Magnetic Reversals and Sea Floor Spreading The greatest mountain chain on earth,

Question

Magnetic Reversals and Sea Floor Spreading The greatest mountain chain on earth, 40, 000 miles long, exists under the oceans. This mountain chain i, erred to as the mid-ocean ridge system. Your instructor will show you maps of this system. Geologist discovered in the early 1960's that if you towed a magnetometer (a device that measures the intensity of the earth's magnetic field) across the ocean's surface perpendicular to a mid-ocean ridge, the intensity of the earth's magnetic field varied in a regular manner. In fact, the variation in intensity was symmetrical about the mid-ocean ridge. It was also discovered that the polarity of the earth's magnetic field reverses through geologic time. It was suggested that the observed variations in magnetic intensity were due to sea floor rock that were formed at different times. If a particular piece of sea floor was formed when the magnetic polarity of the earth's field was the same as it is today (normal polarity) the measured magnetic field would be anomalously high. If a particular piece of sea floor was formed when the magnetic polarity of the earth's field was opposite from what it is today (reversed polarity) the measured magnetic field would be anomalously low. Using radiometric dating techniques it was possible to determine the age of the various magnetic anomalies. The diagram below (Figure 3) shows the pattern of magnetic anomalies for ocean X Open areas rep-normal polarity, shaded areas represent reversed polarity Figure 3. Sea-floor magnetic anomalies as a function of distance from the center of the mid-ocean ridge and age.

Explanation / Answer

in the 1960s electronic magetometers were developed. Unlike older instruments, based on the compass needle, these could be towed behind an airplane or a ship. Oil companies were soon using them abroad airplanes to locate oil deosits. on land, the patterns of this magnetism seemed jumbled with no meaningful order.

Extending those measurements to the oceans,around 1960, revealed a surprising difference. In the ocean floor the magnetization was orderly,arranged in a long strips. It is marked by the focus - points of earthquakes and by some volcanic islands. Not only were the magnetic strips lined-up with the central ridge, but then structure and distribution seemed remarkably symmetric on the sides.

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