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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK:
PART I OLYMPIC GEOLOGY

magnetic stripes on ocean floor
Fig. 24. Comparing the magnetic pattern of basalts on the land to magnetic striping on the ocean bottom. After Cox, "Geomagnetic Reversals" (1969), and Raft and Mason,"Magnetic Survey off the West Coast of North America (1961).

Rock Magnetics

Most rocks that are rich in iron, and basaltic rocks in particular, are naturally magnetic. Evidently, when a melted rock (such as a lava flow) cools, the atomic characteristics of its iron-bearing minerals are influenced by the earth's magnetic field; the rock becomes a very weak magnet, and its poles are aligned with the earth's field. In the geologic past the earth has periodically reversed polarity, that is, the north magnetic pole has switched places with the south pole Lavas that erupted during a time of reversed polarity retain reversed polarity; those that erupted when the earth's polarity was like today's which is called "normal," retain normal polarity. The north and south poles of the lavas can be detected with a sensitive instrument. Working in lava piles in areas of little crustal disturbance, geologists have been able to develop a schedule or time scale of these past polarity reversals (fig. 24).

Ocean Floor Patterns

While geologists and geophysicists were climbing about the lava beds on land, oceanographers were cruising back and forth across the ocean, measuring the magnetic properties of rocks on the ocean floor. Although they could measure only the grossest kinds of magnetic properties from the surface of the ocean, their traverses revealed a startling striped pattern of magnetic changes. The pattern parallels prominent ridges on the ocean floor; furthermore, the pattern on one side of the ridges is a mirror image of the pattern on the other side (fig. 24). The ridges had long been recognized as impressive, if little understood, features of all the world's oceans. Amazingly, the pattern of magnetic changes on the sea floor match the vertical magnetic reversal patterns of progressively older lava layers on land. Thus the ocean floor stripes seem to represent the same polarity reversals found in the lavas on the land.


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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