Lesson 4: Fire, Rock, and Water
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| Mount St. Helens 2 years before its cataclysmic eruption. When the volcano exploded on May 18, 1980, huge volumes of snow and ice quickly melted and contributed to devastating floods and mudflows. |
As hot volcanic debris melted snow and glacier ice on the upper slopes of Mount St. Helens, mudflows - fast-moving mixtures of volcanic debris and water - developed within minutes of the beginning of the May 18 eruption. By 10:10 a.m. Pacific Daylight Time, a mudflow had traveled 43 kilometers (27 miles) downstream in the South Fork Toutle River valley. And before the day ended, nearly all the streams that had their sources on Mount St. Helens were affected by mudflows.
Fortunately, the major mudflow took hours to reach populated areas, giving people time to evacuate. As a result, only a few deaths were attributable to mudflows. Volcanic mudflows are also called lahars, a term borrowed from Indonesia, where mudflows are a major volcanic hazard.
Mudflows' Destructive Force
The largest and most destructive mudflow came down the valley of the North Fork of the Toutle River. It originated from the hot debris from the avalanche, lateral blast, and ash falls that had been deposited in the upper part of the river valley during the first few minutes of the eruption. By afternoon, water from melting snow and glacial ice, and from within the debris itself began to flow.
The mudflow steamed with hot volcanic materials (pyroclastic debris). Thick, like freshly mixed cement, the mudflow enveloped almost anything that it picked up along its path. Eyewitnesses reported seeing everything from ice chunks to a fully loaded logging truck in the flowing mixture. As debris, mud, and fallen trees choked the Toutle River, the river overflowed its banks and flooded, cresting at 6.4 meters (21 feet) above its normal stage.
Snow and Ice Compound Dangers
Mudflows are particular hazards at snow-capped volcanoes such as Mount St. Helens. Even small eruptions of hot volcanic material can very quickly melt large volumes of snow and ice. The resulting surge of water erodes and mixes with volcanic rock to become mudflows. For example, the 1985 eruption of Nevada del Ruiz in Colombia was a very small eruption - ejecting only about 3 percent as much magma as Mount St. Helens - yet it generated high-volume mudflows because of the presence of snow and glacial ice on the volcano. The mudflows that swept down from Nevada del Ruiz buried the town of Armero, killing more than 23,000 people. Nevada del Ruiz, like Mount St. Helens, has snow and ice year round at its highest elevations.
The Risk of Mudflows Continues
Even without a major eruption, mudflows and floods remain potential hazards of Mount St. Helens. As a result of the May 18, 1980, eruption, huge volumes of volcanic debris dammed preexisting streams. Because these dammed streams are composed of loose materials, they are structurally weak. If the dams fail, mudflows and floods can occur. Loose volcanic debris on steep slopes is also vulnerable to flowing during or after heavy rainfalls. The risk of mudflows and floods is greatest on Mount St. Helens during the winter months when precipitation is heaviest and the snowpack is thickest.
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