Mudflows are masses of fluid mud moving downhill under the influence of gravity. Mudflows commonly occur on volcanoes with large deposits of ice and snow on their summits. Mudflows typically begin with the rapid melting of a large amount of ice--the melting may be caused by an eruption or simply by friction in an ice avalanche that has broken free because of an earthquake or collapse of an overloaded ice mass. As the meltwater flows down the volcano's flank, it mixes with the usually abundant loose soil and ash to form a muddy liquid about the consistency of wet cement. The mud follows stream and river valleys down and away from the volcano to become a fast-moving (40-50 mph) wall of mud that will carry away anything in its path. These two images show a house in the Philippines before and after a mudflow from Mount Pinatubo. Only the pointed roof is visible in the second image. Mudflows at Mount St. Helens carried huge trucks and machinery many miles and tore bridges and houses from their foundations. A mudflow roared down a river valley on the side of Nevado del Ruiz in Columbia in 1985 and swept through a town of 25,000 people, killing nearly everyone and leaving nothing standing.
Mudflows are a particular problem on high volcanoes that have glaciers on them, such as those in the Andes Mountains in South America and the Cascades in the western United States. Mount Rainier's "cap" and drapings of ice are clearly visible in this image of its west side. Photo: Dr. Stephen M. Pompea, Pompea & Associates
As you might expect, the larger the amount of ice available to melt, the larger the potential mudflows. About 3.5 billion cubic feet of ice melted during the 1980 Mount St. Helens eruption. Mudflows from that eruption flowed many tens of miles down local river drainages. Mudflows around Mount Rainier, with its much larger mass of glaciers, are an ongoing problem. Some Rainier mudflows have traveled 50 to 100 miles away from the mountain. Mudflows are a significant danger during eruptions because the heat from the inside of the mountain or from falling hot ash is capable of melting large amounts of ice. However, mudflows can also occur when there is no eruption. An earthquake may shake off a mass of ice that melts because of friction as it rolls down the mountain, mixing with loose material to make a mudflow. A heavy snowfall or rapid spring melt may trigger a burst of ice or water from a glacier that may also form a mudflow.
Glacial outbursts, as the name suggests, are masses of water or ice suddenly released from a glacier. Outbursts may be caused by rapid melting, an earthquake, or heat from lava moving inside a volcano. Glacial outbursts are primarily water, but they can turn into mudflows if they flow over ground with abundant soil or gravel. Glacial outbursts and mudflows can occur on any mountain with glaciers or heavy snowpack, but since many volcanoes grow to altitudes at which glaciers form, outbursts and mudflows are frequently significant volcano-related hazards as well.
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