Water and debris are a recipe for lahars at California’s volcanoes
Debris flows or mudflows that form from volcanic material are called lahars. The word lahar comes from Javanese, the language spoken on the island of Java in Indonesia. Lahars are hot or cold mixtures of water and rocks and ash that flow down the slopes of a volcano, usually in a river or stream valley.
Volcanic eruptions can trigger lahars by melting snow and ice in a crater or by ejecting water from a crater lake. Hot pyroclastic flows can generate lahars by melting snow and ice as they travel rapidly down steep slopes. Finally, lahars can also be formed when large amounts of rainfall occur on the material deposited during eruptions. Lahars usually happen on the flanks of stratovolcanoes, such as in Alaska and the Cascade Range, but can occur anywhere there are volcanic slopes with a water source.
In California, lahars are known to have happened at several volcanoes. Lahar deposits are found in drainages around Lassen Peak in Northern California, and the biggest of these was deposited during the eruption in May 1915 .The eruption generated a snow and hot-rock avalanche and a pyroclastic flow, which transformed into lahars as they traveled down Lassen Peak’s slopes.. Current hazard maps of Lassen Peak (first image) reflect the possibility of future lahars, given the right conditions (for example, an eruption during an exceptionally snowy winter).
Another place where lahar deposits are found is at Mount Shasta to the west; eruptions at the Hotlum and Shastina vents on the volcano about 10,000 years ago produced many lahars. About 20 percent of these lahars traveled for more than 20 kilometers (12.4 Miles) from the summit of Mount Shasta and spread out on fans around the base of the volcano. The hazard map for Mount Shasta (second image) highlights some of the stream and river drainages that could see lahars during any future eruptions.
Finally, in the Long Valley region of Eastern California, an eruption at Inyo Craters sometime before 1472 created a lahar which flowed into the Owens River and traveled at least 30 kilometers (18.6 miles) to near the shoreline of present-day Lake Crowley.
None of California’s volcanoes have monitoring equipment specifically designed to detect lahars. However, a lahar detection system designed by scientists at the USGS Cascades Volcano Observatory operates on the flanks of Mount Rainier in Washington where past lahars inundated areas that are now densely populated and contain important infrastructure. Watch this YouTube video to see how this system works, and how it might be deployed on other Cascade volcanoes.
Visit Lahar—River of volcanic mud and debris to read more about lahars and their hazards