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Title: Collected reprints / Atlantic Oceanographic and Meteorological Laboratories (and) Pacific Oceanographic Laboratories
Identifier: collectedreprin1972v2atla (find matches)
Year: 1968 (1960s)
Authors: Atlantic Oceanographic and Meteorological Laboratories; Pacific Oceanographic Laboratories; United States. Environmental Science Services Administration. Research Laboratories; Environmental Research Laboratories (U. S. )
Subjects: Oceanography Periodicals.
Publisher: Washington, D. C. : U. S. Dept. of Commerce, Environmental Science Services Administration, Research Laboratories : For sale by Supt. of Docs. U. S. G. P. O.
Contributing Library: Penn State University
Digitizing Sponsor: LYRASIS Members and Sloan Foundation

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32 Reprinted from the joubnal of college science teaching, October 1972. © 1972 The National Science Teachers Association. Plate Tectonics, Sea-Floor Spreading, and Continental Drift Robert S. Dietz The debate between the fixists and the mobilists which dragged along for a few decades was suddenly marked by a wholesale conversion to drift in 1967. Over the past decade a geologic revolution has tran- spired. It can be summarized in the expression, "the new global tectonics," or more succinctly as plate tec- tonics. This, in turn, involves sea-floor spreading, de- scending lithospheric slabs, transform faulting, and con- tinental drift. It has added a fourth dimension to classical geotectonics which was formerly concerned with uplift, subsidence, and time; but now we have added great hori- zontal shifts as well. Of course, geologists have always accepted a limited amount of horizontal displacement by strike-slip faulting, but never before as the dominant dimension of tectonics. Consider the Bahama platform, where post-mid-Jurassic carbonates all deposited in shallow water have a thickness of about six kilometers. This reveals a history of great subsidence. But during this time, according to the plate tectonic-continental drift solution, the Bahama platform has drifted 5,000 km from an original geographic position near Ascension Island in the equatorial central Atlantic Ocean. Consider also the displacement of 6.5 cm/yr along the San Andreas fault, a transform fault separating the North American plate from the Pacific plate. This rate may seem ponderous, but it is a remarkably rapid geologic process compared to the upheaval of mountains or the peneplanation of continents. It is sufficient to shunt Los Angeles (on the Pacific plate) with respect to San Fran- cisco (on the North American plate) entirely around the circumference of the earth in 600 million years, the time span of the Phanerozoic era. Earth's Mosaic Carapace The plate tectonics con- cept holds that the earth is divided into about eight rigid spherical caps 100 km thick, riding on the weak asthen- osphere and in which the continents are embedded and drift as passive passengers. We can visualize the ideal plate as rectangular, although perhaps only the Indian plate attains this simplicity. Along one edge there is a subduction zone, usually marked by a trench, where the cold crustal plate dives steeply into the earth's mantle, reaching a depth of 700 km before being fully resorbed. Opposing the subduction zone is a mid-ocean rift, or pull- apart zone. As the rift opens, the gap is quickly healed by the inflow of liquid basalt and quasi-solid mantle rock. The other two antithetical sides of the plate, connecting the rifts to the trenches, are crust-piercing shears called transform faults. Three types of plate boundaries are pos- sible: (1) divergent junctures, the mid-ocean rifts where new simatic crust is created; (2) shear junctures, the transform faults where plates slip laterally past one another so that crust is conserved; and (3) convergent junctures, the trenches where two plates overlap, with one plate descending and being consumed. This subducting
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Figure 1—The African plate has drifted north and counterclock- wise, closing up the Tethys seaway and leaving the Mediterranean Sea as a remnant. The northern margin of the plate has been sub- ducted into the Tethyan trench; the mid-Atlantic ridge has been a spreading ridge; the Indian rift has acted mainly as a transform fault along which shearing has occurred. Dr. Dietz is a marine geologist in the National Oceanic and Atmospheric Administration, Atlantic Oceanographic and Meteor- ological Laboratories, Miami, Florida. Dr. Dietz' paper, "Plate Tectonics, Sea-Floor Spreading, and Continental Drift," was pre- sented under the Sunoco Science Seminar Program at the 1972 national convention of the National Science Teachers Association. The author wishes to thank John Holden for the preparation of the figures which appear in this paper. 490

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