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Which sounds more dangerous, lava or mud? The answer may surprise you...

EarthWords is an on-going series in which we shed some light on the complicated, often difficult-to-pronounce language of science. Think of us as your terminology tour-guides, and meet us back here every week for a new word!

Image shows damage from a lahar, including a submerged bridge
Bridge destroyed by lahar in North Fork Toutle River during eruption of Mount St. Helens, May 18,1980. Credit: Richard Waitt, USGS.(Public domain.)

The EarthWord: Lahar


  • A lahar is a hot or cold mixture of water and rock fragments that flow quickly down the slopes of a volcano. They move up to 40 miles per hour through valleys and stream channels, extending more than 50 miles from the volcano.

  • Lahars can be extremely destructive and are more deadly than lava flows.


  • Lahar is a Javanese word for the volcanic mudflows common in that part of Indonesia.

Image shows damage from a lahar from Pinatubo's 1991 eruption
Lahar devastation after the eruption of Mount Pinatubo, Philippines.(Public domain.)

Use/Significance in the Earth Science Community:

  • Lahars generally occur on or near stratovolcanoes, such as those of the Aleutian volcanic arc in Alaska and the Cascade Range in the Western U.S.

  • Although commonly associated with volcanic eruptions, they can happen when no eruption is going on. Sometimes, heavy rainfall or snowfall can easily erode and transport fine-grained, loose volcanic sediment and form a slurry, especially if vegetation has not had time to grow back on recent volcanic deposits.

  • Large lahars can crush, abrade, bury, or carry away almost anything in their paths. Buildings and valuable land may be partially or completely buried. By destroying bridges and roads, lahars can also trap people in areas vulnerable to other hazardous volcanic activity, especially if the lahars leave fresh deposits that are too deep, too soft, or too hot to cross.


  • USGS studies the dangers of lahars as part of its Volcano Hazards Program.

  • USGS scientists study where and how lahars can form and provide that information to emergency managers.

  • In addition, USGS monitors active volcanoes in the United States to give warning when eruptions may be imminent.

Next EarthWord: This week’s EarthWord is straight up steampunk...

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