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USGS/Cascades Volcano Observatory, Vancouver, Washington

PROJECT:
Geologic Map of Mount St. Helens


Objective:

The intent of this project is to produce the first detailed (1:24,000-scale), published geologic map of Mount St. Helens.


Rationale:

Mount St. Helens is the most active volcano in the 48 contiguous states. Since 1480 A.D., it has generated four large tephra-producing eruptions, several lava domes, and numerous lava flows, pyroclastic flows, and lahars. The most recent large tephra- producing eruption, in 1980, provided first-hand experience with the danger of such eruptive activity and the severity of its social and economic impacts. A detailed geologic map of the volcano will show the distribution of the various volcanic deposits formed by these and other past eruptions, and the work required to generate the map will provide an enhanced account of the volcano's very active eruptive history.

We can't change the volcano's behavior; instead, we must understand its behavior (based upon the history of its past eruptions--their frequency, style, and reach) as fully as possible to guide us in mitigating the effects of future eruptions. A detailed geologic map is a tool for evaluating the volcano's past behavior and determining hazard-mitigation measures. Continuing development of the U.S. Forest Service Mount St. Helens National Volcanic Monument makes such mitigation increasingly important; with Mount St. Helens as its centerpiece, the Monument hosts more than a million visitors each year within the 1980 blast zone!

Mount St. Helens is a unique laboratory for studying the processes of explosive volcanoes in the United States and around the world. Its recent (1980s) eruptive activity made possible careful analysis of eruptive mechanisms and the volcano's magma-plumbing system, thereby improving our ability to interpret volcanic deposits formed by past eruptions. Because Mount St. Helens is so young and has been so active and so varied in eruptive style, the deposits from its past eruptions are well preserved and yield a rich record of the volcano's history. We combine our understanding of that record, acquired through detailed geologic mapping, with our new-found knowledge about the volcano's magma-plumbing system and eruption mechanisms to place the recent eruptions in a historical context and to enhance our insight of the evolution of volcanic processes over the long term at Mount St. Helens. Insights thus gained are critical to exploring the broader question: How do volcanoes work? The answers to that question, in turn, are fundamental for assessing the hazards of active volcanoes throughout the Cascades, Alaska, and the world.


Strategy:

Geologic mapping is a fundamental tool for exploring and documenting a volcano's geologic record. At Mount St. Helens, we map, in the field, by tracing the distribution of individual volcanic deposits (for example, individual lava flows, pyroclastic-flow or lahar deposits from a single eruptive episode, or a fan of fragmental debris shed from a once-active growing volcanic dome) on color aerial photographs. We note the characteristic features of the deposits, interpret their origins, and document stratigraphic (sequential) relationships between deposits. The resulting portrayal of the geology is transferred to 1:24,000-scale topographic maps, where it forms an essential framework for unraveling and documenting the volcano's history. Once complete, maps will be published in traditional paper form at a scale of 1:24,000 as well as in digital form.

The geologic map data will provide an unparalleled resource for interpreting the three-dimensional structure of Mount St. Helens. Rapid erosion since 1980 has resulted in new exposures into the flanks of the volcano, and correlation of stratigraphic units from the flanks into the 1980 crater walls is now being undertaken. In combination with geophysical and petrologic data acquired since 1980 on the deep plumbing system of the volcano, we expect to be able to construct unusually accurate cross sections and three-dimensional models of the volcano's internal structure.

Our geologic mapping is complemented by stratigraphic analysis of local rock sequences, petrologic studies to evaluate the nature and evolution of the magma that erupted to form the deposits, and radiocarbon and tree-ring dating to establish the chronology of eruptive events. Indeed, the geologic-mapping effort benefits appreciably from a wealth of previous stratigraphic, petrologic, and geochronologic work (for example: Mullineaux, 1986; Crandell, 1987; Major and Scott, 1988; Scott, 1989; Pallister and others, 1992; Yamaguchi and Hoblitt, in press) as well as unpublished smaller-scale geologic mapping by C.A. Hopson prior to the 1980 eruption.


Funding:

The Mount St. Helens geologic-map project is funded by the U.S. Geological Survey Volcano Hazards and Geothermal Studies Program.


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12/11/02, Lyn Topinka