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Lahars: Understanding Volcano Mudslides
| Article
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12257 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
2 / 1987 |
4,650 Words |
| Author
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Marc J. Defant, Robert H. Stewart, and Joanne L. Stewart
Marc J. Defant holds a doctorate in geology and is an
assistant professor at the University of South Florida. He
specialized in the geochemistry of volcanic eruptions and is
currently doing volcanic research in the Philippines, Greece,
and Panama. He was in Armero, Colombia, shortly after the
lahar devastated the city.
Robert H. Stewart holds a master's degree in geology and is a
research associate at the University of South Florida. His
specialties are engineering geology and volcanology. He has
worked for the last thirty-six years in Panama and Central
America.
Joanne L. Stewart graduated from Kansas State University in
physical science and geology. She was employed by the Panama
Canal for twenty-five years working in engineering and
structural geology. She spent the last eight years as a
supervisory geologist. |
The ash began to fall as gently as snow on Armero, Colombia, at about 5:30 P.M., November 13, 1985. An hour later, a torrential downpour mixed with the ash and the pungent smell of sulfur permeated the city. Early that evening, no one thought that Nevado del Ruiz, the Andes' northernmost volcano, had tragedy in store for them. (Earlier that year, after 140 years of dormancy, the 17,716-foot-high peak had come to life with small earthquakes and minor ash eruptions.)
When the earth rumbled at 9:09 P.M. from an explosive eruption that blew pyroclastic material - volcanic ash and rock - seventeen miles above the volcano, the memories began to stir anxiety in Armero. The area is called the Valley of Sorrow because in 1845 volcanic mud flows - lahars in the lexicon of geologists - from Nevado del Ruiz buried more than 1,000 people alive. The fields that the farmers cultivated consisted of rich topsoil from debris left behind from past lahars. In fact, Armero had been built on portions of these ancient lahars.
The heat from hot gases and ash that escaped form Nevado del Ruiz partially melted the large alpine glacier that had developed on the volcano from the buildup of snow during periods of volcanic quiescence. The water from the glacier mixed with the ash that had built up along the slopes of the mountain from previous eruptions. This ash-laden water roared down the slopes of the mountain, picking up momentum along with more ash and debris. At the base of Nevado del Ruiz, the thirty-foot-high wall of mud spread out over the town of Armero.
Almost 25,000 people died that night, most in the town of Armero. In terms of lives lost, it was the most devastating volcanic disaster since the 1902 eruption of Mt. Pelee in Martinique, where nearly 28,000 people were killed when a fiery cloud of gases and pyroclastic debris - a nuee ardente -swept over the city. Measured in terms of the loss of life, Armero ranks as the fourth worst volcanic disaster of all time, behind the eruptions of Tambora (in Indonesia in 1815, when 92,000 were killed), Krakatoa (Indonesia, 1883, 40,000 killed), and Pelee.
Lahar is an Indonesian word for volcanic mudflow. The need to understand lahars is imperative because of their destructive power. Lahars have resulted in large death tolls, particularly around the shores of the Pacific Ocean, especially in Japan (more than 12,000 killed), South America (more than 26,000 killed), Indonesia (more than 10,000 killed), and Central America (more than 1,500 killed). Volcanic disasters are among the most
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