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This book is the sequel to Palaeomagnetism and Plate Tectonics written by Michael W. McElhinny, first published in 1973. The aim of that book was to explain the intricacies of paleomagnetism and of plate tectonics and then to demonstrate that paleomagnetism confirmed the validity of the new paradigm. Today it is no longer necessary to explain plate tectonics, but paleomagnetism has progressed rapidly over the past 25 years. Furthermore, magnetic anomaly data over most of the oceans have been analyzed in the context of sea-floor spreading and reversals of the Earth's magnetic field. Oceanic data can also be used to determine paleomagnetic poles by combining disparate types of data, from deep-sea cores, seamounts, and magnetic anomalies. Our aim here is to explain paleomagnetism and its contribution in both the continental and the oceanic environment, following the general outline of the initial book. We demonstrate the use of paleomagnetism in determining the evolution of the Earth's crust.
Our intention has been to write a text that can be understood by Earth-science undergraduates at about second-year level. To make the text as accessible as possible, we have kept the mathematics to a minimum. The book can be considered a companion volume to The Magnetic Field of the Earth by Ronald T. Merrill, Michael W. McElhinny, and Phillip L. McFadden, which was published in the same series in 1996. There is inevitably some overlap between the books, occurring mostly in Chapter 4. However, the emphasis is different, with this text concentrating more on the geological aspects.
Chapter 1 introduces geomagnetism and explains the basis of paleomagnetism in that context. It follows the original book quite closely. Chapter 2 is about rock magnetism and the magnetic minerals that are important in paleomagnetism. The theory of rock magnetism is an essential part of understanding how and why paleomagnetism works. Chapter 3 deals with field and laboratory methods and techniques. The chapter concludes with a summary of some methods for identifying magnetic minerals. Chapter 4 describes the evidence for magnetic field reversals and their paleomagnetic applications. The development of the geomagnetic polarity time scale and its application to magnetostratigraphy are highlighted, together with the analysis of reversal sequences.
Oceanic paleomagnetism, including the modeling and interpretation of marine magnetic anomalies, is discussed in Chapter 5. Methods for determining pole positions using oceanic paleomagnetic data are also covered. Global maps in color show the age of the ocean floor and of the evolution of the Pacific Ocean. Chapter 6 summarizes the results from continental paleomagnetism and includes methods of data selection and combination to produce apparent polar wander paths. Reference apparent polar wander paths are then compiled and presented for each of the Earth's major crustal blocks.
Chapter 7 puts it all together and relates the results to global tectonics. Here we emphasize only the major features of global tectonic history that can be deduced from paleomagnetism. Van der Voo (1993) gives an excellent detailed account of the application of paleomagnetism to tectonics, and it is not our intention, in a single chapter, to provide readers with that level of detail and analysis. Color paleogeographic maps illustrate continental evolution since the Late Permian. A new and exciting development in global tectonics is the hypothesis of a Neoproterozoic supercontinent named Rodinia. Paleomagnetism is playing and will continue to play an important role in determining its configuration and evolution. With this in mind we discuss Earth history from 1000 Ma to the present through a combination of geology with paleomagnetism.
In writing the book we have had discussions with many colleagues. We thank Jean Besse, Dave Engebretson, Dennis Kent, Zheng-Xiang Li, Roger Larson, Dietmar Muller, Andrew Newell, Neil Opdyke, Chris Powell, Phil Schmidt, Chris Scotese, Jean-Pierre Valet, and Rob Van der Voo for their assistance in providing us with materials. Our special thanks go to Charlie Barton, Steve Cande, Jo Lock, Helen McFadden, Ron Merrill, and Sergei Pisarevsky, who read parts of the book and made valuable comments. Mike McElhinny thanks Vincent Courtillot and the Institute de Physique du Globe de Paris for providing financial assistance for a visit to that institute in 1997, during which time he commenced writing the book. Phil McFadden thanks Helen Hunt and Christine Hitchman for their assistance in preparing the manuscript, and Neil Williams and Trevor Powell for their continued support.
Hat Head and Canberra April 1999
Michael W. McElhinny Phillip L. McFadden
Geomagnetism and Paleomagnetism
The properties of lodestone (now known to be magnetite) were known to the Chinese in ancient times. The earliest known form of magnetic compass was invented by the Chinese probably as early as the 2nd century B.C., and consisted of a lodestone spoon rotating on a smooth board (Needham, 1962; see also Merrill et al., 1996). It was not until the 12th century A.D. that the compass arrived in Europe, where the first reference to it is made in 1190 by an English monk, Alexander Neckham. During the 13th century, it was noted that the compass needle pointed toward the pole star. Unlike other stars, the pole star appeared to be fixed in the sky, so it was concluded that the lodestone with which the needle was rubbed must obtain its "virtue" from this star. In the same century it was suggested that, in some way, the magnetic needle was affected by masses of lodestone on the Earth itself. This produced the idea of polar lodestone mountains, which had the merit at least of bringing magnetic directivity down to the Earth from the heavens for the first time (Smith, 1968).
Roger Bacon in 1216 first questioned the universality of the north-south directivity of the compass needle. A few years later Petrus Peregrinus questioned the idea of polar lodestone deposits, pointing out that lodestone deposits exist in many parts of the world, so why should the polar ones have preference? Petrus Peregrinus reported, in his Epistola de Magnete in 1269, a remarkable series of experiments with spherical pieces of lodestone (Smith, 1970a). He defined the concept of polarity for the first time in Europe, discovered magnetic meridians, and showed several ways of determining the positions of the poles of a lodestone sphere, each method illustrating an important magnetic property. He thus discovered the dipolar nature of the magnet, that the magnetic force is both strongest and vertical at the poles, and became the first person to formulate the law that like poles repel and unlike poles attract. The Epístola bears a remarkable resemblance to a modern scientific paper. Peregrinus used his experimental data as a source for new conclusions, unlike his contemporaries who sought to reconcile observations with pre-existing speculation. Although written in 1269 and widely circulated during the succeeding centuries, the Epístola was not published in printed form under Peregrinus' name until 1558.
Magnetic declination was known to the Chinese from about 720 A.D. (Needham, 1962; Smith and Needham, 1967), but knowledge of this did not travel to Europe with the compass. It was not rediscovered until the latter part of the 15lh century. By the end of that century, following the voyages of Columbus, the great age of exploration by sea had begun and the compass was well established as an aid to navigation. Magnetic inclination (or dip) was discovered by Georg Hartmann in 1544, but this discovery was not publicized. In 1576 it was independently discovered by Robert Norman. Mercator, in a letter in 1546, first realized from observations of magnetic declination that the point which the needle seeks could not lie in the heavens, leading him to fix the magnetic pole firmly on the Earth. Norman and Borough subsequently consolidated the view that magnetic directivity was associated with the Earth and began to realize that the cause was not the polar region but lay closer to the center of the Earth.
In 1600, William Gilbert published the results of his experimental studies in magnetism in what is usually regarded as the first scientific treatise ever written, entitled De Magnete. However, credit for writing the first scientific treatise should probably be given to Petrus Peregrinus for his Epístola de Magnete; Gilbert, whose work strongly influenced the course of magnetic study, must certainly have leaned heavily on this previous work (Smith, 1970a). He investigated the variation in inclination over the surface of a piece of lodestone cut into the shape of a sphere and summed up his conclusions in his statement "magnus magnes ipse est globus terrestris" (the Earth globe itself is a great magnet). Gilbert's work, confirming that the geomagnetic field is primarily dipolar, thus represented the culmination of many centuries of thought and experimentation on the subject. His conclusions put a stop to the wild speculations that were then common concerning magnetism and the magnetic needle. Apart from the roundness of the Earth, magnetism was the first property to be attributed to the body of the Earth as a whole. Newton's theory of gravitation came 87 years later with the publication of his Principia.
If a magnetic compass needle is weighted so as to swing horizontally, it takes up a definite direction at each place and its deviation from geographical or true north is called the declination (or magnetic variation), D. In geomagnetic studies D is reckoned positive or negative according as the deviation is east or west of true north. In paleomagnetic studies D is always measured clockwise (eastwards) from the present geographic north and consequently takes on any angle between 0° and 360°. The direction to which the needle points is called magnetic north and the vertical plane through this direction is called the magnetic meridian. A needle perfectly balanced about a horizontal axis (before being magnetized), so placed that it can swing freely in the plane of the magnetic meridian, is called a dip needle. After magnetization it takes up a position inclined to the horizontal by an angle called the inclination (or dip), I. The inclination is reckoned positive when the north-seeking end of the needle points downwards (as in the northern hemisphere) or negative when it points upwards (as in the southern hemisphere).
The main elements of the geomagnetic field are illustrated in Fig. 1.1. The total intensity F, declination D, and inclination I, completely define the field at any point. The horizontal and vertical components of F are denoted by H and Z. Z is reckoned positive downwards as for I. The horizontal component can be
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