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North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism

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by Gillian Turner Phd


  mantle convection Very slow movement of Earth’s mantle material, due primarily to upward heat flow from the core–mantle boundary and heat loss through the crust. The mantle is extremely viscous (almost solid), and convects very slowly in response to the temperature gradient. Hot buoyant material wells up at mid-ocean ridges, and cooler denser material is drawn back into the mantle along subduction zones.

  Maxwell’s equations Set of four mathematical equations describing the interactions between electric and magnetic fields, electric charge and current. Formulated by James Clerk Maxwell, they encapsulate the work of Faraday, Ampère and Gauss, and are the cornerstone of electromagnetism.

  meridians (geomagnetic) Great circles passing through both geomagnetic poles.

  meridians (geographic) Great circles passing through both geographic poles; lines of longitude.

  mid-ocean ridge/rise Underwater mountain range rising several thousand meters above the seafloor, where upwelling mantle material forms new seafloor, which is then carried away in both directions by the process of plate tectonics.

  natural remanent magnetization In situ remanent magnetization of a natural material, for example a rock.

  Navier-Stokes equations Mathematical equation used to describe the dynamics of fluid flow.

  non-dipole field (of Earth) Part of the geomagnetic field remaining when the field of the best-fitting tilted geocentric dipole is subtracted from the total field.

  nutation Small nodding-like oscillation of Earth’s rotation axis.

  ocean trench Trench or valley that occurs on the ocean floor at a plate boundary, where one plate is drawn down beneath the other. Also known as subduction zone.

  Ohm’s Law Physical law which states that the electric current flowing through a conductor varies in proportion to the electric potential difference or voltage across it. The ratio of potential difference to current is called the electrical resistance, and is measured in ohms.

  outer core Outer part of the Earth’s core, between 2900 and 4100 kilometers below the surface. The outer core has a molten metallic composition, mainly iron, nickel and small amounts of various lighter materials.

  P wave Longitudinal seismic wave, often referred to as the primary wave because it is the first wave felt after an earthquake. P waves can travel through elastic solids and fluids, including the molten outer core of the Earth. Also known as pressure wave.

  paleolatitude Latitude at which a rock or fossil formed; because of continental drift, this may be different from the rock or fossil’s present latitude. A rock or fossil’s paleolatitude may be determined from the inclination of magnetization acquired at its formation.

  paleomagnetic pole Ancient position of the geomagnetic pole relative to rocks of a particular continent. If the continent has drifted since the rock formed or was magnetized, the paleopole may no longer coincide with the geographic pole: the difference indicates the amount of continental drift that has taken place. Also known as paleopole.

  paleomagnetism Study of the history of Earth’s magnetic field and related phenomena through measurement and analysis of the remanent magnetization of rocks, sediments and ancient fired materials.

  paleopole See paleomagnetic pole.

  paramagnetism Magnetic property whereby, when an external magnetic field is applied to a substance, a weak magnetization is induced in it parallel to the external field; this magnetization is lost when the inducing field is removed.

  plate tectonics Theory that the Earth’s surface comprises a number of rigid plates extending through the crust and into the uppermost mantle; the plates move relative to each other, driven by the process of mantle convection, new crustal material being formed at mid-ocean ridges and old material returning to the mantle at ocean trenches or subduction zones.

  polar wander path/apparent polar wander path Trajectory on the surface of the Earth indicating the apparent motion of one of the paleomagnetic poles relative to the present position of the corresponding geographic pole. Differences between the polar wander paths derived from the rocks of different continents led to the acceptance of the theories of continental drift and plate tectonics.

  polarity chron Major interval of the geomagnetic polarity timescale, comprising a period of predominantly normal polarity or one of predominantly reversed polarity; each chron corresponds to a major feature of seafloor magnetic anomaly profiles, and is typically around one million years in duration.

  polarity reversal See geomagnetic field reversal.

  pole (of Earth) See geographic poles ; geomagnetic poles ; magnetic poles.

  pole (of magnet) Point near the end of a magnet, or on a terrella or globe, from which magnetic field lines diverge (north pole) or to which they converge (south pole).

  poloidal magnetic field Magnetic field that emerges through the core–mantle boundary, and constitutes the geomagnetic field observed at and above the surface of the Earth.

  portolan Medieval nautical navigation chart, showing compass bearings to be followed between sea ports.

  potassium−argon dating method Radiometric dating technique important in the development of the geomagnetic polarity timescale. The radio active isotope potassium-40 (40K) makes up a small but measurable fraction of the potassium contained in many rocks; it decays radioactively to argon-40 (40Ar). Comparison of the amount of 40Ar trapped in a rock with the amount of 40K remaining can be used to determine how long ago the rock was formed.

  precession (of equinoxes) Slow periodic rotational motion of Earth’s axis in space, due to gravitational forces acting between Earth and other planetary bodies.

  pressure wave See P wave.

  prime meridian Meridian from which longitude is measured; today this is the meridian through Greenwich, England. In earlier times other meridians were favored—for example, that through the Azores, where the declination was zero in about AD 1600.

  radiometric/isotopic age estimation Any of several techniques used to date materials from the relative abundances of certain naturally occurring radioactive isotopes and their radiogenic decay products in the material—for example, potassium−argon.

  remanent magnetization Magnetization of a material or sample over and above any induced magnetization. In other words, the magnetization retained by a sample when it is removed from any magnetic field. Remanent magnetization is carried by grains of ferri- or ferromagnetic minerals.

  S wave Transverse seismic wave that travels more slowly than a P wave, and so is felt later. S waves cannot propagate through fluids, and so do not enter Earth’s liquid outer core. Also known as secondary or shear wave.

  secondary or shear wave See S wave.

  secular variation Gradual changes in the declination, inclination and/or intensity of the (internal) geomagnetic field that take place over timescales ranging from tens to thousands of years.

  self-reversal Rare magnetic property whereby a material acquires a stable remanent magnetization in the opposite direction from the prevailing magnetic field.

  self-sustaining dynamo Dynamo in which the induced current or flow of a conductive medium generates a magnetic field that reinforces the original field inducing the current or flow.

  solar wind Stream of electrically charged particles continually emitted by the sun, and flowing past the Earth at supersonic speeds.

  spherical harmonic analysis Mathematical technique by which a series of measurements made on the surface of a sphere is represented in terms of a sum of wavelike functions; used in geomagnetism and other branches of physics and geophysics, such as gravity.

  subduction zone See ocean trench.

  sunspot Cooler region of the sun’s surface appearing as a dark spot, and associated with intense magnetic fields.

  terrella Solid sphere of lodestone that naturally takes on a uniform magnetization. Shown by Petrus Peregrinus and William Gilbert to have two poles, and later shown to have a dipolar magnetic field.

  thermoremanent magnetization Magnetization acquired by a material as it cools through
the Curie temperature of its constituent ferri- or ferromagnetic minerals.

  toroidal magnetic field Magnetic field that is confined within the Earth’s core and does not emerge through the core–mantle boundary; cannot be observed at the Earth’s surface.

  torsion balance Device employed for measuring weak forces. It consists of a bar suspended horizontally from its center of mass by a fine thread or wire. When a force is applied perpendicular to the end of the bar, the bar rotates; the angle of rotation is proportional to the force.

  transform fault Boundary between two tectonic plates where the plates slide past each other. Transform faults occur mainly at ocean ridges, where they connect sections of the ridge system.

  variation Original term for declination; used only rarely today.

  westward drift General, predominantly westward, drift of features of Earth’s magnetic field due to secular variation.

  Select Bibliography

  My journey through geomagnetism has led me to many hundreds of books, review and research papers, historical publications, encyclopedias and, of course, websites. In a book of this nature it is impractical to give a full bibliography: I have restricted myself to major sources of information, and books that will be accessible to the general reader who is interested in reading more widely on the subject.

  De Magnete, William Gilbert, 1600; translated by P. Fleury Mottelay: Courier Dover Publications, New York, 1958

  The Earth: Its Origin, History and Physical Constitution, Harold Jeffreys: Cambridge University Press, Cambridge, 1976

  “Earth’s Core and the Geodynamo,” Bruce A. Buffett: Science 288, 2000

  Encyclopedia Britannica: www.britannica.com

  Encyclopedia of Geomagnetism and Paleomagnetism, David Gubbins and Emilio Herrero-Bervera, editors: Springer, Dordrecht, 2007

  Faraday, Maxwell and Kelvin, D.K.C. MacDonald: Doubleday, London, 1964

  The Fellowship: The Story of a Revolution, John Gribben: Allen Lane, London, 2005

  Foundations of Modern Physical Science, Gerald Holton and Duane H.D. Roller: Addison-Wesley, Reading, Massachusetts, 1958

  Fundamentals of Geophysics, William Lowrie: Cambridge University Press, Cambridge 1997

  Geomagnetism, S. Chapman and J. Bartels: Clarendon Press, Oxford, 1940; second edition 1962

  Latitude and the Magnetic Earth: The True Story of Queen Elizabeth’s Most Distinguished Man of Science, Stephen Pumfrey: Icon Books, Cambridge, 2002

  Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, Dava Sobel: Fourth Estate, London, 1995

  The Magnetic Field of the Earth: Paleomagnetism, The Core, and The Deep Mantle, Ronald T. Merrill, Michael W. McElhinny, and Phillip L. McFadden: Academic Press, San Diego, 1998

  “A Millenium of Geomagnetism,” David P. Stern: Reviews of Geophysics 40, 2002

  Michael Faraday: A Biography, L. Pearce Williams: Chapman and Hall, London, 1965

  The Origins and Growth of Physical Science, Volume 2, D.L. Hurd and J.J. Kipling: Pelican, London, 1970

  Plate Tectonics and Geomagnetic Reversals: Selected Readings and Introductions, A. Cox, editor: W.H. Freeman and Co. Ltd., San Francisco, 1973

  Remarkable Physicists: From Galileo to Yukawa, Ioan James: Cambridge University Press, Cambridge, 2004

  The Road to Jaramillo: Critical Years of the Revolution in Earth Science, William Glen: Stanford University Press, Stanford, 1982

  The Royal Institution of Great Britain: www.rigb.org

  The Royal Society: www.royalsociety.org

  St. Andrews University, Scotland: www-groups.dcs.st-and.ac.uk/~history/Biographies

  Science and Civilization in China, Volume 4: Physics and Physical Technology, Joseph Needham: Cambridge University Press, Cambridge, 1962

  A Source Book in Physics, William Francis Magie: Harvard University Press, Cambridge, 1935

  “A three-dimensional self-consistent computer simulation of a geomagnetic field reversal,” G.A. Glatzmaier and P.H. Roberts: Nature 377, 1995

  Treatise on Geophysics, Volume V: Geomagnetism, Masaru Kono, editor: Elsevier, Amsterdam, 2007

  Illustration Credits

  Illustrations are listed by page number.

  Reproduced from Science and Civilization in China, vol. 4, pt I, plate CXXII (fig. 344) by Joseph Needham: Cambridge University Press, Cambridge, 1962. Reprinted with permission.

  From Cosmographia by Peter Apian, 1524

  Courtesy European Geosciences Union

  Attributed to Joan Oliva (1580–1615), pen-and-ink and watercolor on vellum, G1059.O4 1590, Library of Congress Geography and Map Division Washington, D.C. 20540-4650, USA

  From De Magnete by William Gilbert, 1600; reproduced from P. Fleury Mottelay’s 1893 translation thereof: Courier Dover Publications Ltd, New York, 1958

  From De Magnete by William Gilbert

  (top) From De Magnete by William Gilbert

  (bottom) From De Magnete by William Gilbert

  Reproduced from Reports of the British Association for the Advancement of Science, vol. 5, 1836

  Illustration courtesy of the author

  Reproduced from An account of the Cause of the Change of the Variation of the Magnetical Needle; With an Hypothesis of the Structure of the Internal Parts of the Earth: As It Was Proposed to the Royal Society in One of Their Late Meetings by Edmond Halley: Philosophical Transactions of the Royal Society of London, vol. 16, pp. 563–578, 1692

  Reproduced from The Three Voyages of Edmond Halley in the Paramore, 1698–1701 edited by Norman J.W. Thrower: Hakluyt Society, London, 1981

  From De Magnete by William Gilbert

  From Neu-entdeckte Phaenomena von bewunderswürdigen Würkungen der Natur by A. Doppelmayr, Nuremberg, 1774, reproduced in Electricity in the 17th and 18th Centuries: A Study of Early Modern Physics by J.L. Heilbron: University of California Press, Berkeley, 1979

  Reproduced from Magnetism and Electricity for Students by H.E. Hadley: MacMillan & Company, London, 1924

  Reproduced from Foundations of Modern Physical Science by G. Holton and D.H.D. Roller: Addison-Wesley, Reading, Massachusetts, 1958

  Reproduced from Foundations of Modern Physical Science by G. Holton and D.H.D. Roller: Addison-Wesley, Reading, Massachusetts, 1958

  (top) The Royal Institution, London

  (bottom) The Royal Institution, London/The Bridgeman Art Library

  Natural Philosophy Collection, University of Aberdeen

  Science Museum/Science and Society Photo Library (ref. 10302107)

  Courtesy of Dr. F. E. M. Lilley

  Reproduced from Reports of the British Association for the Advancement of Science, vol. 5, 1836

  (top) From Magnetismus der Erde by Christopher Hansteen, 1819, reproduced in Reports of the British Association for the Advancement of Science, vol. 5, 1836

  (bottom) From Magnetismus der Erde by Christopher Hansteen

  From a lithograph by Siegfried Bendixen, 1828

  Compiled from a program written by Dr P.L. McFadden, Australian Geological Survey Organization, 2000

  Reproduced from Magnetism and Electricity for Students by H.E. Hadley: MacMillan & Company, London, 1924

  Reproduced from Magnetism and Electricity for Students by H.E. Hadley

  Drawing by W. Alexander, reproduced courtesy of Hulton Archive/Getty Images

  Illustration courtesy of the author

  Illustration courtesy of the author

  Illustration courtesy of the author; after Inge Lehmann, 1936

  (top) Illustration courtesy of the author

  (bottom) Cut-away of Earth showing interior structure Mehau Kulyk/Science Photo Library

  Reproduced from Comptes Rendus de L’Académie des Sciences, series II (Earth and Planetary Sciences), vol. 328, 1999, p. 143

  AIP Emilio Segre Visual Archives, Weber Collection

  Courtesy of Edward Irving

  From Time magazine, September 27, 1954

  Illustration by Matthias Me
yer

  Reproduced from Plate Tectonics and Geomagnetic Reversals edited by Allan Cox: W.H. Freeman & Co. Ltd, San Francisco, 1973; based on an original in Raff & Mason, 1961, Geological Society of America Bulletin 72, pp. 1267–1270

  Illustration by Matthias Meyer

  Illustration courtesy of the author

  Reproduced from Fundamentals of Geophysics by William Lowrie, Cambridge University Press, Cambridge, 1997, based on an original in Heirtzler et al, Deep Sea Research 13, pp. 427–443, 1966

  Illustration courtesy of the author

  Courtesy The Royal Society

  Illustration by Matthias Meyer

  Reproduced with permission of the Computer Laboratory, University of Cambridge, England

  Courtesy of Gary Glatzmaier and Paul Roberts

  (left) Courtesy of Gary Glatzmaier

  (right) Courtesy of Paul Roberts

  Courtesy of Gary Glatzmaier and Paul Roberts

  Created by K. Endo, courtesy of Prof. Yohsuke Kamide, National Geophysical Data Center Boulder, Colorado

  Acknowledgments

  Researching and writing North Pole, South Pole has been one of the biggest challenges of my career. Among all the tasks of a university academic it is not easy to dedicate the time necessary to produce a book that is neither a text nor a research manual, but the history of a scientific quest that has spanned several millennia. It has, nonetheless, been one of the richest experiences of my life, and this is in no small part thanks to the tremendous help and support of many friends, colleagues and family members, and the wonderful people at Awa Press.

  My preliminary research was carried out during study leave at the National Oceanography Center and University of Southampton, where I was hosted by Professor Andrew Roberts. I have fond memories of evenings sitting outside Andrew’s office, gazing across Southampton Water towards the New Forest and discussing geophysical complexities such as precession of the equinoxes and the Chandler Wobble, not to mention our discoveries regarding the personal lives of great scientists such as Coulomb, Gauss and Sabine. Thanks also to Professors Kathy Whaler and Ken Creer of Edinburgh University for entertaining me at short notice and generously sharing their wisdom and stories on, respectively, the geodynamo and the seminal discoveries of the Cambridge paleomagnetism group in the 1950s.

 

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