Most active volcanoes occur in areas where there has been a long history of previous volcanism and tectonism. The active volcanoes of the Washington Cascades (Mount Baker, Glacier Peak, Mount Rainier, Mount Adams, and Mount St. Helens) occur in the northern part of the Cascades arc, which extends as a long band of sedimentary, intrusive, and volcanic rock formed during the past 40 million years along the western coast of North America from southern Canada (Meager Mountain) southward into northern California (Lassen Peak).
[Graphic,15K,GIF]
Figure 1:
Cascade Eruptions During the Last 4000 Years
(from:
USGS Open-File Report 94-585, Preparing for The Next Eruption in the
Cascades)
Twenty years ago, there were no large-scale, detailed geologic maps covering contiguous areas in the Washington Cascades. Knowledge of the Cascades was restricted to reconnaissance mapping for the state geologic map, scattered checks and prospecting for mineral resources, and few isolated Masters and Ph.D. theses (see: Bibliography of Other Maps and Bibliography of Related Topical Studies). Therefore, there were relatively few data from past volcanism and tectonism to apply to interpretation of recent volcanism and seismic activity in the region, which is growing rapidly in population and has long been a strong contributor to the nation's economy from its forest product industry and hydroelectric power generation.
In 1975, however, geologists of the U.S. Geological Survey, with support from the Washington State Department of Natural Resources, began mapping a series of 7.5-minute quadrangles near Mount St. Helens (Banks and others , 1978, 1979; Evarts and Ashley (1993a-d)). Five years later, in 1980, Mount St. Helens erupted catastrophically.
[Image,40K,GIF]
Figure 2:
Map of southwestern Washington showing 7.5-minute quadrangles with published
geologic mapping (green), completed mapping being compiled for publication
(yellow), and mapping in progress (blue). Unshaded quadrangles indicate areas
of future study. Thick shaded line marks the edge of Southern
Washington Cascades Conductor (SWCC, Stanley and others, 1987)
relative to Holocene (red) and late Pleistocene (blue) composite volcanoes.
Not shown are maps at smaller scale
published for Mount Adams (Hildreth and Fierstein (1995)) and currently being
compiled for publication for the Mount St. Helens Volcanic Monument.
This event brought home the need for funding to provide improved forecasting of volcanic events in the Cascades through improved monitoring and detailed mapping of Mount St. Helens, Mount Rainier, and Mount Adams (published by Hildreth and Fierstein in 1995). Mount Rainier is identified as the primary U.S. volcano for the IAVCEI-sponsored Decade Volcano Program and is potentially the most dangerous volcano in the Cascades, because of dense population in drainage valleys susceptible to torrential meltwater mudflows from even small eruptions at the ice-mantled summit region. Mount St. Helens, currently in a state of restless sleep, was active from 1980 through 1986. Mount Adams has not erupted in more than 4,000 years but is a large volcano with a long eruptive history and currently active fumaroles, and thus still has a potential for future eruptions.
The eruption of Mount St. Helens also elevated the importance of geologic and geophysical understanding of the terrane between the active volcanoes, and in the 15 years since 1980, the detailed quadrangle mapping progressed eastward and northward from Mount St. Helens to abut distal products of Mount Adams and include the southern exposures of Mount Rainier (Figure 2). In addition, a number of geophysical studies of the Cascades by USGS and other workers (see: Bibliography of Related Topical Studies). revealed that the study area is astride a large electrical conductivity anomaly, the Southern Washington Cascades Conductor (SWCC; Stanley and others, 1987, 1992), around which the large Holocene and late Pleistocene volcanoes of southern Washington to cluster (Swanson, 1993b) (Figure 2). The detailed information being produced by the geologic studies of the structure, petrochemistry, and volcanism within and bounding the SWCC will provide useful keys to the understanding of the SWCC as well as to interpretation of current and future earthquake and eruptive activity in southwest Washington.
Geologic Snapshot of the Study Area:
Volcanism in the Cascades arc of Washington began in the late Eocene about 40 million years ago. Within the study area, the volcanism produced basalt, andesite, dacite and rhyolite lava flows with locally abundant andesitic to dacitic pyroclastic flows contributed from a number of volcanic centers both within and outside of the mapped area. Interbedded with the oldest volcanic rocks are mica-bearing stream- and lake-deposited sandstones and mudstones (from sources in eastern Washington) and a wide variety of volcaniclastic rocks from a number of local areas. These rocks are cut by many intrusions of basalt, andesite, dacite, and more silicic composition (and their coarser-grained equivalents). Locally sills, dominantly andesite and diorite, significantly inflate the section, which has been subjected to low-grade regional metamorphism, local contact metamorphism, folding, and erosion.
The regional metamorphism is zeolite facies and related to burial during accumulation of the thick Tertiary sequence (Fiske and others, 1962; Wise, 1970, Hammond, 1980). Volcanic glass is nearly everywhere replaced by iron-bearing smectite, which gives the rocks their characteristic green to brown color. Olivine and orthopyroxene are also usually replaced by smectite and other minerals; plagioclase is variably replaced with albite, calcite, and various clay and zeolite minerals; clinopyroxene is least affected. Contact metamorphism around intrusions produced epidote hornfels and locally, amphibole hornfels. The folds are generally broad and trend north to northwest and most predated 12Ma (Swanson, 1992, 1993a; Hagstrum and Swanson, 1994) (see: Bibliography of Related Topical Studies). Faulting is difficult to detect because of local provenance and limited distribution of marker beds in the thick sequence of otherwise nondescript flows, sediments, and volcaniclastics. However, joints and faults, with slight to several tens of meters of offset, are locally common. Generally those oriented N and NW experienced dextral slip while those oriented E and NE show sinstral slip (Swanson, 1989, 1991, 1992, 1993a, 1994a; Swanson and others, 1989; Evarts and Ashley (1990a, 1990b, 1991, 1992, 1993a, 1993b, 1993c, 1993d) (see: Bibliography of Project Maps).
Pleistocene and Holocene volcanism produced flows and cinder cones that locally overlie the Tertiary section with marked erosional and structural unconformity. Mudflows and lava flows from Mount Rainier can be found in the northern tier of quadrangles of the study area, as well as down the Cowlitz and Nisqually Rivers. Holocene and historic mudflows and pyroclastic deposits also occur around Mount St. Helens. Numerous lava flows and mudflows entered drainages radiating from the Pleistocene volcano center in the Goat Rocks Wilderness area. Late Pleistocene glaciers of at least two ages carved most of the present high relief of the area and left glacial drift covering large parts of the study area.
Non-commercial and marginally commercial deposits of copper, gold, and silver have generated some exploration effort and small- scale mining at times within the area,. Many of these occurrences appear to have formed by shallow, acidic, hydrothermal systems related to the volcanism and intrusion that formed the host rocks. Semi-precious gemstone (agate, etc.) and collectable mineral specimens are locally present.
Dense forests, some of old growth, cover the steep, dissected topography in a patchwork with areas clear-cut by logging. Access along the network of logging roads that sprout from surfaced State and Forest Service roads is generally good, particularly in the several years following timber harvest. However, there remain large roadless tracts within and outside of formally designated Wilderness areas.
Geologic mapping and petrochemical studies are fundamental tools for exploring and documenting the history and behavior of a volcanic province and forecasting what to expect in the nature of future volcanism and tectonism in a given area. The study seeks to provide this detailed information for the geologic terrane between the three active volcanoes in southwest Washington. The map and age distribution of units, their petrology and geochemistry, and definition of the structural evolution of the area will provide a tectonic and petrogenic framework that will assist interpretation of past and present geophysical studies across the SWCC, the abutting Holocene and older volcanoes, and the multi-discipline studies of the Grays Harbor-Columbia Plateau geophysical research corridor, which crosses the Cascades crest within the study area (Wells and others 1993).
The primary goal of these interlinked studies is better understanding of the Cascades arc -- how it formed, when and where it was folded, faulted, intruded, and mineralized, and how it relates to active volcanism and seismicity in southwestern Washington. Among other objectives are development of a petrogenetic understanding of the genesis of the rocks of the Cascade arc of southern Washington, and unraveling how the Cascade arc developed in relation to other sections of the volcano-tectonic belt that rims the Pacific Basin. The studies also produce other geologic information such as mineralized areas, structural information, and geologic sites of interest useful to more general use by the public, industry, and academic researchers and teachers.
The study area includes geologic terrane between the three Holocene composite volcanoes in the southern Washington Cascades. Several projects contribute to the study area, each preparing geologic maps and cross sections at scales of 1:24,000 to l:100,000, with supporting text, as well as topical studies of the hazards, petrology, geochemistry, and structure related to the mapping. Because of the absence of many widespread stratigraphic marker beds, and to assess the natural hazards, complex stratigraphy, and volcanic/intrusive history, the field mapping is at a scale of 1:24,000. Generally, the geologic maps are published at mapping scale as Open-File Reports, usually within a year of completion of the field work. Final versions of some of these maps will be at mapping scale, while others will be compiled at smaller scales (to 1:100,000) for final publication. Samples are being collected for petrographic, radiometric, and chemical analyses. When necessary, geophysical studies are employed. These data are being included with the Open File Maps and/or are published in topical papers that support or derive from the mapping studies.
Geologic maps and accompanying detailed text have been published for the Randle quadrangle by Moore and others (1994); French Butte, Greenhorn Buttes, Tower Rock, McCoy Peak, Blue Lake, East Canyon Ridge, and Hamilton Buttes quadrangles by Swanson, (1989, 1991, 1992, 1993a, 1994a, 1996); and Elk Mountain, Goat Mountain, Lakeview Peak, and Cougar quadrangles by Evarts and Ashley (1992, 1990a, 1991, and 1990b). In addition, final maps of Vanson Peak, Cowlitz Falls, Spirit Lake West, and Spirit Lake East quadrangles have been published by Evarts and Ashley (1993a, 1993b, 1993c, 1993d). (Figure 2). Full references to published maps from the several projects that comprise the cooperative program, as well as derivative products and results of associated geophysical studies, are found below.
The maps and other publications published directly by the USGS can be obtained from the U.S. Geological Survey's Publications Office and a number of commercial sources.Geological maps for Sawtooth Ridge, Purcell Mountain, and Packwood quadrangles are currently being compiled. Work is in progress in the Elk Rock, Hoffstadt Mountain, Coyote Mountain, Winters Mountain, Ohanapecosh Hot Springs and Packwood Lake quadrangle (Figure 2). Nine additional 7.5 minute quadrangle are currently planned in the study area (Figure 2).
Funding and personnel engaged in the 20-year study area have come from a number of USGS and Washington State programs including the National Cooperative Geologic Mapping Program, Earthquake Hazards Reduction Program, Geologic Framework and Processes Program, Minerals Resources Program, and Volcano Hazards Program, as well as the Department of Natural Resources of Washington State.
