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.
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.
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