Lahar, an Indonesian word for volcanic mudflow, is a mixture of water, mud, and volcanic rock flowing swiftly along a channel draining a volcano. Lahars can form during or after eruptions, or even during periods of inactivity.
Lahars form in many ways. They commonly occur when eruptions melt snow and ice on snow-clad volcanoes; when rains fall on steep slopes covered with fresh volcanic ash; when crater lakes, volcano glaciers or lakes dammed by volcanic debris suddenly release water; and when volcanic landslides evolve into flowing debris. Lahars are especially likely to occur at erupting or recently active volcanoes.
Lahars can occur with little to no warning, and may travel great distances at high speeds, destroying or burying everything in their paths. Because lahars are so hazardous, USGS scientists pay them close attention. They study lahar deposits and limits of inundation, model flow behavior, develop lahar-hazard maps, and work with community leaders and governmental authorities to help them understand and minimize the risks of devastating lahars.
Read more and download this new USGS Fact Sheet, Lahar—River of volcanic mud and debris.
From 2009 to 2015, researchers systematically monitored hydrothermal behavior at selected Cascade Range volcanoes in order to define baseline hydrothermal and geochemical conditions. Gas and water data were collected regularly at 25 sites on 10 of the highest-risk volcanoes in the Cascade Range. These sites include near-summit fumarole groups and springs/streams that show clear evidence of magmatic influence. The archived monitoring data are housed on a new webpage and are available for researcher to use as a retrospective comparison with other continuous geophysical monitoring data or for context during future episodes of volcanic unrest.
Download the data from Hydrothermal monitoring data from the Cascade Range, northwestern United States and read more in a new publication, Multi-year high-frequency hydrothermal monitoring of selected high-threat Cascade Range volcanoes.
Scientists at the USGS-Cascades Volcano Observatory developed a MultiGAS analyzer to continuously monitor gas plumes at Mount St. Helens. Take a tour of the crater and the "SNIF" station in this new 7-minute video narrated by USGS-CVO Research Geologist Peter Kelly.
While Mount St. Helens is currently at normal background levels of activity, by continuously monitoring volcanic gases at Mount St. Helens and other volcanoes, scientists hope to pick up on the earliest signs of unrest. The data will be used in combination with other monitoring data such as seismicity and ground deformation to piece together a comprehensive model for what we think is going on at the volcano. The information will be used to issue warnings of impending eruptions and deliver eruption updates to local governments, public officials, the media and the public.
Watch the video Continuous Gas Monitoring Tracks Volcanic Activity at Mount St. Helens or download from the USGS Multimedia Gallery.