Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
The Dangers of Lahars
Mt. Rainier is a massive volcano located in west-central Washington. Several
major cities in Washington-Seattle, Tacoma, Yakima and Portland, Oregon are
within 200 miles of Mt. Rainier.
The cities located in
the regions near the base of Mt. Rainier are in danger of being destroyed but
not because of an impending eruption but because of lahars (a volcanic debris
A lahar is a slurry of volcanic material and water. Despite the fact that lahars
can be very thick, like concrete, they can move incredibly quickly, and they are
often deadly. Because a lahar can occur on a volcano which is not actively
erupting, it is a significant risk, since one may occur with minimal warning.
Geologists have studied lahars in an attempt to be able to predict their path so
that people can evacuate to safety quickly when it becomes apparent that a lahar
The word lahar is borrowed from the Javanese language used in Indonesia. In
Javanese, lahar means lava, or lava flow, and geologists adopted the term to
refer specifically to flows of volcanic rock and water. In some regions of the
world, the term “lahar” also refers to a dry landslide of volcanic material,
which can be no less deadly. A lahar will always flow down slope, following the
path of least resistance, which makes predicting the paths of potential lahars
The precise contents of a lahar vary, depending on the volcano. Typically, it
contains pyroclastic material, meaning rocks and debris of volcanic origin,
along with debris picked up as the lahar travels. The water may come from rain,
snow, packed ice, or a diverted river, and a lahar can be cool or hot. Hot
lahars are usually caused by active volcanoes melting packed snow, while cool
lahars are caused by torrential rains which loosen enough material to form a
A lahar can build up quite a head of steam, making it rather
devastating. Lahars have leveled and buried entire towns, along with neighboring
forests and fields. When the lahar finally stops, it will dry and harden much
like concrete, making the ground underneath useless until fresh dirt is
deposited on top. A lahar will also leave large chunks of debris like uprooted
trees and chunks of rock as it travels. These pieces can be used as clues to
find historic lahars.
With an ever-growing human population, lahars have become a matter of grave
concern. Many populations settle around volcanoes and former volcanic sites,
since the soil is rich, making it ideal for farming. Unfortunately, bad weather
could cause a lahar at any time, putting these populations at great risk. Since
a lahar cannot be outrun, it may be difficult to get to safety, especially in a
heavily congested area. Many people who live around active volcanoes are already
aware of the inherent risk in their choice of living environment, but people who
live on and near dormant volcanoes may not be aware of the risk of lahars.
The tree stump below was encased in deposits of the Electron lahar and excavated in 1993 during construction of a housing subdivision in the Puyallup River Valley. Clearly, an old growth forest grew on the valley floor when the lahar swept from Mount Rainier about 500 years ago. The lahar deposit is about 5 m thick at this location, a few kilometers downstream from Orting, Washington. Mount Rainier is visible in the distance. Like most lahars generated by large landslides, the Electron deposit is rich in clay-sized particles (6-11 percent). The most distinctive rock in the deposit is a scoriaceous, black andesite lava, which is as large as 1.5 m in diameter. The largest boulders, however, are a reddish-brown breccia as large as about 10 m in diameter. Both rock types were derived from Mount Rainier's cone.
A lahar carries away a bridge spanning the Toutle River about 55 km downstream from Mount St. Helens volcano on May 18, 1980. Before arriving at the bridge, the lahar swept through a logging camp and picked up thousands of neatly cut and stacked logs from along the river. This lahar originated from the huge landslide that started the eruption at 8:32 in the morning.