Abstract

Geomagnetism is the study of the magnetic field of the Earth. It originated in ancient China and Greece, where the properties of the natural magnet (lodestone) were recognized for the first time. Since the publication of De Magnete of Gilbert, geomagnetism as a science progressed by various observations and then by the theory of electromagnetism, which was formulated in the nineteenth century.

 

The present field is observed by magnetic observatories, surveys, and magnetic satellites.

But there are ways other than the direct measurements to observe the magnetic field.

Paleomagnetism, which studies the remanent magnetization in rocks and sediments to infer the magnetic field of ancient times, has greatly expanded the time span covered by observations.

By this method, we can trace what we do not see in the present-day field, that is, magnetic reversals and polar wander.

The exploration of solar system bodies by spacecraft opened up an exciting possibility of observing the magnetic field in planets and satellites. In such studies, the presence or absence of the magnetic field gives us a unique means to examine the internal structure of the planets.

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Long-Term Behaviors of the Geodynamo

Geomagnetism provided a very useful tool for recovering past plate motions through the analysis of oceanic magnetic anomalies.

There was a great reward for this service;

the resulting intense study of sea-floor spreading in the world oceans led to the establishment of the polarity reversal timescale.

When plate tectonics was initiated, the polarity timescale was available only for the last 3.5 My from the paleomagnetic and geochronological studies of the volcanic rocks on land.

Following Vine’s first attempt to extend this timescale to tens of millions years in the past

(Vine, 1966), Heirtzler et al. (1968) produced a timescale that covered all the Cenozoic era, based on comparison of the magnetic anomaly patterns in the Atlantic, Pacific, and Indian Oceans.

This was a tremendous step for exploring the evolution of oceanic plates, because it provided a way to date the ocean floor, relying mostly on the pattern of magnetic anomalies.

The reversal timescale ended abruptly at about 80 Ma due to the ‘Magnetic Quiet Zone’ where magnetic anomaly was absent.

Later, Larson and Pitman (1972) succeeded in establishing the polarity timescale for the parts of older oceans beyond the Quiet Zone.

With Cenozoic and Mesozoic timescales, it is possible to determine the age of the all ocean floors (Figure 16).

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Figure 16. Digital isochrons of oceans based on magnetic chronology.

From Mueller RD, Roest WR, Royer J-Y, Gahagan LM, and Sclater JG Digital isochrons of the world’s ocean floor. Journal of Geophysical Research 102: 3211–3214.

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The polarity timescale has been refined and updated many times. The current standard timescale is the one presented by

Cande and Kent (1992, 1995).

Further development is discussed in Chapter 5.12.

Polarity timescales are the source of very important information about the nature of the geomagnetic field.

The Quiet Zone is the part of the ocean where magnetic anomalies are absent because polarity reversals did not occur.

The long period without reversals is called the Cretaceous Long Normal (CLN), and is speculated that the core dynamo state was significantly different at that time from both before

(Jurassic and early Cretaceous) and after that time (Tertiary and Quaternary).

From the analysis of the polarity timescale, it was concluded that the reversals occur without memories of the past (Poisson process).

The reversal process is not steady; the probability of a reversal increases monotonically from the end of the CLN to the present (McFadden, 1984).

The typical timescale of the dynamo process in the core is considered to be about 15 000 years (e.g., Kono and Roberts, 2002). The geomagnetic polarity timescale provides the record of field behavior for time intervals much longer than this time.

Thus there is a strong possibility that the change in the dynamo behavior over such long times represents the changes in the environment in which the dynamo process operates

(McFadden and Merrill, 1984).