Since first publication of Guide to the Geology of Olympic National Park in 1975, many ideas have changed in the field of geology, and new geologic mapping on the Olympic Peninsula has clarified the geologic story. Rather than rewrite the book which is partially presented on this website, I summarize in this preface some of the more significant new ideas.
In the discussion of plate tectonics, my tone was somewhat tentative in 1975, but today the theory of plate teconics has been substantiated many times. Several papers and books have been published that provide a regional plate tectonic context and history of the Pacific Northwest (see references below: Wells and others, 1984; Engebretson and others, 1985; Wells and others, 1998; Orr, 2002; Haeussler and others, 2003).
The Olympic Mountains are regarded as a prime example of a subduction complex (Tabor and Cady, 1978b), that is, a thick wedge of rocks (an accretionary wedge) produced by the progressive offscraping of ocean floor sediments during subduction and their accretion to the continental margin (Tabor and Cady, 1978). The forces that drive plate movement are no longer viewed as the result of simple convection in the Earth's mantle (like a pot of boiling water), but earth scientists now think plate motion is driven by gravity acting on the plates themselves-the young, hot plates near the spreading centers tending to float high; the old, cold oceanic plates far from their place of birth tending to sink at the subduction zones. On this website some of the archaic discussion in the book has been left out.
With publication of a general geologic map of the Olympic Peninsula (Tabor and Cady, 1978a), geologists began adding detail to this complex area. Of particular note are maps by Rau (1975, 1979, 1986), Snavely and others (1993), and Gerstel and Lingley (2000), which have improved our understanding of the core rocks. The latter, complete with a wealth of data, and a paper by Stewart and Brandon (2004), confirms the earlier work of Rau and Stewart and supports the structural history presented here.
By studying paleomagnetic directions of Eocene basalt around the Olympic Peninsula, Geophysists Beck and Engebretson (1982) have lent some credence to the idea that the bend of the basaltic horseshoe is due to the jamming of the Olympic rocks into the inside corner formed by the older North Cascade Range and Vancouver Island.
Origin of the basalt of the Crescent Formation
Recent chemical investigations of the basaltic horseshoe of the Crescent Formation) has refined ideas of where the basalt erupted on the sea-floor. Babcock and others (1992, 1994) conclude that the voluminous basalt pile has not been thickened much by faulting and that it erupted in what geologists call a rifted marginal ocean basin. This conclusion could call for a revision of the structural history presented here, at least in detail, but neither the above authors or other workers have yet detailed directly the emplacement of the basalt along the continental margin.
Age of the Olympic Core Rocks
A monumental study of fission-track ages of core strata by Brandon and Vance (1992) indicated that some core rocks presented here as probably Eocene in age actually might be Miocene, considerably younger. This finding called for an important modification of the structural history presented in figure 26 here, but, most recently, studies by Stewart and others (2003) utilizing the radiometric clock of uranium and lead in zircon crystals from core rocks, suggests that some of the Eocene ages may be correct after all. Details may be forthcoming.
Uplift and erosion
A number of scientists (Roden-Tice and others, 1995; Brandon and others, 1998; Pazzaglia and Brandan, 2001) have been examining the uplift history of the Olympics by studying stream terraces and noting the cooling history of certain minerals as they were uplifted from depth to the Earth's surface. Pazzaglia and Brandon suggest that the Olympics reached a steady state of erosional lowering just balanced by continuing uplift since about 14 million years ago. The uplift is not strictly due to gravitational floating up of the low-density thickened sedimentary rock wedge, but by continued stuffing of material under the range by subduction. The implication is that the mountains would probably not get much higher, or lower for that matter, in the near geologic future. This hypothesis does not explain the old erosion surface described in Field trip stop 12, which is probably much younger than 14 million years and formed when the Olympics were eroded down to more gentle uplands.
Babcock, R.S., Burmester, R.F., Engebretson, D.C., Warnock, A. and Clark, K.P., 1992, A rifted margin origin for the Crescent Basalts and related rocks in the northern Coast Range volcanic province, Washington and British Columbia: Journal of Geophysical Research, v. 92, p. 6799-6821.
Babcock, R.S., Suczek, C.A. and Engebretson, D.C., 1994, The Crescent "Terrane", Olympic Peninsula and Southern Vancouver Island: Washington Division of Geology and Earth Resources Bulletin, v. 80, n. p. 141-157.
Beck, M.E. and Engebretson, D.C., 1982, Paleomagnetism of small basalt exposures in the west Puget Sound area, Washington, a speculations on the accretionary origin of the Olympic Mountains: Journal of Geophysical Research, v. 95, p. 3755-3760.
Brandon, M.T. and Calderwood, A.R., 1990, High-pressure metamorphism and uplift of the Olympic subduction complex: Geology, v. 18, n. 12, p. 1252-1255.
Brandon, M.T., Roden-Tice, M.K. and Garver, J.I., 1998, Late Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State: Geological Society of America Bulletin, v. 110, n. 8, p. 985-1009.
Brandon, M.T. and Vance, J.A., 1992, Tectonic evolution of the Cenozoic Olympic subduction complex, Washington State, as deduced from fission track ages for detrital zircons: American Journal of Science, v. 292, p.565-636.
Engebretson , D. C., Cox, A., Gordon, R. G, 1985, Relative motions between oceanic and continental plates in the Pacific Basin: Geological Society of America Special Paper 206, 59 p.
Gerstel, W.J. and Lingley, W.S.J., 2000, Geologic Map of the Forks 1:100,000 Quadrangle, Washington, Washington Division of Geology and Earth Resources, Open File Report 2000-4, scale 1:100,000.
Haeussler, P. J., Bradley, D. C., Wells, R. E., Miller, M. L., 2003, Life and death of the Resurrection Plate; evidence for its existence and subduction in the northeastern Pacific in Paleocene-Eocene time: Geological Society of America Bulletin, vol.115, no.7, p.867-880.
Haeussler, P.J., Yount, J. and Wells, R., 1999, Preliminary Geologic Map of the Uncas 7.5' Quadrangle, Clallam and Jefferson Counties, Washington, U.S. Geological Survey, Open-File Report 99-421, scale 1:24000.
Pazzaglia, F.J. and Brandon, M.T., 2001, A fluvial record of long-term steady-state uplift and erosion across the Cascadia Forearc High, Western Washington State: American Journal of Science, v. 301, p. 385-431.
Orr, W.N., 2002, Geology of the Pacific Northwest, 2nd Edition: McGraw-Hill, p. 337.
Rau, W.W., 1975, Geologic Map of the Descruction Island and Taholah Quadrangles, Washington, Division of Geology and Earth Resources, Geologic Map GM-13, scale 1:62,500.
Rau, W.W., 1979, Geologic Map in the Vicinity of the Lower Bogachiel and Hoh River Valleys, and the Washington Coast, Washington Division of Mines and Geology, Geologic Map GM-24, scale 1:62,500.
Rau, W., W., 1986, Geologic Map of the Humptulips Quadrangle and Adjacent Areas, Grays Harbor County, Washington, Washington Division of Geology and Earh Resources, Geologic Map GM-33, scale 1:62,500.
Roden-Tice, M.K., Garver, J.I., Brandon, M.T., Pickering, M.J. and Schlidge, C.B., 1995, Timing and rate of modern denudation in the Olympic Moutains of the Cascade Forearc, Washington State, based on apatite fission-track thermochronology, in International Workshop on Fission -Track Dating, Gent, Belgium, August 26-30, 1996.
Stewart, R.J., Wooden, J.L., Brandon, M.T., Vance, J.A. and Wells, R.E., 2003, U-PB SHRIMP ages from detrital zircons in the Grand Valley and Western Olympic Assemblages, Olympic Subduction Complex, Washington: Geological Society of America Abstracts with Programs, v. 35, n. 6, p. 512.
Stewart, R. J. and Brandon, M. T., 2004, Detrital-zircon fission-track ages for the "Hoh Formation": implications for late Cenozoic evolution of the Cascadia subduction wedge: Geological Society of America bulletin, v. 116, n. 1/2, p. 60-75.
Snavely, P.D., Jr., MacLeod, N.S. and Niem, A.R., 1993, Geologic map of the Cape Flattery, Clallam Bay, Ozette Lake, and Lake Pleasant Quadrangles, Northwestern Olympic Peninsula, Washington, U.S. Geological Survey, Miscellaneous Investigations Series I-1946, with major contributions by D.L. Minasian, J.E. Pearl, and W.W. Rau; scale 1:48,000.
Tabor, R.W. and Cady, W.M., 1978a, Geologic map of the Olympic Peninsula, U.S. Geological Survey, I-994, scale 1:125,000.
Tabor, R.W. and Cady, W.M., 1978b, The structure of the Olympic Mountains, Washington--analysis of a subduction zone: U.S. Geological Survey Professional Paper 1033, 133 p.
Wells, R. E., Engebretson, D. C., Snavely, P. D., Jr., and Coe, R. S., 1984, Cenozoic plate motions and the volcano-tectonic evolution of western Oregon and Washington: Tectonics, v. 3, p. 275-294.
Wells, R. E., Weaver, C. S., and Blakely, R. J., 1998, Fore arc migration in Cascadia and its neotectonic significance; Geology, v. 26, p. 759-762.
On to Introduction
Material in this site has been adapted from Guide to the Geology of Olympic National Park by Rowland W. Tabor, of the USGS. It is published by The Northwest Interpretive Association, Seattle.