Surface and subsurface mapping of lahar and lahar runout deposits from Glacier Peak volcano has contributed important geologic information for land-management planning and emergency preparedness in the lower Skagit Valley.
Defining the Problem
Active volcanoes, such as Glacier Peak (Fig. 1), pose a variety of potential hazards. Like Mount Rainier (Fig. 2) and Mount St. Helens, the history of Glacier Peak includes explosive eruptions and lahars. Eruptions, earthquakes, or precipitation can trigger landslides that with the incorporation of water become lahars or mudflows. Lahars and lahar runouts (the dense deposits they form) have been known to travel more than 160 km, and they place infrastructure and people living in valleys draining the volcano at risk (Fig. 3).
Figure 1 (top): Although Glacier Peak normally can not be seen from any urban areas, this active volcano periodically erupts in an explosive catastrophic manner that could affect the lower part of the populated Skagit River Valley. Credit: D. Mullineaux, USGS
Figure 2 (middle): Mount Rainier threatens Tacoma, WA. Credit: Lyn Topinka, USGS
Figure 3 (bottom): Lahar racing down the slopes of Mount St. Helens. A lahar is a mud flow consisting of a thick mixture of water and volcanic debris. Credit: USGS
Applying the Geologic Map
The cities of Lyman, Sedro-Woolley, and Burlington are built on terraces composed of lahar-runout deposits from Glacier Peak volcano. Near the town of La Conner and Puget Sound, 135 km down the valley from Glacier Peak, the lahar-runout deposits are 3 to 18 m thick. Although the frequency of large destructive volcanic events from Glacier Peak is once every few thousand years, the impact of such an event could be catastrophic. Lahars could bury entire towns and valuable agricultural land in the lower Skagit Valley, as well as I-5, a critical transportation corridor between Seattle and Vancouver, B.C. Lahar deposits could also block tributary stream valleys, causing upstream flooding. The lahar dam itself could then be breached by water from the inundated valley causing sudden downstream flooding.
Figure 4: Note the location of Glacier Peak volcano and the path lahars have taken down stream valleys to reach the plain of the lower Skagit Valley. Gray rectangles indicate geologic mapping study area. Credit: Dragovich et al. (2000)
Because lahar deposits are typically composed of saturated deposits of uncompacted sand and gravel, areas underlain by such materials have an increased susceptibility to become lahars during strong earthquakes. Surface and subsurface mapping of lahar and lahar-runout deposits from Glacier Peak volcano contributed important geologic information relevant to land-management planning and emergency preparedness in the lower Skagit Valley.
Figure 5: The geologic map of the lower Skagit River Valley shows the extent of exposed lahar deposits from Glacier Peak volcano. This information is vital to regional and local land planning / emergency preparedness in the area. Credit: Dragovich et al.
Case study authors: Joe D. Dragovich and David K. Norman (State of Washington Department of Natural Resources, Division of Geology and Earth Resources)
Case study from: Thomas, W.A. 2004. Meeting Challenges with Geologic Maps, p. 48-49. Published by the American Geosciences Institute Environmental Awareness Series. Click here to download the full handbook.