.S. Geological Survey were dispatched to Colombia, to quickly establish a seismic and tiltmeter network at the volcano to help Colombian scientists assess the likelihood of future eruptions and lahars. At the same time, a painstaking search began for the circumstances that led to the disaster.
It soon became clear that no single factor was responsible for the disaster. Contributing factors were a lack of a timely hazards evaluation (a hazard map took nearly a year to complete after the first signs of volcanic unrest and was available for distribution only days before the eruption), an inadequate monitoring system at the volcano, and ineffective procedures for communicating information and making decisions during the emergency. In hindsight, the disaster at Nevado del Ruiz could have been prevented.
The realization that disasters like that at Nevado del Ruiz might be prevented launched the Volcano Disaster Assistance Program (VDAP) in August 1986. With support from USAID through its Office of Foreign Disaster Assistance, the U.S. Geological Survey created VDAP to assist developing countries during volcanic crises. During its short existence, VDAP has assisted Ecuador, Colombia, Guatemala, the Philippines, and other countries to reduce the loss of life and property from volcanic eruptions and to prepare for future volcanic crises. The successful response to the 1991 eruption of Mount Pinatubo in the Philippines stands as VDAP's most extraordinary contribution to volcano-hazard mitigation.
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