Mud Flows

Information

Mud Flows

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.

If Volcanic Ash begins to fall...

Stay indoors.
If you are outside, seek shelter such as a car or building.
If you cannot find shelter, breathe through a cloth, such as a handkerchief, preferably a damp cloth to filter out the ash.
When the air is full of ash, keep your eyes closed as much as possible.
Heavy falls of ash seldom last more than a few hours -- only rarely do they last a day or more.
Heavy fall of ash may cause darkness during daylight hours and may temporarily interfere with telephone, radio, and television communications.
Do not try to drive a car during a heavy fall of ash -- the chance of accident will be increased by poor visibility.
The thick accumulation of ash could increase the load on roofs, and saturation of ash by rain could be an additional load. Ash should be removed from flat or low-pitched roofs to prevent a thick accumulation.

Volcanic Mudflows (Lahars)

Valleys that head on the volcano may be the routes of mudflows which carry boulders and resemble wet flowing concrete. Mudflows can move faster than you can walk or run, but you can drive a car down a valley faster than a mudflow will travel. When driving along a valley that heads on a volcano, watch up the river channel and parts of the valley floor for the occurrence of mudflows.

Before crossing a highway bridge, look upstream.
Do not cross a bridge while a mudflow is moving beneath it.
The danger from a mudflow increases as you approach a river channel and decreases as you move to higher ground.
Risk of mudflows also decreases with increasing distance from a volcano.
If you become isolated, do not stay near a river channel, move upslope.