Land Subsidence
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LAHAR Richard B. Waitt U.S. Geological Survey, Vancouver, WA, USA
Synonyms Volcanic mudflow; Debris flow Definition A lahar is a flowing slurry of rock debris and water originating on the slopes of a volcano. The term may also mean the deposit of such a flow. Discussion Characteristics: Lahars contain grains from clay to large boulders. The flowing material is water-lubricated sand or mud, but the whole mixture maybe more than half of incorporated cobbles and boulders. Volumes can reach 109 m3 and peak discharge may exceed 107 m3/s. Depending on proportions of freshly erupted volcanic debris and snow or water, temperatures range from nearly 100 C to 0 C but are typically below 50 C. Flowing lahars peak swiftly and wane more slowly, passing any one place within minutes to a couple hours but sometimes lasting several hours. Origin: Lahars can originate by eruption of hot fragmental debris onto snow or ice. A melting mass then flows swiftly downslope, incorporating more and more ash and rock from the volcano’s slopes. Some large lahars initiate from volcanic debris avalanches. If the avalanche consists of rock highly altered to clay, almost all the flowing mass becomes lahar, for instance, the Osceola Mudflow off Mount Rainier, USA. But if the avalanche is rocky, its groundwater can escape, incorporate loose sand, and flow downvalley – as in May 1980 at Mount St. Helens, USA. Lahars may initiate by eruption through a crater lake, as at
Kelut in Indonesia or by breakout of a crater lake, as at Ruapehu in New Zealand. Lahars may originate from torrential rainfall during or after eruption of voluminous ash, as at Merapi in Indonesia, Pinatubo and Mayon in the Philippines, and Unzen in Japan. Large lahars occasionally originate by breakout of a debris-dammed lake on a volcano’s lower flank. Downstream evolution: An initially watery lahar may incorporate more debris from the channel and increase the initial flow volume by many times. Lahars moving down a volcano’s flanks flow at 10–40 m/s but slow downvalley. A dense lahar may at first overrun river water, incorporating enough to dilute the mass into an intermediate (hyperconcentrated) flow or even muddy-water flow. Distribution: Lahars radiate from a volcano. They scour and drape steep valley reaches and accumulate in gentler downstream reaches far beyond the volcano’s flanks. Here they may spread widely and inundate areas far beyond the delivering valley. Empirical runout models can estimate areas inundated by future lahars. Hazard: A lahar can sluice down a valley and burst fast and deep upon a town, smashing or removing almost all structures. Tens of thousands of people have perished from lahars, most infamously at Nevado del Ruíz in Colombia in 1985. Close monitoring of an impending eruption, assessing probable runout in valleys at risk, and effective and timely communication to people in the way can mitigate the hazard to human life if not to infrastructure.
Bibliography Major, J. J., and Newhall, C. G., 1989. Snow and ice perturbation during historical volcani
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