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Volcano Hazards: | Types and Effects
| Location | | Gas | Lahars | Landslides | Lava Flows | Pyroclastic Flows | Tephra | |
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A lahar carries away a bridge spanning the Toutle River about 55 km downstream from Mount St. Helens volcano 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 from along the river. This lahar originated from the huge landslide that started the eruption at 8:32 in the morning. |
Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys. When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter. Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second--much too fast for people to outrun.
As a lahar rushes downstream from a volcano, its size, speed, and the amount of water and rock debris it carries constantly change. The beginning surge of water and rock debris often erodes rocks and vegetation from the side of a volcano and along the river valley it enters. This initial flow can also incorporate water from melting snow and ice (if present) and the river it overruns. By eroding rock debris and incorporating additional water, lahars can easily grow to more than 10 times their initial size. But as a lahar moves farther away from a volcano, it will eventually begin to lose its heavy load of sediment and decrease in size.
Numerous terms are used by scientists to describe the properties of lahars (for example, mudflows, debris flows, hyperconcentrated flows, and cohesive and non-cohesive flows).
Eruptions may trigger one or more lahars directly by quickly melting snow and ice on a volcano or ejecting water from a crater lake. More often, lahars are formed by intense rainfall during or after an eruption--rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys. Some of the largest lahars begin as landslides of saturated and hydrothermally altered rock on the flank of a volcano or adjacent hillslopes. Landslides are triggered by eruptions, earthquakes, precipitation, or the unceasing pull of gravity on the volcano.
 Mount Rainier, Washington |
Lahars almost always occur on or near stratovolcanoes because these volcanoes tend to erupt explosively and their tall, steep cones are either snow covered, topped with a crater lake, constructed of weakly consolidated rock debris that is easily eroded, or internally weakened by hot hyrothermal fluids. Lahars are also common from the snow- and ice-covered shield volcanoes in Iceland where eruptions of fluid basalt lava frequently occur beneath huge glaciers (for example, at Vatnajokull, Iceland). |
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The scenarios listed below illustrate most of the mechanisms by which lahars are generated. For convenience, we've grouped the mechanisms according to whether a volcano is erupting, has erupted, or is quiet. Each mechanism is illustrated with one or more case studies.
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Lahars racing down river valleys and spreading across flood plains tens of kilometers downstream from a volcano often cause serious economic and environmental damage. The direct impact of a lahar's turbulent flow front or from the boulders and logs carried by the lahar can easily 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 force of a lahar, buildings and valuable land may become partially or completely buried by one or more cement-like layers of rock debris. By destroying bridges and key roads, lahars can also trap people in areas vulnerable to other hazardous volcanic activity, especially if the lahars leave deposits that are too deep, too soft, or too hot to cross.
After a volcanic eruption, the erosion of new loose volcanic deposits in the headwaters of rivers can lead to severe flooding and extremely high rates of sedimentation in areas far downstream from a volcano. Over a period of weeks to years, post-eruption lahars and high-sediment discharges triggered by intense rainfall frequently deposit rock debris that can bury entire towns and valuable agricultural land. Such lahar deposits may also block tributary stream valleys. As the area behind the blockage fills with water, areas upstream become inundated. If the lake is large enough and it eventually overtops or breaks through the lahar blockage, a sudden flood or a lahar may bury even more communities and valuable property downstream from the tributary.
Lahars can... |
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| ...destroy by direct impact. | ...lead to increased deposition of sediment. | ...block tributary streams. | ...bury valleys and communities with debris. |
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What's that cloud upriver? An eyewitness account of a lahar triggered by rainfall in Guatemala. |