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Geology of the National Parks


Plate tectonic scheme in cross secion
Fig. 25. Plate tectonics scheme showing subduction and the formation of major rock types: igneous, sedimentary, and metamorphic.

The Spreading Ocean Floor Meets the Continent

As geologists began to realize that the ocean floor was progressively older farther away from the ridges, they concluded that new crustal rock must be forming at the ridges and moving out to either side. Knowing that the oceanic crust is basalt, at least on the surface, scientists realized that molten rock had been added to the ridge as the ridge pulled apart at the center. Some of the rock erupted as undersea lava flows, but most cooled deep in the crack of the split ridge. It seems as if the crack in the oceanic crust, although continually opening, is also continually caulked by hot rock from below. The process has gone on so long that much of the oceanic crust is made up of long strips of caulking, and these strips take on the magnetic polarity of the earth's field, normal or reversed, as they form (fig. 24). But if the floor moves away from the ridges, where does it go? The striped pattern of magnetic variation ends rather abruptly against the continents and in some places appears to have been swept under them. The distribution of earthquakes deep beneath the margins of some continents suggests an answer. The points where these deep earthquakes originate crudely define planes dipping deep under the continents. These planes seem to represent the moving plates of oceanic crust that are moving down beneath the continental plates (fig. 25).

Using the plate tectonic scheme, we can visualize the earth's whole crust as consisting of gigantic plates, moving away from each other at the oceanic ridges and riding over or colliding with each other along continental margins. The area between the ridge and the continental margin is one plate, and the continent is another The magnetic pattern off the coast of Washington suggests that much of the Pacific oceanic crust plunged beneath the North American continent.

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.

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