Paleomagnetic Data and Tectonic Plates

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Introduction:

The study of paleomagnetism, along with the knowledge of plate tectonics can help predict what happened on Earth millions of years ago. Before the knowledge that earth’s crust contained rocks with definite magnetic orientation, there was no sufficient evidence to explain the origin of the continents. Past theories suggested that mid-ocean regions were ancient, unmoving land masses. With the discovery of magnetic orientations came observations that many rocks are not lined up with earth’s current magnetic field. This led to a new era in plate tectonics.

Paleomagnetism:

The study of paleomagnetism is the study of the earth’s magnetic field as it existed in the past. We need to study this, because earth’s magnetic field is constantly changing - strength and polarity. The source of earth’s magnetic field is the it’s molten core. In normal simple dipole magnets, the magnetic material produces field lines that travel from the north pole to the south pole. The earth, however is not a simple dipole, because the extreme temperatures of the core cause the iron to lose it’s magnetic properties (Curie point). The magnetic field is instead created by another means.

Physicists know that moving electrical current creates a magnetic field, and that current is induced into a conductor moving in a magnetic field (Foster, 1973). It is believed that the liquid outer core flows in a way resembling convection currents. Compositional differences in the earth’s core could possibly have created a small battery effect, producing some electrical current. This current would produce a magnetic field, which in turn would induce a current into a conductor (the core) moving through the magnetic field. This cycle would repeat, like a dynamo (Figure 1), amplifying the current and magnetic field. Normally in the dynamo effect, friction and other sources of resistance prevent perpetual motion of the system. In earth’s system, the rate of energy loss can be supplemented by a variety of sources, such as the core’s intense heat, or the earth’s rotation.

Since the magnetic field is produced by a liquid source, it is constantly changing. Averaging the positions of the magnetic poles over time, they appear to be centered on the rotational axis of earth, due to the effects on the field by earth’s rotation. Another change in the magnetic field is a phenomenon not yet understood.

Over time, it has been discovered that earth’s magnetic field would suddenly change polarity. In fact, over the past four million years, earth’s magnetic field has reversed polarity nine times. Perhaps the field dies down to nothing, experiences destructive interference, or the poles physically migrate to opposite ends of earth. There does not exist any evidence to prove any theory. However, when the field starts up again, the polarity is reversed (Figure 2). This phenomenon, though not quite understood, allows scientists to more accurately study plate tectonics.

Plate Tectonics: The study of Earth’s broad structural features

Earth’s surface is composed of many separate plates that migrate over time. This tectonic plate shifting has been responsible for isolation and evolution of animal species, volcanoes, earthquakes, and today’s continents. The cause of this shifting is in the mantle.

Earth’s mantle is composed of silicates that are so plastic, they flow with a slow convective pattern. Since the lithosphere (area between surface of crust and Mohorovicic discontinuity) is less dense than the asthenosphere (upper mantle), the former floats on the latter. The convection currents push the plates around, constantly changing the appearance of the earth’s surface. The movement of these plates follows a cyclical history of rifting, drifting and collision, as outlined by the Wilson cycle (Figure 3)

Earlier on, however, it was believed that earth’s crust was a rigid body, and that the oceanic basins were ancient unmoving features. A man by the name of Alfred Wegener proposed that the continents could move, with a theory that suggested Africa and South America were once together as part of a larger supercontinent. He proposed that all of the continents were once part of the supercontinent, which he called Pangaea. It wasn’t until the 1950’s, when paleomagnetic data and the discovery that seismic activity was concentrated in narrow zones, that Wegener’s theory was seriously considered. To explain the seperation of the continents, H. H. Hess in 1962 proposed that mid-oceanic regions were sources of generation of new oceanic crust due to upwelling lava at continental rifts. The crust then migrates parallel to earth’s surface until it reaches a continent, where it travels back down into the mantle due to subduction.

Comparison:

This proposal led to the theory of plate tectonics (Flint, 1974), which helps explain changes in earth’s lithosphere over time. This theory has three tests which validate it. First is the paleomagnetism test.

The paleomagnetism test uses the notion that earth’s polarity is constantly changing, and that this change is recorded in igneous rocks. Lava expelled at mid-ocean ridges acquires the magnetic polarity at the time of hardening. If the lithosphere is indeed moving, then one should observe strips of magnetic rock, each with opposite polarities as new rock would be constantly forming. Evidence shows that the lithosphere is moving, and that it moves away from the mid-ocean ridge with the same velocity on both sides.

The second test of the theory of plate tectonics is the sea-floor-sediment test. If the lithosphere is indeed moving, then sediment should be thicker further away from mid-ocean ridges. Also, for example, Africa and South America are moving apart at about 4 cm per year. If this rate is constant, than one can deduce that approximately 150 million years ago, they were in contact with each other, and that the current rock between them cannot be more than 150 million years old. Drilling conducted by the Glomar Challenger, an oceanic research vessel designed to drill into rock at the deepest parts of the ocean, conducted some drills and proving that this basalt is indeed younger than 150 million years old. It was also noted that the thickness of strata increased as distance from the mid-ocean ridge increased. This is because the older rock had more time for sediment to settle on it. By this, we can assume that all present ocean basins are young features.

The third test is the earthquake-focus test. Since paleomagnetism proves the source of new lithosphere, and sedimentation proves that it is moving, there should be some evidence that the old lithosphere returns to the mantle, completing the cycle. As the rigid lithoshpere slides into the hot asthenosphere, it fractures, causing earthquakes. Since most earthquakes occur beneath sea-floor trenches, one can assume that this is a point of re-entry of the lithosphere into the mantle.

These three tests verify the theory of plate tectonics. From this theory, we can see that paleomagnetism plays an important role in studying the motions of the ancient and present earth.

Paleomagnetism is also playing a role in providing evidence supporting a new theory of the ‘snowball earth’ (Geotimes, 2000). In the 1960’s Cambridge geologist Brian Harland had been curious about glacial rocks having nearly parallel magnetic lines, indicating equitorial origin. This evidence thus suggests that the glacier must have originated somewhere near the equator.

Closing Remarks:

Now that geologists have a better understanding of earth’s history, they can develop new ideas as to how earth is continuously evolving. Information gathered from the knowledge of paleomagnetic data and plate tectonics can be used not only on earth, but on other planets like Venus or Mars. Geologists can study these planets, and maybe discover exciting new clues about our early solar system.

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