Global Volcanism Program | Image GVP-03110
Electronic Distance Measurements (EDM) by the U.S. Geological Survey at South Sister volcano, with Middle Sister to the left, are conducted routinely to monitor these Cascades volcanoes for potential eruptive activity. By measuring the distance between two fixed points, these instruments can detect minor changes in the surface of the volcanic edifice that can occur prior to eruptions.
Photo by Lyn Topinka, 1985 (U.S. Geological Survey).
This image is made available as a Public Domain Work, but proper attribution is appreciated.
Galleries: Volcano Monitoring