Palaeomagnetism: Principles and Applications in Geology, Geophysics and Archaeology

387 Pages·1983·8.98 MB·English

by  D. H. Tarling (auth.)

Palaeomagnetism
Palaeomagnetism
Principles and Applications in
Geology, Geophysics and Archaeology
D. H. TARLING
LONDON NEW YORK
CHAPMAN AND HALL
First published 1983 by
Chapman and Hall Ltd
11 New Fetter Lane, London EC4P 4EE
Published in the USA by
Chapman and Hall
733 Third Avenue, New York NY 10017
© 1983 D. H. Tarling
Softcover reprint of the hardcover 1st edition 2007
ISBN-13: 978-94-009-5957-6 e-ISBN-13: 978-94-009-5955-2
001: 10. \007/978-94-009-5955-2
This title is available in both hardbound and paperback editions.
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All rights reserved. No part of this book may be reprinted, or
reproduced or utilized in any form or by any electronic,
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the Publisher.
British Library Cataloguing in Publication Data
Tarling, D. H.
Palaeomagnetism.
1. Paleomagnetism
I. Title
523.01'887 QE501.P35
Library of Congress Cataloging in Publication Data
Tarling, D. H. (Donald Harvey)
Palaeomagnetism: principles and applications in
geology, geophysics, and archaeology.
Bibliography: p.
Includes index.
1. Palaeomagnetism. I. Title.
QE501.4.P35T37 1983 538'.7 83-5176
Contents
Preface page IX
1 Introduction 1
1.1 Scope of the book
1.2 Historical perspective 1
1.3 Data presentation, magnetic units and geological ages 8
l.3.1 Data presentation 8
1.3.2 Magnetic units 12
1.3.3 Geological ages 12
2 The physical basis 15
2.1 Magnetization on an atomic scale 15
2.2 Magnetic domains and anisotropy 16
2.3 Temperature, grain size and time 23
2.3.1 Thermoremanent magnetization (TRM) 25
2.3.2 Chemical remanent magnetization (CRM) 29
2.3.3 Viscous remanent magnetization (VRM) and partial
thermal demagnetization 29
2.3.4 Coercivity, alternating magnetic fields and
laboratory remanences 31
3 Magnetic mineralogy and magnetic identification of minerals 35
3.1 Introduction 35
3.2 Magnetic minerals 37
3.2.1 Titanomagnetites 37
3.2.2 Ilmenohaematites 39
3.2.3 Iron hydroxides and carbonates, etc. 40
3.2.4 Pyrrhotite 41
3.2.5 Iron and nickel 42
3.2.6 General comments 42
3.3 Identification of magnetic minerals 42
3.3.1 Isothermal remanence 44
3.3.2 Magnetic hysteresis and coercivity spectra 44
3.3.3 Low-temperature transitions 46
3.3.4 Curie temperature 46
3.3.5 Chemical analyses 48
3.3.6 Magnetic study of chemical changes during heating 49
4 The magnetization of natural materials 51
4.1 Introduction 51
Contents
4.2 Igneous rocks 53
4.3 Unconsolidated sediments and sedimentary rocks 56
4.3.1 Physical orientation processes and
unconsolidated sediments 56
4.3.2 Chemical processes and consolidated sediments 62
4.4 Metamorphic rocks 65
4.5 Archaeological materials 67
4.5.1 Heated materials 67
4.5.2 Deposited materials 68
4.5.3 Chemically changed materials 70
4.6 Secondary magnetizations 70
4.7 Magnetic inhomogeneity and anisotropy 72
4.7.1 Inhomogeneity 72
4.7.2 Anisotropy 73
4.8 Summary 74
5 Sampling, measurement and procedures 76
5.1 Introduction 76
5.2 Sampling and orientation 77
5.2.1 Sampling consolidated materials 78
5.2.2 Sampling unconsolidated materials 79
5.2.3 Orientation of samples 80
5.2.4 Tectonic corrections 82
5.3 Measurements of remanence, low-field susceptibility
and anisotropy 84
5.3.1 Magnetometers 84
5.3.2 Low-field susceptibility meters 88
5.3.3 Magnetic anisotropy meters 88
5.4 The stability of remanence 89
5.4.1 Thermal demagnetization 90
5.4.2 Alternating magnetic field demagnetization 91
5.4.3 Direct current demagnetization 93
5.4.4 Other stability indicators 95
5.5 The age of the remanence 95
5.5.1 Viscous remanence 95
5.5.2 Consistency 98
5.5.3 Folds, tilts and great circles 98
5.5.4 Identification of the carriers of remanence 100
5.6 Palaeo intensities 102
5.7 Summary 106
6 Statistical and mathematical analyses 108
6.1 Introduction 108
6.2 Intensity of remanence and susceptibility 109
VI
Contents
6.3 Directional analyses 111
6.3.1 Mean directions and poles 111
6.3.2 Precision 117
6.3.3 Scatter estimates 122
6.3.4 Combining and comparing groups of vectors 124
6.3.5 Fischer and non-Fischer distributions and error estimates 125
6.4 Analyses of vector components and their stability 130
6.5 Levels of direction analyses and sampling numbers 138
6.6 Magneticfabric 140
7 Archaeological applications 145
7.1 Introduction 145
7.2 Archaeomagnetic dating 145
7.2.1 Intensity 149
7.2.2 Directions 150
7.2.3 Master curves and assessment of standard methods 153
7.2.4 Viscous remanence and 'alignment' dating 156
7.3 Other applications 157
7.3.1 Sourcing (provenance) of archaeological materials 158
7.3.2 Technological and other potential applications 160
8 Geomagnetic applications 162
8.1 Introduction 162
8.2 The present geomagnetic field and historical observations 163
8.3 Secular variations and the drift of the non-dipole field 171
8.4 Polarity reversals, transitions and excursions 181
8.4.1 Polarity reversals 181
8.4.2 Transitions of polarity 187
8.4.3 Geomagnetic excursions 189
8.5 The general nature of the geomagnetic field 190
9 Geological applications 197
9.1 Introduction 197
9.2 Magnetic dating 197
9.2.1 Secular variations 198
9.2.2 Reversals and excursions 200
9.2.3 Palaeomagnetic poles 216
9.3 Magnetic fabric 242
9.3.1 Sediments and sedimentary rocks 243
9.3.2 Igneous rocks 247
9.3.3 Metamorphic rocks 248
9.4 Sedimentological applications 250
9.4.1 Sandstones and siltstones 253
9.4.2 Carbonates 256
9.4.3 Coal 256
Vll
Contents
9.4.4 Evaporites 257
9.4.5 'Sedimentary' ores 257
9.5 Igneous and metamorphic rocks 258
9.5.1 Composition, redox conditions and oceanic rocks 258
9.5.2 Emplacement temperatures 261
9.5.3 Metamorphic aureoles, thermal contacts and
depth of burial 262
9.6 Structural applications 265
9.6.1 Introduction 265
9.6.2 Intracontinental movements 267
9.6.3 Large-scale intracontinental tectonics 274
9.6.4 Small-scale tectonic applications 280
9.6.5 Very large-scale applications 282
9.7 Biological, weather, climatic, palaeontological and
palaeogeographic aspects 284
9.7.1 Biological aspects 285
9.7.2 Weather and climatic aspects 287
9.7.3 Palaeontology, palaeolatitudes and palaeogeography 290
9.8 Extraterrestrial studies 296
Index 369
Vlll
Preface
Palaeomagnetism and archaeomagnetism are fascinating specialized studies
because they are applicable to such a wide range of problems in geology,
archaeology and geophysics. They can also be undertaken cheaply, when
compared with most other geophysical techniques, and, at first sight, simply.
In fact, real comprehension of the magnetic processes that have occurred in
rocks and other types of material over several thousands or many millions of
years is still extremely difficult to assess and measure. On this basis, this book
cannot explain all such features, nor can it attempt to cover all the actual and
potential applications of the method. All that can be attempted is to give an
impression of the ways in which such techniques can be used in a wide variety
of fields, and how these techniques are usually applied. The magnetization of
rocks is, in fact, one of the earliest of the true sciences, but we are still not in a
position to answer many of the problems posed. Consequently some of the
examples given of applications are, essentially, state-of-the-art comments,
rather than being a review as such. The changing position of the geomagnetic
poles with time is still not adequately defined, for example, and some of the
more recent conventional views are given, although the emphasis is placed on
more subjective, probably more controversial, evaluations. In these, I have
possibly been too pessimistic, although, as with any pessimist, I think that the
assessments are realistic! In any case, the data base is expanding rapidly in
most fields and the evidence evaluated here must be under constant review.
It is difficult to know where to start with the acknowledgements. Obviously
I have probably learnt more from past and present students than they have
learnt from me, and many colleagues have been involved in discussions and
reading some of the sections. Specifically to mention A. Stephenson, W.
O'Reilly, D. W. Collinson, R. Thompson, E. A. Hailwood and F. J. Lowes
runs the risk of omission of many others of my mentors such as J. A. Clegg,
P. M. S. Blackett, K. M. Creer, S. K. Runcorn and, above all, E. Irving. On
the technical side, Lynn Whiteford, Dorothy Cooper and Marie Summersby
were always co-operative with the typescript, drawings and computing - even
when I was not!
Newcastle upon Tyne
May 1982
IX
Chapter One
Introduction
1.1 SCOPE OF THE BOOK
Magnetism is one of the oldest of the true sciences (Section 1.2) although its
remarkable properties have only recently achieved recognition and many of
the ways in which it can be applied to a range of geological, geophysical and
archaeological problems have still to be assessed and developed. Its unique
feature is that it is the only geophysical property of the Earth that can be
satisfactorily measured and evaluated throughout time. Seismicity, gravity
and electrical properties are transient features that leave no clear trace of
their previous values. The strength and direction of the geomagnetic field can,
however, be studied over archaeological, geological and even cosmic time
scales. Studies within an archaeological context, archaeomagnetism, do not
differ in principle from longer-term studies, palaeomagnetism, and so these
terms mainly reflect the purpose of the research rather than any intrinsic
difference. The general principles of magnetization (Chapter 2) and the
minerals that retain ancient magnetizations (Chapter 3) are therefore con
sidered before the specific materials that may be of interest to geologists,
geophysicists, and archaeologists (Chapter 4). Methods of sampling and
measurement (Chapter 5) and of statistical analyses (Chapter 6) are again of
common interest, but the interests of archaeologists, geophysicists and geo
logists tend to differ in the ways in which they can apply such palaeomagnetic
observations. Archaeological interests are specifically concerned with the last
few thousand years (Chapter 7) and are considered before geomagnetic
observations (Chapter 8). Most of the knowledge of geomagnetic field
behaviour over periods longer than ~OO years is based on archaeomagnetic
observations. However, the reliability of some archaeological and most geo
logical studies depends on the validity of models for the long-term behaviour
of the geomagnetic field and so the geological and cosmic time-scales are
discussed last (Chapter 9).
1.2 HISTORICAL PERSPECTIVE
Magnetized stones showing attraction and repulsion may have been known
to Early Man, but such properties would be difficult to see unless the rocks
were almost entirely formed of magnetite and had previously been struck by
1
Palaeomagnetism
lightning. Even then it is likely that such properties could not be studied until
the discovery of iron. The ability of such magnetic stones, lodestones, to
induce a magnetization by rubbing and stroking iron needles would certainly
be regarded as magical and such attractive and repulsive forces were probably
discovered very early in the Iron Age. The phenomenon of magnetic repulsion
was certainly known in Ancient Egypt and both repulsive and attractive
properties were described by Thales of Miletus, c. 624-565 BC (Hesse, 1961).
Greek legends also included, obviously exaggerated, stories of the ability of
some rocks to pull the nails out of passing vessels. However, the ability of
magnetized needles to point in specific directions was not known in Western
Europe, although such properties were well known to the Chinese in the 1st
century AD (Needham, 1962) and had almost certainly been known for at
least 300 years previously. The Chinese were also aware, by at least 720 AD,
that compasses did not point due south (the Chinese prime meridian was
south), but at an angle to it.
In Europe, the earliest record of compasses is by Neckham (1187) and it
seems probable that such vital navigational instruments reached Europe via
Arabic traders during the 12th century. During the European 'Dark Ages', it
was thought that the compass pointed towards the Pole star, i.e. along the axis
of the Universe, but the 'Dark Ages' also saw the beginnings of experimental
science, even though the main development was delayed until the 17th
century. In particular, the work of Roger Bacon (1267) established the
principles of experimental science and Petrus Peregrinus (Pierre de
Maricourt), a French military engineer, described his experiments with
magnets in a widely circulated letter (1269). These experiments included the
construction of spheres of lodestone on which he defined lines of equal force
and also found that such lines of force converged to two diametrically
opposite points, which he termed poles. He also went on to show how like
poles repelled and unlike attracted (Chapman, 1967; Smith, 1970).
Peregrinus's Epistola has, in fact, been described as 'the first original scientific
work of western Christendom' (Bernal, 1965).
The European 'discovery' of declination, i.e. that the compass does not
point to true north but at an angle to it, is not clear. The Flemish cartographer,
Gerhard Mercator, and the Portuguese explorer, Joao de Castro, reported
differences between the magnetic direction and true north in the mid-16th
century (Hellmann, 1896), but some sundials of around 1450 AD are thought
to have markings indicating compass settings that differ from true north, as do
some pre-1500 road maps (Chapman, 1967). Similarly, the 'discovery' of
inclination, the angle which a suspended magnetic needle will make to
horizontal, is unclear. It was recorded by Hartmann (1544) and indepen
dently discovered by Norman in 1576 (Norman, 1581). Nonetheless, the
philosophy of the 'Scientific Revolution' was laid by Francis Bacon (1605),
and William Gilbert (1600) is generally recognized as the founder of experi
mental science, although much of his work, De Magnete, probably relied on
2