Paleomagnetism

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Paleomagnetism

The magnetization of minerals in rocks

Magnetization of minerals

Measurement of paleomagnetism

Applications of paleomagnetism

Paleomagnetism and plate tectonic theory

Resources

Paleomagnetism is the study of magnetism in ancient rocks. The phenomenon was first discovered by the French physicist Achilles Delesse (1817–1881) in 1849, who observed that certain magnetic minerals in rocks were aligned parallel to Earth’s magnetic field. A related discovery was made by the French physicist Bernard Brunhes (1867–1910) in 1906. Brunhes observed that the magnetic minerals in some rocks are oriented in exactly the reverse position than would be expected if they were acting as simple compasses. That is, some of these minerals were oriented with their north poles pointing to Earth’s north magnetic pole, and their south poles to Earth’s south magnetic poles.

The first treatise on experimental science by thirteenth century scholar Petrus Peregrinus of Marincourt dealt with magnetism (“Epistola de Magnete”). However, direct observations of the geomagnetic field were not recorded until the late sixteenth century, when the magnetic compass became a widespread tool for navigation. In order to understand nature and origin of Earth’s magnetic field, however, much longer records are necessary. Paleomagnetic research draws this information from rocks that acquire a remnant magnetization upon formation.

The magnetization of minerals in rocks

The phenomena observed by Delesse and Brunhes can be explained because of the fact that certain iron-containing minerals are affected by any magnetic field, including that of Earth. Two of the most important of these minerals are the oxides of iron: magnetite (Fe₃O₄) and hematite (Fe₂O₃). When these minerals occur in molten rock, their atoms are free to move in such a way as to align themselves with Earth’s magnetic field. When the rocks cool, the minerals are then frozen in position and oriented along Earth’s magnetic north-south axis.

Magnetic minerals found in rocks today, however, are not necessarily oriented along Earth’s present magnetic north-south axis. They may have shifted in a vertical direction (their inclination or dip) or in a horizontal direction (their declination). The deviation of a mineral’s orientation to the present magnetic field is of value in determining changes in Earth’s structure in the past.

For example, a magnetic mineral originally laid down along the equator would have an inclination of 0^°, while one laid down at one or the other of the poles would have an inclination of 90^°. If one were to find a rock lying at 40^° north latitude with minerals that have an inclination of 0^°, he or she might conclude that the rock originally formed along the equator and, by some means, was transported northward to 40^° latitude.

Magnetization of minerals

The natural magnetization of a rock is parallel to the ambient magnetic field. It is carried by small amounts of ferrimagnetic minerals and can be stable over geological time scales.

Minerals can be magnetized and oriented with Earth’s magnetic field in a variety of ways. One of these methods was described above. Igneous rocks are formed when molten rock escapes from beneath Earth’s surface and cools sufficiently to form new rock. As long as the original rock is molten, minerals are too hot to hold a magnetic field or to stay in a permanent position. As the rock cools, however, it reaches a point where it can retain a magnetic field and assume a fixed position. At this point, the minerals are frozen into place as compass like indicators of the direction of Earth’s magnetic field.

Magnetic minerals can also be found in sedimentary rocks. As sand, silt, clay, and other such materials are moved from place to place by wind, water, waves, and other forces, the magnetic minerals are constantly reoriented. However, when these materials finally settle out and form permanent accumulations, the minerals orient themselves with Earth’s magnetic axis as they settle. Therefore, these sediments, which may eventually become sedimentary rocks, can preserve the orientation of Earth’s magnetic field just as igneous rocks do.

Magnetization of minerals also occurs within rocky material during the chemical changes that result from metamorphism, or exposure to highly elevated temperature and pressures, which produces metamorphic rocks. Again, freedom of movement allows the minerals to become magnetized along Earth’s existing magnetic lines of force.

The study of the orientation of magnetic minerals is further complicated by the fact that more than one episode of magnetization may have affected a sample. For example, an igneous rock might be worn away by erosion and then re-deposited as a sedimentary rock. Then this sedimentary rock may be metamorphosed to produce a metamorphic rock, and then this rock may be exposed to another episode of metamorphism. Each of the metamorphic episodes has the potential to reorient the original sediments, or it may leave them relatively undisturbed. Recognizing the changes in the magnetic materials that occurred over millions of years within such a rock can be difficult.

Measurement of paleomagnetism

The study of paleomagnetism started in the 1940s when the British physicist Patrick M.S. Blackett (1897–1974) invented a device for measuring the very small amount of magnetic fields associated with magnetic minerals. The astatic magnetometer consisted of a number of tiny magnets suspended on a thin fiber. The magnetometer was rotated around a sample and the amount of magnetism measured by changes in the fiber.

Today, two other devices are more commonly used to study paleomagnetic materials: the spinner magnetometer and the cryogenic magnetometer. Each of these devices represents a significant improvement in the ability of a researcher to detect and measure the magnetic field associated with a mineral.

Applications of paleomagnetism

Sequences of rocks can act like a magnetic tape of geologic history, but the original record is usually altered secondarily through time and various weathering processes. Paleomagnetic methods must be employed to remove this magnetic noise and extract a true primary magnetization.

The results of paleomagnetic studies over the past four decades have had an important influence on our understanding of Earth history. The most significant finding is that the orientation of magnetic minerals in rocks is often very much out of phase with Earth’s present magnetic field. At least two possible explanations for this phenomenon are possible and have been proposed by scientists.

First, Earth’s magnetic field itself changes over time. Differences in orientation result from changes in the magnetic poles, not in the orientation of the minerals.

Second, variations in the orientation of magnetic minerals have been caused by the movement of the minerals themselves. Since the minerals are now—and have for a long time been—frozen into the rocks, this theory would suggest that it is the rocks themselves that are moving across Earth’s surface.

In fact, scientists now know that both of these explanations are correct; Earth’s magnetic poles have wandered from place to place over time and the rocks in which magnetic minerals are found have traveled across Earth’s surface. In addition, there is strong evidence that the polarity of Earth’s magnetic field has shifted (the north pole changing to the south pole, and vice versa) at least 171 times in the past 76 millions years. These reversals of polarity take place rather slowly, over a period of 5,000-10,000 years. They then remain fixed for a period of up to a million years.

Earth’s magnetic field has been a dipole field for more than 99.9% of Earth’s history. Its shape resembles that of the field of a bar-magnet. The field lines emerge at one pole and re-enter at the other pole. Earth’s magnetic field, however, is not caused by a mass of iron with a remanent magnetization. Its origin lies in the outer fluid core where convective motion generates the magnetic field in a self-sustaining dynamo action. This dynamic origin of the geomagnetic field is the main reason why its shape and orientation are not constant but subject to temporal variations on time scales that range from millions of years to days. Averaged over time spans greater than 100,000 years, the dipole axis is parallel with Earth’s spin axis.

In addition to these dramatic reversals of polarity, Earth’s magnetic poles have also wandered. About 300 million years ago, for example, the north magnetic pole was located in the eastern region of Siberia. It then traveled northward to the northern coast of Siberia, along to the coastline to Alaska, and then northward to its present location.

Paleomagnetism and plate tectonic theory

Even when the effect of reversal and change of location of Earth’s magnetic poles are taken into consideration, deviations of magnetic minerals in rocks from true north are still observed. In some cases, this deviation is very great. Since the 1960s, scientists have believed that the reason for these variations is that large chunks of Earth’s surface have moved significant distances across the planet’s face over millions of years.

In accordance with plate tectonic theory, Earth’s crust and upper mantle, which together constitute the lithosphere, consists of about 20 large plates that are about 60 mi (100 km) thick and thousands of miles wide. These plates move back and forth on top of a lower layer of material known as the asthenosphere. The plates collide with each other, slide past each other, and pull apart from each other. Significant geological events, such as volcanoes and earthquakes, are in most cases the result of plate movements.

One of the strongest pieces of evidence for plate tectonics has been paleomagnetism. Evidence has shown, for example, that some rocks in Alaska have magnetic minerals oriented in such a way that they must have been laid down at or near the equator. The fact that they are now at 70^° north latitude suggests strongly that the plate on which they are riding must have migrated a very long distance during Earth history.

Paleomagnetism can also be used to match up land masses that are now separated from each other, but which must once have been joined. For example, the orientation of magnetic minerals along the eastern coast of South America very closely matches that of similar minerals on the western coast of Africa. This correlation, taken with other evidence, provides strong support for the notion that South America and Africa were once joined together as a single land mass.

One of the great successes of paleomagnetism has been in the study of sea floor spreading. Mid-oceanic ridge-rift systems are areas in the oceans where the edges of two plates, and any continents that may be on them, are being forced away from each other by currents in the underlying asthenosphere. Magma from the asthenosphere is pushed up from below the rift to fill in the void created by spreading and to create new ocean floor.

Strong evidence for this theory has come from the study of paleomagnetism on either side of mid-ocean ridges. Magnetometers towed by ships sailing above the rifts have found that the patterns of orientation of magnetic minerals on either side of a rift form stripes that are mirror images of each other. Patterns of high and low intensity and specific inclination and

KEY TERMS

Compass —A device for detecting the presence and direction of a magnetic field.

Declination —The vertical deviation of a compass needle from true magnetic north.

Inclination —The horizontal deviation of a compass needle from true magnetic north.

Magnetic field —The region in space in which a magnetic force can be felt.

Magnetic pole —A space in which magnetic force appears to be concentrated. The two opposing magnetic poles are designated as the north and south poles of a magnetic.

declination running parallel to the rift on one side are exactly matched by similar patterns on the opposite side. This pattern could exist only if new rock were being formed simultaneously on either side of the rift, as suggested by the above theory.