| MadSci Network: Earth Sciences |
The terms refer to the direction of earth's magnetic field. The earth has a field very much like a standard bar magnet, with a north and a south magnetic pole. Unlike a bar magnet, whose field is locked into the solid crystals of the magnet, the earth's field is generated by moving molten iron in the outer core. This motion is very complex and at times, the complexity results in the north and south end of the earth's field trading places. It's as if you turned the bar magnet upside down, but did it without actually moving the bulk of the earth (your compass's north needle would point to Antarctica instead of Siberia). This happens at time intervals of a few hundred thousand years to tens of millions of years and is not at all periodic or cyclical. Because the current situation is what humans have always known, we call it NORMAL polarity (North magnetic pole near north geographic pole). The opposite situation is known as REVERSED polarity (north magnetic pole near south geographic pole). Except for the direction of the poles, there is no difference between normal and reversed polarity magnetic fields and no significant effects on life or on geologic processes. The magnetic fields have reversed and then switched back to normal over and over again throughout the history of the earth. Geologists have developed a time scale for these reversals by correlating them to radioactive ages in rocks. A good overview of the existing time scales is provided by the Woods Hole Oceanographic Institute's web pages at: http://deeptow.whoi.edu/gpts.html The rocks of the ocean floor are mostly igneous rocks (covered with a thin layer of sediments). When they erupt at a mid-ocean ridge, they take on the prevailing magnetic polarity (either normal or reversed) as they cool. Now the rocks are themselves very weak magnets. As the earth's present-day magnetic field passes through these rocks it gets the little magnetic field from the rocks added to it (if the rocks formed during a normal interval, or subtracted from it (if the rocks formed during a reversed interval). A very sensitive detector can be towed across the ocean bottom to measure these rises and drops in the strengths of earth's field. The resulting maps showed a series of stripes - the best evidence that the ocean floor forms at and then moves away from the mid-ocean ridges. See the chapter on "Developing the Theory" in the USGS's wonderful on-line book, This Dynamic Earth: http://pubs.usgs.gov/publications/text/dynamic.html or the Short course notes at: http://earthguide.ucsd.edu/seafloorscience/index.html In summary: Earth's magnetic field flips end over end every so often. Normal is when magnetic north aligns with geographic north. Reversed is when magnetic north aligns with geographic south. Rocks of the ocean floor (or anywhere) acquire a small magnetic field of their own as they form and that field aligns with the earth's field at that time, forming a sort of fossil record of magnetic directions. On the ocean floor, normal intervals have strong magnetic strengths above them, and reversed sections have weaker magnetic strengths above them. Alternating strong and weak zones form stripes that are symmetrical baout the mid-ocean ridge and get older as they get farther away. All of this is strong evidence for sea-floor spreading. Good luck on your test! David Smith, Ph.D. La Salle University, Philadelphia, PA, USA
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