Magnetic Striping

During World War II submarines were equipped with very sophisticated magnetometers that were designed to measure the magnetic field emanating from the steel hulls of enemy submarines. In essence, it measured the strength of the magnetic field. To the surprise of many, when the submarines measured the magnetic field strength close to the ocean floor they found that there were symmetric stripes on either side of the midocean ridges. The alternating stripes of relatively high and low magnetic strength at first puzzled scientists, and later became one of the key pieces in solving the puzzle of plate tectonics. In the early days of the concept of continental drift, it was thought that the oceans were very old and the continents new and mobile, plowing across the top of the oceanic crust.

The magnetic field of the earth at any time is preserved in many igneous rocks because magnetically susceptible minerals like magnetite are frozen into the rock pointing towards the magnetic north pole of the earth at the time of crystallization. The minerals point towards the pole as they crystallize in the magma and that orientation is locked into the rock as the rest of the magma crystallizes.

Isostacy

The way in which continental crust floats higher in the mantle than does oceanic crust is due to something called isostacy. According to the theory of isostacy, lighter crustal material floats on the denser mantle material so that the pressure at the base of any crust at a particular depth is equal. This means that if a particular continental landmass has variable thickness it will float so that each high standing area has a deep root that extends into the mantle. The principle of isostacy also explains why continental crust floats higher than oceanic crust. Oceanic crust is denser than continental crust, and so it floats lower than continental crust.

Conclusions

Hopefully, by now you know a great deal more about plate tectonics than you did at the beginning of the class. You probably are now beginning to see the connections between things you have learned in other modules and plate tectonic processes. for example large sediment deposits and sedimentary rock formations are usually formed when large mountain ranges, formed by tectonic activity, are eroded, transported and deposited as sediments and rocks.

The most impressive aspect of plate tectonics is that it provides a framework through which much of what we observe on the earth may be explained. This is why it is one of the great scientific revolutions of all time.

To test your comprehension of the material in this module, click the link below.

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