Lecture 9 Volcanic Hazards

A lahar carries away a bridge spanning the toutle

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vegetation and other debris, and gains water from melting snow and ice and the river it overruns. A lahar carries away a bridge spanning the Toutle River about 55 km downstream from Mount St. Helens on May 18, 1980. Before arriving at the bridge, the lahar swept through a logging camp and picked up thousands of neatly cut and stacked logs. This lahar originated from the huge landslide that started the eruption at 8:32 in the morning. 9.5.1. Causes of Lahars Lahars may be triggered during an eruptions when pyroclastic flows quickly melt a large volume of snow and ice on a volcano. Lahars may also occur after an eruption when precipitation erodes bare slopes covered by loose volcanic debris left by an eruption. Some of the largest lahars occur without an eruption as landslides of saturated and altered rock on the flank of a volcano triggered by earthquakes, precipitation, or the steady pull of gravity on a steep slope.
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Pyroclastic flows from the 1985 eruption of Ruiz volcano, Columbia, melted about 10% of the volcano’s ice cap triggering a devastating lahar that killed 25,000 people. Hundreds of lahars swept the river valleys draining Mt. Pinatubo for years after the 1991 eruption. Each lahar was triggered by heavy rainfall on thick ash deposits left by the eruption. Lahars almost always occur on or near stratovolcanoes because these volcanoes tend to erupt explosively, are constructed of weakly consolidated rock debris that is easily eroded or internally weakened by hot hydrothermal fluids, and their tall, steep cones are often covered by snow and glacier ice.
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Mount Rainier, with its steep, unstable slopes and massive volume of snow and glacier ice, is the most dangerous volcano in the Cascades. This aerial view shows the west flank of Mount Rainier which collapsed in a massive landslide about 6500 years ago, truncating about 2000 feet off the height of the mountain. The resulting lahar traveled all the way to Puget Sound. 9.5.2. Lahars from Mount Rainier A massive lahar could occur at any time if part of the volcano collapses. An eruption could melt the mountain’s ice cap and trigger a series of lahars. Hundreds of thousands now live in path of future lahars from Mount Rainier.
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This hazard map generated from topographic data shows areas inundated by lahars of different volumes. Note that the risk of lahars extends much farther from volcano than the area at risk from pyroclastic or lava flows. 9.5.3. Effects of Lahars Lahars traveling down river valleys and spreading across flood plains far downstream from a volcano cause economic and environmental damage. The direct impact of a lahar and the boulders and logs it carries crush, abrade, or shear off at ground level just about anything in the path of a lahar. Even if not crushed or carried away by the lahar, buildings and land may be partially or completely buried by cement-like layers of rock debris. Lahars triggered after a volcanic eruption by rainfall mobilize large volumes of volcanic debris and bury entire towns and valuable agricultural land.
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9.5.4. Ash Fall and Lahars, Mount Pinatubo The eruption of Mount Pinatubo (1991) was the largest volcanic eruption anywhere on Earth in at least the last 95 years. Pinatubo is a composite volcano and so the major hazards associated with its
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  • Fall '08
  • GODDARD
  • Volcano, eruption, pyroclastic flows

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