Praise for North Pole, South Pole
“A wonderful read that put me in mind of Dava Sobel’s books. This is an insightful and lively account of a complex subject that deftly weaves the story of Earth’s magnetic field through vignettes of physicists, mathematicians, and explorers through the ages, culminating in the persuasive observations of modern paleomagnetists and theorists.”
—Dennis Kent, Board of Governors Professor of Earth and Planetary Sciences, Rutgers University
“[An] engaging appreciation of science at work discovering the mysteries of magnetism.”
—Kirkus Reviews
“A fantastic story, highly readable.”
—Simon Lamb, author of Devil in the Mountain
“A compelling narrative of the two-thousand-year scientific struggle to unlock the innermost secrets of the cosmic speck of dust we call home. Engagingly written in a lively style accessible to all.”
—M. E. (Ted) Evans, Professor Emeritus, Department of Physics, University of Alberta
“A wonderful, joyful, lucid book. Turner is a natural storyteller.”
—Ted Irving, Geological Survey of Canada
“Clearly written and beautifully illustrated.”
—Sir Paul Callaghan, Alan MacDiarmid Professor of Physical Sciences, Victoria University
“A fascinating read.” —Kenneth Creer, University of Edinburgh
“Gillian Turner has a great gift for writing about science, and personal knowledge of many of the modern giants of geomagnetism. This book will enthuse anyone, young or old, about the physics of the world around them.”
—Ted Lilley, Australian National University
“In recent years, many very good books for interested non-scientists have been published: Richard Dawkins’s Climbing Mount Improbable and The Ancestor’s Tale, Stephen Jay Gould’s The Lying Stones of Marrakech, and Dava Sobel’s Longitude and The Planets, to name some of them. North Pole, South Pole . . . is a worthy addition to that list. . . .Gillian Turner has a great story to tell, and she tells it well.”
—The Press (New Zealand)
North Pole, South Pole
North Pole, South Pole
The Epic Quest to Solve the Great Mystery of Earth’s Magnetism
Gillian Turner
North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth’s Magnetism
Copyright © Gillian Turner, 2010, 2011
All rights reserved. Except for brief passages quoted in newspaper, magazine, radio,
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The Experiment, LLC
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www.theexperimentpublishing.com
First edition published by Awa Press, Wellington, New Zealand. First North American
edition published by arrangement.
The Experiment’s books are available at special discounts when purchased in bulk
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Library of Congress Control Number: 2010934222
E-book ISBN 978-1-61519-132-1
Cover design by Michael Fusco | michaelfuscodesign.com
Typeset by Jenn Hadley, Wellington, NZ
Author photograph by Robert Cross, Victoria University of Wellington Image Services
Cover illustrations: “Magnetic variations at sea,” an engraving from De Magnete by
William Gilbert, 1600 (Science and Society Picture Library). “The Earth’s figure
and dimensions,” a drawing from The Beauty of the Heavens by Charles F. Blunt, 1849
(Mary Evans Picture Library).
Manufactured in the United States of America
First printed January 2011
Published simultaneously in Canada
10 9 8 7 6 5 4 3 2 1
To my family–
my parents, my husband, my children
Contents
Illustrations
Main Characters
Introduction
1 The Mystery of Magnetism
2 Voyages of Discovery
3 Magnus Magnes
4 The Wandering Compass Needle
5 Measuring the Force
6 Of Forces and Fields
7 The Third Element
8 The Magnetic Crusade
9 The Core of the Matter
10 Reading the Rocks
11 Poles Flipped, Continents Adrift
12 The Story on the Seafloor
13 Unraveling the Record
14 In Search of a Solution
15 The Geodynamo
Epilogue
Glossary
Select Bibliography
Illustration Credits
Acknowledgments
Index
About the Author
Illustrations
Thales of Miletus (c. 624–546 BC)
Reconstruction of the earliest Chinese magnetic compass
Plan of part of the Chinese town of Shandan
Medieval concept of a geocentric universe
Petrus Peregrinus Medal of the European Geosciences Union
Medieval “portolan” chart of the eastern Mediterranean and Black Sea region
Map of the Septentrionalium Terrarum (Northern Lands or Lands of the Seven Stars by Gerardus Mercator
Robert Norman’s demonstration of the inclination, or dip, of a magnetized needle
Portrait of William Gilbert (c. 1544–1603)
Gilbert’s demonstration of the orientations of short iron needles placed around a lodestone terrella
Gilbert’s sketch of the inclination of a dip needle placed at various latitudes around the Earth
Chart of magnetic declination around 1600
Magnetic meridians due to a geocentric axial dipole and due to a tilted geocentric dipole
Edmond Halley’s concept of Earth’s internal structure
Earliest chart of magnetic declination, or “variation,” over the Atlantic Ocean, published by Edmond Halley, 1701
William Gilbert’s electrostatic versorium
Stephen Gray’s “charity boy” demonstration of electric conduction
Sketch of Coulomb’s magnetic torsion balance
Portrait of Michael Faraday (1791–1867)
Faraday’s apparatus to demonstrate magnetic forces between magnetic poles and current-carrying wires
Faraday’s sketch of his electromagnetic induction coils
Michael Faraday’s electromagnetic induction coils
Faraday disc dynamo
Portrait of James Clerk Maxwell (1831–1879)
Dip needle used on d’Entrecasteaux’s expedition
Chart of geomagnetic intensity around the world c. 1830
Chart of geomagnetic declination around the world, 1787
Chart of geomagnetic inclination around the world, 1780
Portrait of Carl Friedrich Gauss (1777–1855)
Diagrammatic representation of spherical harmonic components
Kew pattern magnetometer
Kew pattern dip circle
Portrait of Henry Cavendish (
1731–1810)
Formation of a seismic shadow zone
Seismic shadow zone of 1929 Murchison earthquake, New Zealand
Seismograms of 1929 New Zealand earthquake from eastern Europe and Asia
Seismic ray paths through the Earth
Internal structure of the Earth
Bernard Brunhes (1867–1910)
Louis Néel (1904–2000)
Edward Irving (born 1927)
Apparent polar wander path of Creer, Irving and Runcorn, 1954
Mantle convection
Marine magnetic anomaly chart off the west coast of North America
Block diagram of seafloor spreading/magnetic anomaly formation
Early steps in the development of the geomagnetic polarity timescale
Marine magnetic anomalies over the Reykjanes Ridge, North Atlantic Ocean
Modern version of the geomagnetic polarity timescale for the past 80 million years
Joseph Larmor (1857–1942)
Single- and double-disc dynamos
Differential analyzer at Cambridge University
Glatzmaier and Roberts’ simulation of geomagnetic field during period of stable, normal polarity
Gary Glatzmaier (born 1949)
Paul Roberts (born 1929)
Glatzmaier and Roberts’ simulation of geomagnetic field during polarity reversal
Artist’s impression of Earth’s magnetic field as if visible from space
Main Characters
Ampère, André-Marie (1775–1836). French mathematician and physicist; founder of electrodynamics.
Brunhes, Bernard (1867–1910). French geophysicist who, with Pierre David, discovered lava flows and baked clays magnetized in the opposite direction to Earth’s magnetic field.
Bullard, Edward (Teddy) Crisp (1907–1980). English geophysicist; early researcher on dynamo theories of Earth’s magnetic field.
Coulomb, Charles-Augustin de (1736–1806). French military engineer who discovered the inverse square laws of electrostatic and magnetostatic attraction and repulsion.
Creer, Kenneth (born 1925). Member of the group of Cambridge paleomagnetists who, in the 1950s, discovered apparent polar wander, contributing to the confirmation of polarity reversals and continental drift; later professor of geophysics at Edinburgh University.
d’Entrecasteaux, Bruni (1739–1793). French explorer who, with Elisabeth de Rossel, made the first measurements of relative geomagnetic intensity.
Elsasser, Walter Maurice (1904–1991). American geophysicist; early researcher of geomagnetic dynamo theories.
Faraday, Michael (1791–1867). English experimental physicist and chemist; director of the Royal Institution; discoverer of electromagnetic induction.
Gauss, Carl Friedrich (1777–1855). German mathematician instrumental in establishing a worldwide network of geomagnetic observatories, and developing first mathematical representation of geomagnetic field.
Gellibrand, Henry (1597–1636). Professor of astronomy at Gresham College, London, who discovered that declination, the angle of deviation of a compass needle from true north, changes with time—the phenomenon known as geomagnetic secular variation.
Gilbert, William (1544–1603). Sixteenth-century experimentalist; physician to Queen Elizabeth I, and author of the classic work De Magnete.
Glatzmaier, Gary (born 1949). American solar physicist and geophysicist; with Paul Roberts, developed the first internally consistent computer simulation of a magnetohydrodynamic, self-exciting dynamo in the Earth’s outer core to undergo spontaneous polarity reversals.
Graham, George (1675–1751). English compass-maker who discovered the geomagnetic diurnal (daily) variation.
Graham, John American paleomagnetist who, in the mid twentieth century, designed methods to test the authenticity and antiquity of magnetization in rocks; a skeptic of field reversal theory.
Halley, Edmond (1656–1742). English astronomer and explorer who produced the first chart of magnetic declination, covering the Atlantic Ocean, and developed a four-pole theory of Earth’s magnetic field.
Hansteen, Christopher (1784–1873). Norwegian astronomer and physicist who advocated and elaborated on Edmund Halley’s four-pole theory of Earth’s magnetic field, and produced the first charts of geomagnetic intensity (isodynamic charts).
Hartmann, Georg (1489–1564). Vicar of Nuremberg; keeper of early records of declination (circa 1510), and possibly inclination (circa 1544).
Hospers, Jan (1925–2006). Cambridge paleomagnetist who, in the early 1950s, discovered sequences of normally and reversely magnetized lava flows, and proposed the geocentric axial dipole hypothesis. Later professor of applied geophysics at Norwegian Institute of Technology, University of Trondheim.
Irving, Edward (Ted) (born 1927). Member of the group of Cambridge paleomagnetists who, in the 1950s, discovered apparent polar wander, contributing to the confirmation of polarity reversals and continental drift. Established the first paleomagnetism laboratory at the Australian National University, Canberra, and later another at the Pacific Geoscience Center in British Columbia, Canada.
Larmor, Joseph (1857–1942). Anglo-Irish physicist and Fellow of St. John’s College, Cambridge University, whose idea of a solar magnetic dynamo eventually led to dynamo theories of Earth’s magnetic field; his work on nuclear magnetic moments led to modern nuclear resonance and magnetic resonance imaging techniques, and to a range of magnetometers based on the same principle.
Lehmann, Inge (1888–1993). Danish seismologist who discovered the solid inner part of the Earth’s core.
Magnes. Legendary Greek shepherd whose iron-tipped boots and staff were attracted to magnetized lodestone rocks, and whose name is reputedly the origin of the word “magnetism.”
Maricourt, Pierre Pèlerin, de. See Peregrinus, Petrus.
Matthews, Drummond (1931–1997). Cambridge University marine geophysicist who, with his student Fred Vine, first published an explanation of marine magnetic anomalies in terms of geomagnetic polarity reversals and seafloor spreading.
Matuyama, Motonori (1884–1958). Japanese paleomagnetist who first suggested that normally and reversely magnetized rocks follow an age sequence.
Maxwell, James Clerk (1831–1879). Scottish professor of theoretical physics who founded the laws of electromagnetism, invariably known as Maxwell’s Equations.
Mercanton, Pierre (1876–1963). French paleomagnetist who showed that reversely magnetized rocks occur all over the globe.
Morley, Lawrence (born 1920). Toronto-based geophysicist whose theory combining seafloor spreading and geomagnetic field reversals to explain observed patterns of marine magnetic anomalies, proposed at the same time as that of Vine and Matthews, was originally rejected for publication; eventually acknowledged as the co-discoverer of the Vine-Matthews-Morley theory.
Neckam, Alexander (1157–1217). English monk from St. Albans; the first European to describe the use of a compass for navigation.
Néel, Louis (1904–2000). French physicist; developed a theory to explain the stable (thermo-) remanent magnetization of volcanic rocks and lavas, carried by tiny grains of magnetic minerals.
Norman, Robert. Sixteenth-century English hydrographer who, in 1576, discovered and described the inclination of the geomagnetic field.
Ørsted, Hans Christian (1777–1851). Danish professor who discovered that an electric current has an associated magnetic field.
Peregrinus, Petrus Thirteenth-century French crusader; author of Epistola de Magnete (1269), in which he described the poles of a magnet, the poles of the Earth and magnetic compasses.
Roberts, Paul (born 1929). British theoretical geophysicist who, with Gary Glatzmaier, developed the first internally consistent computer simulation of a magnetohydrodynamic, self-exciting dynamo in the Earth’s outer core to undergo spontaneous polarity reversals.
Runcorn, S. Keith (1922–1995). Member of the group of Cambridge paleomagnetists who, in the 1950s, discovered apparent polar wander, contributing to
the confirmation of polarity reversals and continental drift; dynamo theorist, later professor of physics at University of Newcastle upon Tyne.
Sabine, Edward (1788–1883). Anglo-Irish scientist and explorer; supervisor of British colonial geomagnetic observatories.
Thales of Miletus (c. 624–546 BC). Greek philosopher and reputed founder of many branches of mathematics and science, including electricity and magnetism.
Thellier, Émile (1904–1987). French experimental paleomagnetist who, with his wife Odette Thellier, worked extensively on the determination of the intensity of the paleomagnetic field from lavas and archaeological artifacts.
Vine, Fred (born 1939). Cambridge geophysicist who, with Drummond Matthews, was the first to publish a theory combining seafloor spreading and geomagnetic field reversals to explain the patterns of marine magnetic anomalies.
Von Humboldt, Alexander (1769–1859). Prussian naturalist and explorer who was the first to recognize the magnetization of rocks, and to publish measurements of the (relative) intensity of the geomagnetic field.
Introduction
If there is anything I share with the twentieth-century genius Albert Einstein, it is a fascination with the magnetic compass. It was this that led both of us to one of the greatest problems in physics: finding the origin of the force that draws the compass needle unerringly towards the north. The story I am about to tell has grown from a notion sown in my mind by my publisher in 2005, the International Year of Physics and the centenary of Einstein’s annus mirabilis.
Einstein became entranced by magnetism at the age of seven, when his father gave him a compass. For me the moment of truth struck during a short series of lectures in my third year of undergraduate physics at Cambridge University. Cambridge had a curious assessment exercise, the “prepared essay” exam. By the time my essay, “Our Magnetic Planet,” was honed and practiced, no one could have crammed more information into the two hours of solid writing we were allowed. I was well and truly hooked.
A long journey of discovery lay ahead. It took me first to Edinburgh, where as a research student I mucked about in small boats collecting long cores of lake-bottom mud from such beauty spots as Scotland’s Loch Lomond and the Lake District’s Windermere. From my lake mud I uncovered the wanderings and variations of Earth’s magnetic field that had lain hidden for 10,000 years. I shared an office with Stavros, a Greek ex-army officer; Eric, a sports fanatic who somehow fitted in his research between seemingly endless games of squash, tennis, soccer and badminton; Julie, a quiet, industrious student, who zipped around town on a tiny scooter; and Ruth, who, were it not for her meager budget, would have owned a full-blown Harley-Davidson. Together we ruled the roost in the James Clerk Maxwell Building, taunted the professor, blissfully unaware of his pre-eminence in the foundations of our subject, and gradually unraveled the secrets of geomagnetic secular variation— the curious way in which Earth’s magnetic field keeps changing.
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