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Evidence for Sea-Floor Spreading

Marine Magnetic Anomalies

Recognizing anomalies

For a hypothesis to become a theory, researchers must demonstrate that the idea really works. During the 1960s, geologists found that the sea-floor spreading hypothesis successfully explains several previously baffling observations. Here we discuss two: (1) the existence of orderly variations in the strength of the measured magnetic field over the sea floor, producing a pattern of stripes called marine magnetic anomalies; and (2) the variation in sediment thickness on the ocean crust, as measured by drilling.

Geologists can measure the strength of Earth’s magnetic field with an instrument called a magnetometer. At any given location on the surface of the Earth, the magnetic field that you measure includes two parts: one produced by the main dipole of the Earth generated by circulation of molten iron in the outer core, and another produced by the magnetism of near-surface rock. A magnetic anomaly is the difference between the expected strength of the Earth’s main dipole field at a certain location and the actual measured strength of the magnetic field at that location. Places where the field strength is stronger than expected are positive anomalies, and places where the field strength is weaker than expected are negative anomalies.

The discovery of marine magnetic anomalies.

Geologists towed magnetometers back and forth across the ocean to map variations in magnetic field strength (figure above a). As a ship cruised along its course, the magnetometer’s gauge might first detect an interval of strong signal (a positive anomaly) and then an interval of weak signal (a negative anomaly). A graph of signal strength versus distance along the traverse, therefore, has a sawtooth shape (figure above b). When geologists compiled data from many cruises on a map, these marine magnetic anomalies defined distinctive, alternating bands. If we color positive anomalies dark and negative anomalies light, the pattern made by the anomalies resembles the stripes on a candy cane (figure above c). The mystery of this marine magnetic anomaly pattern, however, remained unsolved until geologists recognized the existence of magnetic reversals.

Recall that Earth’s magnetic field can be represented by an arrow, representing the dipole, that presently points from the north magnetic pole to the south magnetic pole. When researchers measured the paleomagnetism of a succession of rock layers that had accumulated over a long period of time, they found that the polarity (which end of a magnet points north and which end points south) of the paleomagnetic field preserved in some layers was the same as that of Earth’s present magnetic field, whereas in other layers it was the opposite  (figure above a, b).

At first, observations of reversed polarity were largely ignored, thought to be the result of lightning strikes or of local chemical reactions between rock and water. But when repeated measurements from around the world revealed a systematic pattern of alternating normal and reversed polarity in rock layers, geologists realized that reversals were a worldwide, not a local, phenomenon. They reached the unavoidable conclusion that, at various times during Earth history, the polarity of Earth’s magnetic field has suddenly reversed! In other words, sometimes the Earth has normal polarity, as it does today, and sometimes it has reversed polarity (figure above c). A time when the Earth’s field flips from normal to reversed polarity, or vice versa, is called a magnetic reversal. When the Earth has reversed polarity, the south magnetic pole lies near the north geographic pole, and the north magnetic pole lies near the south geographic pole. Thus, if you were to use a compass during periods when the Earth’s magnetic field was reversed, the north-seeking end of the needle would point to the south geographic pole. Note that the Earth itself doesn't turn upside down it is just the magnetic field that reverses.

In the 1950s, about the same time researchers discovered polarity reversals, they developed a technique that permitted them to measure the age of a rock in years. Geologists applied the technique to determine the ages of rock layers in which they obtained their paleomagnetic measurements, and thus determined when the magnetic field of the Earth reversed. With this information, they constructed a history of magnetic reversals for the past 4.5 million years; this history is now called the magnetic-reversal chronology. The time interval between successive reversals is called a chron.

A diagram representing the Earth’s magnetic-reversal chronology (figure above d) shows that reversals do not occur regularly, so the lengths of different polarity chrons are different. For example, we have had a normal-polarity chron for about the last 700,000 years. Before that, a reversed-polarity chron occurred. The youngest four polarity chrons (Brunhes, Matuyama, Gauss, and Gilbert) were named after scientists who had made important contributions to the study of magnetism. As more measurements became available, investigators realized that some short-duration reversals (less than 200,000 years long) took place within the chrons, and they called these shorter durations “polarity subchrons.” Using isotopic dating, it was possible to determine the age of chrons back to 4.5 Ma.

Interpreting marine magnetic anomalies

The progressive development of magnetic anomalies and the long-term reversals chronology.

Why do marine magnetic anomalies exist? In 1963, researchers in Britain and Canada proposed a solution to this riddle. Simply put, a positive anomaly occurs over areas of the sea floor where underlying basalt has normal polarity. In these areas, the magnetic force produced by the magnetite grains in basalt adds to the force produced by the Earth’s dipole the sum of these forces yields a stronger magnetic signal than expected due to the dipole alone (figure above a). A negative anomaly occurs over regions of the sea floor where the underlying basalt has a reversed polarity. In these regions, the magnetic force of the basalt subtracts from the force produced by the Earth’s dipole, so the measured magnetic signal is weaker than expected.

The sea-floor-spreading model easily explains not only why positive and negative magnetic anomalies exist over the sea floor, but also why they define stripes that trend parallel to the mid-ocean ridge and why the pattern of stripes on one side of the ridge is the mirror image of the pattern on the other side  (figure above b). To see why, let’s examine stages in the process of sea-floor spreading (figure above c). Imagine that at Time 1 in the past, the Earth’s magnetic field has normal polarity. As the basalt rising at the mid-ocean ridge during this time interval cools and solidifies, the tiny magnetic grains in basalt align with the Earth’s field, and thus the rock as a whole has a normal polarity. Sea floor formed during Time 1 will therefore generate a positive anomaly and appear as a dark stripe on an anomaly map. As it forms, the rock of this stripe moves away from the ridge axis, so half goes to the right and half to the left. Now imagine that later, at Time 2, Earth’s field has reversed polarity. Sea-floor basalt formed during Time 2, therefore, has reversed polarity and will appear as a light stripe on an anomaly map. As it forms, this reversed-polarity stripe moves away from the ridge axis, and even younger crust forms along the axis. The basalt in each new stripe of crust preserves the polarity that was present at the time it formed, so as the Earth’s magnetic field flips back and forth, alternating positive and negative anomaly stripes form. A positive anomaly exists over the ridge axis today because sea floor is forming during the present chron of normal polarity.

Closer examination of a sea-floor magnetic anomaly map reveals that anomalies are not all the same width. Geologists found that the relative widths of anomaly stripes near the Mid-Atlantic Ridge are the same as the relative durations of paleomagnetic chrons (figure above d). This relationship between anomaly-stripe width and polarity-chron duration indicates that the rate of sea-floor spreading has been constant along the Mid-Atlantic Ridge for at least the last 4.5 million years. If you assume that the spreading rate was constant for tens to hundreds of millions of years, then it is possible to estimate the age of stripes right up to the edge of the ocean.

Evidence from Deep-Sea Drilling

In the late 1960s, a research drilling ship called the Glomar Challenger set out to sail around the ocean drilling holes into the sea floor. This amazing ship could lower enough drill pipe to drill in 5-km-deep water and could continue to drill until the hole reached a depth of about 1.7 km (1.1 miles) below the sea floor. Drillers brought up cores of rock and sediment that geoscientists then studied on board.

On one of its early cruises, the Glomar Challenger drilled a series of holes through sea-floor sediment to the basalt layer. These holes were spaced at progressively greater distances from the axis of the Mid-Atlantic Ridge. If the model of sea-floor spreading was correct, then not only should the sediment layer be progressively thicker away from the axis, but the age of the oldest sediment just above the basalt should be progressively older away from the axis. When the drilling and the analyses were complete, the prediction was confirmed. Thus, studies of both marine magnetic anomalies and the age of the sea floor proved the sea-floor-spreading model.

Credits: Stephen Marshak ( Essentials of Geology)

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