Mount Rainier Active Cascade Volcano (1994) / Chapter Skim
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3 Development and History of Mount Rainier
3 Development and History of Mount Rainier
Pages 36-61
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From page 36... ...
This corridor, which passes through the Mount Rainier region, will be the site of seismic refraction, wide-angle reflection, magnetotelluric, gravity, magnetic, geologic mapping, and geochemical studies. Results from these investigations can be integrated into the regional work to pro 36
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From page 37... ...
North of Mount Rainier, virtually all eruptive activity has been concentrated at the major composite cones of Glacier Peak, Mount Baker, Mount Garibaldi, and Meagher Mountain. Mount Rainier occurs in a dominantly compressional tectonic setting, in contrast to the extensional setting that characterizes the Oregon and California Cascades.
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From page 38... ...
Glacier Peak; R Mount Rainier; GR, Goat Rocks; A, Mount Adams; TH, Indian Hewen; S
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From page 39... ...
Mount Rainier, Mount Adams, and Goat Rocks volcanoes are probably located along the Late Cretaceous continental margin that forms the east side of the SWCC; Mount St. Helens occurs on the western margin of the SWCC.
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From page 40... ...
; He Seattle Fault was recently discovered and is Nought to be active. The dashed box shows He approximate area for the recommended regional studies.
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From page 41... ...
structural features of Me volcanic edifice, including small-scale faulting and dike swarms; and (3) the distribution of hydrothermally altered, structurally weakened rocks.
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From page 42... ...
CIasts in the lahars, as well as Me tephra, contain hornblende phenocrysts and are unlike most products of the modern ~roicano. These volcanic rocks, assigned to the Lily Creek and Puyallup Formations, have reversed magnetic polarities and are older than an 840,000year-old layer of volcanic ash (Easterbrook and others, 1981, 1985~.
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From page 43... ...
The limited thickness data for the tephra layer at Mount Rainier suggest that it may have a comparable volume. The volume of this late Pleistocene tephra is probably at least an order of magnitude greater than that of the most voluminous tephra layer of postglacial age.
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From page 44... ...
Some of the debris flows contain relatively abundant clay-sized material, which evidently was derived from hydrothermally altered rocks excavated by phreatic or phreatomagmatic explosions or by the gravitational collapse of a sector of the edifice weakened by hydrothermal alteration (Frank, in press)
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From page 45... ...
Hydrothermal activity in this area has formed cIay-rich deposits across an area spanning ~ km. (Photo courtesy of David Frank, U.S.
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From page 46... ...
Regional Studies to Assess Volcanic Hazards The Mount Rainier region as defined for the purposes of this report extends from the Seattle-Tacoma metropolitan area on the north to the Columbia River on the south, and includes Mount Rainier, Mount St. Helens, and Mount Adams (Figure 3.2~.
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From page 47... ...
Features of particular interest include extrusive and intrusive rocks, faults, hydrothermal alteration zones, springs, fumaroles, glaciers, and surficial deposits. Geologic mapping of the volcano must be supported by isotopic dating to test geologic correlations and establish the chronology and rates of edifice growth and destruction.
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From page 48... ...
. Sampling for petrologic studies must be coordinated with geologic mapping so that samples represent explicitly mapped geologic units.
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From page 49... ...
Radiometric ages are essential for understanding recurrence intervals of volcanic eruptions at Mount Rainier and in He surrounding region. For Tertiary rocks, paleomagnetic techniques may be useful for placing constraints on ages.
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From page 50... ...
Seismic studies are essential to assess potential earthquake-generated hazards, including eruptions, edifice failure, and glacier outburst floods. The influence of seismic activity on edifice stability must be evaluated carefully because of the sporadic occurrence of large crustal earthquakes in southern Puget Sound, such as the 7.
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From page 51... ...
Demonstration of the usefulness of tomographic studies was provided at Mount St. Helens, where a high degree of detail in the subsurface was obtained win tomographic imaging using local earthquakes recorded on the local seismic network Tees, 1992~.
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From page 52... ...
Geodetic baselines need to be established across suspected strike-slip fault zones and over possible neotectonic features such as the northwest-trending anticline located just west of Mount Rainier. Geologic mapping of brittle-fracture indicators in surficial volcanic rocks and stress studies in boreholes must be a part of neotectonic research in the region, especially west and southwest of Mount Rainier in the area of current seismicity.
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From page 53... ...
Gravity and magnetic data have been used at other Cascade volcanoes to locate buried intrusive bodies. Examples include Goat Rocks Volcano (Williams and Finn, 19X7)
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From page 54... ...
Several northwest-trending alteration zones occur north of Me National Park. Electromagnetic surveys could help resolve whether these zones are expressions of strike-slip faults that extend southward beneath Me volcano.
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From page 55... ...
UVZ-llVl~ O-~1&AV&~ ^~^ ~^~ Hazard Studies In addition to the general studies outlined above to elucidate development of the edifice, special attention needs to be focused on understanding lahars and edifice collapse, two hazards of particular significance for residents in surrounding areas. Chars Lahars probably constitute He greatest hazard to life and property in the Mount Rainier region, and consequently they are an important focus of research.
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From page 56... ...
An improved chronology might also allow mass movements on Me volcano to be correlated with paleoseismic events in the Pacific Northwest (for example, Atwater, 1987; Bucknam and others, 1992; Karlin and AbelIa, 1992~. Edifice Collapse Most of die geologically or hydrologically hazardous events eruptions, collapses, and slope failures have originated, and are likely to originate in the future, from the upper parts of the edifice.
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From page 57... ...
Such altered rocks are weaker than fresh Inca flows and are thus more prone to failure. The distribution of hydrothermally altered rocks on Mount Rainier is clearly significant for assessing the structural integrity of the edifice and can only be determined by detailed geologic mapping and sampling.
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From page 58... ...
Regional studies are needed to address the formation and subsequent development of Mount Rainier within the Cascade volcanic arc environment. Of particular importance in this context are studies of the following: tectonic processes that control the locations of volcanic vents; regional stress fields and their effects on volcanism, faulting, and · · se~sm~c~ty; the crustal deformation field caused by magma injection, subduction, and glacial loading; and ages, distributions, and characteristics of tephras, lavas, and lahars.
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From page 59... ...
Mapping the spatial and temporal distributions of eruptive and intrusive rocks, faults, hydrothermal alteration zones, surficial deposits, springs, fumaroles, and glaciers should be undertaken as part of the effort to understand the development of the Cascade volcanic arc and Mount Rainier edifice. This mapping work should be supported by dating and paleomagnetic studies to establish correlations, chronologies, and rates of edifice grown and failure.
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From page 60... ...
and on Mount Rainier itself to identify areas of present-day hydrothermal activity. Potential field surveys, including gravity and MT surveys to locate magma and other intrusive bodies, faults, and hydrothermally altered rocks.
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From page 61... ...
Edifice collapse is also a significant hazard to life and property in the Puget Lowland area; it warrants careful study. Research on edifice stability should focus on mapping the distributions of hydrothermally altered rocks, faults, and dikes, which are mechanically weak and prone to failure.
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