Lahars and Pyroclastic Flows
Lahars and Mudflows
Lahars are massive flows of water, ash, mud and other debris and are usually a result of a sudden volcanic eruption. Snow provides the liquid for the flow as it is quickly melted by super-heated steam and volcanic gases. These flows can travel as fast as 60 miles per hour. In these images you can see that the gray flow has filled the Toutle River valley near the mountain with ash and mud. The flows destroyed bridges, covered roads and damaged over 200 homes. When cooled and dried the flow is as hard as concrete.
The flow is outlined in red and the arrow shows the direction it flowed through the valley. The road is about 40 feet wide and was covered by the flow just before the trees begin. The Toutle River has since began meandering through the valley and cut a new channel in the flow. Over 18 million trees were planted to reforest the area. In the island area at right in the photo is home to a herd of antelope that have returned to the valley.
A volcano doesn't have to erupt to cause a lahar. For instance, the glaciers on top of Mount Rainier rest on rock and rubble because steam from inside the mountain breaks down the strength of the rock. When the pull of gravity on the glacier overcomes the support of the underlying material the glacier slides down the mountain. The ice melts and combines with rocks, trees and dirt to form a lahar. There have been a number of these lahars around the mountain. As the population around Mount Rainier increases there is the danger of a catastrophic mudflow that would endanger thousands of lives.
A hot, fast moving and high-density (thick like a dust storm) mixture of ash, pumice, rock fragments and gas formed during explosive eruptions is called a pyroclastic flow. Gases released from the volcano vent expand into the atmmosphere and carry the debris. Heavier materials flow along the surface while lighter ash and pumice pieces are thrown up and outward. At Mount St. Helens these flows were strong enough to flow uphill and over Spirit Lake. In the satellite image below the pyroclastic flows moved directly to the right of the mountain, over the Debris Avalanche Deposit and Pumice Plain and beyond Spirit Lake.
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Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Fragments of less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions.
- Volcanic rock caused by partial melting of the Earth's crust.
- A large volcanic depression, commonly circular or elliptical when seen from above.
- Composite Volcano
- A steep-sided volcano composed of many layers of volcanic rocks, usually made from high-viscosity (thick like honey) lava, ash and rock debris (broken pieces).
- A steep-sided mound that forms when viscous (thick like honey) lava piles up near a volcanic vent (opening at the surface).
- A vent that releases volcanic gases and steam.
- A mixture of water and rock debris that forms on the slopes of a volcano. Also known as a mudflow or debris flow. The term comes from Indonesia.
- A light-colored volcanic rock containing lots of bubbles from trapped gases. This rock can sometimes float on water.
- Pyroclastic Flow
- A hot, fast moving and high-density (thick like a dust storm) mixture of ash, pumice, rock fragments and gas formed during explosive eruptions.
- Shield Volcano
- A volcano shaped like a bowl in the middle with long gentle slopes made by basaltic lava flows.
- An opening at the surface where magma, gas and steam erupt.
- A vent at the surface where magma, gas and steam erupt. Also, the landform constructed by volcanic material.
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