When Is Mud Dangerous?
ES1204  Volcanic Mudflows—Lahars

Cynthia Gardner, USGS
Mount Hood, Oregon with mudflow deposits in foreground.

Lyn Topinka, USGS
Debris from mudflows buried huge areas of forested hills surrounding Mount St. Helens, Washington.

Volcanoes are another source of dangerous mud. You might think that the only thing that runs down the sides of volcanoes is lava, but the fine ash erupted from explosive volcanoes can cause destructive mudflows.

When a volcano erupts, ice and snow melted by the heat of the eruption run downhill carrying ash particles and other debris. These volcanic mudflows are called lahars.

The danger of lahars can last for years following an eruption: fine particles of ash deposited on hillsides during the eruption are carried downstream every time it rains. During a rainy season, lahars can sweep downstream with tremendous power, burying entire cities with debris.

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