Memories set in stone

Rock Magnetism - Introduction to Geomagnetic Fields

February 6, 1998

Tell most people that they live on a giant bar magnet, spend their lives cocooned in invisible yet omnipresent electromagnetic fields, and that the rocks they walk on record and store this field in much the same way as information on a computer disk, and you will probably be accused of watching one too many episodes of The X Files. But geomagnetism - the study of the earth's magnetic field - is a very real and tangible scientific discipline that has helped to resolve some of the most debated geological controversies of the century. In particular, the discovery in the early 1960s of magnetic strips on the ocean floor by Fred Vine and Drummond Mathews - arguably the most important discovery in geoscience this century, because it proved that the new sea floor was being created at mid-ocean ridges - effectively confirmed Alfred Wegener's idea of continental drift and led directly to the all-embracing theory of plate tectonics.

According to the works of Hanfucious, the Chinese were exploiting the geomagnetic properties of the earth through use of the compass around 250bc (migrating fish and birds have been doing it for rather longer). William Gilbert in De Magnete, published in 1600, first suggested that the earth may have the properties of a huge bar magnet, and measurements of the magnetic field declination by seafaring European navigators from the 17th century onwards, provide a valuable historical record of changes in its magnetic intensity. The earth's magnetic field has its origin deep inside the planet. Convective motions in metallic fluids in the outer core set up electric currents, producing a dipolar field that extends through the solid earth and atmosphere into space. Although the details of how this happens are still unclear, research in the past few years using computer simulations has shed new light on how the "self-exciting dynamo", as it is known, may work.

Wallace Campbell's Introduction to Geomagnetic Fields is a short, general book that summarises in a non-technical way the basic concepts and physical processes of geomagnetism. It starts with a nice description of the earth's main field. This is a complex topic requiring a proper understanding of Maxwell's equations, Fourier analysis and Legendre polynomials. One of the author's claims is that he provides a less technical coverage of geomagnetism than existing textbooks, and although I would not profess to be fully on top of all books in this category, I think this claim is valid. Mathematics is kept to a minimum, and a useful appendix provides a review and definition of those mathematical topics that do appear.

The earth's magnetic field is not constant, but changes from day to day. Sometimes it is quiet, at other times we are in the midst of a magnetic storm that can play havoc with terrestrial power grids and satellite tracking and communications systems. There is even some evidence to suggest that magnetic storms may affect the mental behaviour of humans. Geomagnetic storms originate in the sun, and their effect on the atmosphere is well covered here, as are the practicalities of measuring the ever-present magnetic field in and out of the laboratory.

Each chapter ends with a succinct summary, and the appendices include a useful listing of world data centres for geomagnetism along with the Internet address of the National Geophysical Data Centre, where the interested reader can download a range of geomagnetism computer software free of charge.

While Campbell provides a good all-round introduction to the topic, David Dunlop and Ozden Ozdemir offer an altogether weightier tome that is the latest in a series of graduate texts on magnetism. The fact that ferromagnesian minerals in rocks act like tiny compasses, trapped and frozen in the earth's magnetic field at the time of their formation thus preserving a fossilised record of their then-latitude, may seem like scientific confirmation of the mystic powers of crystals. Yet since the 1950s, a whole subdiscipline of geophysics called paleo-magnetism has grown up dedicated to reading and interpreting the magnetic signal stored in rocks. By reading this memory in the laboratory, geophysicists have proved that the earth's continents over geological time have drifted away from present-day north, and that the earth's magnetic field is able periodically to reverse itself.

The book has 17 chapters that provide a comprehensive treatment of the basic theory behind rock magnetism and the magnetism of fine particles, from the atomic scale to large-scale plate tectonic motions. No mathematical appendices or computer programs here, but a comprehensive 20-page list of references will keep the specialist happy. Beginning with the basic principles of fine-particle magnetism, they move on to magnetic mineralogy, magnetostatic principles and magnetic domain structure and hysteresis at a technical level suitable for graduate students upwards. I found the section on the application of microscopic techniques including magnetic force microscopy in identifying magnetic domains (areas of reduced magnetostatic energy) particularly enlightening. The hows and whys of thermal, chemical and remnant magnetisation in minerals, a technically challenging subject, are explained clearly and, while this was not for the mathematically faint-hearted, the good use of graphics and the occasional photomicrograph should enable most likely readers to get a quantitative feel for the underlying physics.

Chapters seven to nine are also dedicated to fundamentals including the modifying effects of temperature and time on primary magnetic properties, and recent theoretical advances in micromagnetic computation that allow non-deterministic investigation of domain structure in submicron-sized grains.

One criticism of books on "rock magnetism" is that they never really deal with rocks at all, and concentrate instead on the few small ferromagnesian minerals such as magnetite and haematite that store most of a rock's magnetic moment. Dunlop and Ozdemir counter this by devoting their final four chapters to a review of each main rock type - igneous, metamorphic and sedimentary - in the context of their geological environment. Each of these chapters is a book in itself, and the authors have done a fine job in synthesising a vast amount of literature. Highlights for me include the sections on igneous and altered basaltic rocks, but I was surprised by the lack of any reference to AMS (anistropy of magnetic susceptibility) dating, which has been used to good effect in recent years in mapping out foliation patterns in deformed granitic plutons.

Rock Magnetism ends with a review of extraterrestrial magnetism. Besides the fact that the fields of the other planets are orders of magnitude less than that of the present-day earth (0.3-0.6 gauss), the apparent lack of plate tectonics, low oxygen fugacities during crystallisation and vastly different weathering processes on other planets, make this a challenging area. Even meteorites, the oldest available rocks, which at first glance might seem good clean candidates, suffer from the thermal effects of their scorching passage through the atmosphere, so that the proper meaning of their measured natural remnant magnetism and paleo-intensities remains unclear. Despite this, meteorites and lunar samples collected by the Apollo missions clearly point to higher paleo-field intensities in the early solar system that may be preserved in rocks on the inner planets. Good news for budding astro-paleo-magnetists.

Nick Petford is a senior lecturer in geology, Kingston University.

Rock Magnetism: Fundamentals and Frontiers

Author - David J. Dunlop and Ozden Ozdemir
ISBN - 0 521 32514 5
Publisher - Cambridge University Press
Price - £80.00
Pages - 573

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