Introduction to geomagnetism

353

T~~t~l~~~p~~~si~~s, 109 (I 984) 353-354 Elsevier Srience

Publishers

B.V.. Amsterdam

Printed

in The Netherlands

Book Review

Introduction

to Geomugnetism.

W.D. Parkinson.

Scottish Academic

Press, Edinburgh,

1983. X + 443 pp., fstg 27.50. (Hardcover). This is a textbook covering the field of geomagnetism essentially level and it will also be a useful reference volume for geophysicists

at the graduate in general.

The book is well organized by being logically divided into four major sections. These are the main field, the local field, the external field and the induced field of the earth which are treated in separate chapters. They are preceded by an introductory chapter dealing with the fundamental principles and units of geomagnetism, and magnetic instrumentation including both that are used in mineral exploration and by magnetic observatories. The chapter on the main (or core) field of the earth contains discussion

on the various

hypotheses

for its origin.

not only a useful

but a series of recent

contour

maps are presented showing the variation of the total field, the various components and the inclination and declination of the earth’s magnetic field throughout the world. A series of contour maps showing the secular variation for the components, inclination

and declination

are also presented

for the world.

The chapter on the local (or crustal) field first discusses the various mechanisms by which rocks are magnetized and then reviews palaeomagnetic techniques, magnetic reversals continents. described

and the resulting

Ground together

apparent

polar wandering

curves from the various

and airborne magnetic exploration techniques over land are then with an overview of their interpretation. This is followed by a

review of shipborne magnetic studies which have resulted in the generally accepted ocean-floor spreading hypothesis and finally there is discussion of tectonomagnetism. The chapter on external conductivity of the partially

fields first deals with the basic theory of the electrical ionized gas (or plasma) surrounding the earth and then

the ionosphere and magnetosphere are described. Finally the various categories of diurnal variation and magnetic storms are reviewed. The chapter on induced fields deals with the eddy currents induced in the earth by external field variation and the theory by which subsurface conductivities may be calculated. The book concludes review of evidence

with an interesting

indicating

chapter

on the history of geomagnetism,

that the birds and the bees can orient

themselves

a

using

the geomagnetic field, and several mathematical appendices. It will be apparent from the foregoing description of the book that it covers a rather wide range of topics and it is well nigh impossible for any individual to be current in all aspects of the discipline. However, W.D. Parkinson has been very diligent in reviewing the literature that falls outside his own field of expertise and

354

research

namely

magnetic

can be questioned. local

(or crustal)

compilation formation)

There

anomaly

is about On

necessary

to assume

states

the magnitude

and direction

tion. Such an assumption is not computer curve-fitting optimization Systeme International they have been officially

latitudes

necessary techniques

tional Association of Geomagnetism not used by many geomagneticians

and somewhat

from

the that

of iron

less at low

interpretation

of the causative

it is

body magnetiza-

and a variety of methods such as have been published over the past 20

vector is calculated

(SI) units have been adopted by scientific

clear

the local effects

in quantitative

that

that the

has been removed)

rocks (ignoring that

of points

indicating

125 nT. It is abundantly

2000 nT at high magnetic

p. 177, the author

years in which the magnetization

a number

maps (in which the IGRF

of the field due to crustal

latitudes.

are, however,

on p. 79, there is a statement

field is approximately

of magnetic

the amplitude

variations.

For example

as a parameter.

used throughout the book because organizations such as the Interna-

and Aeronomy. However, these units are still and the author perhaps unwittingly provides an

example of why this is so. On p. 199, the author is describing strong seafloor anomalies of 1000 gammas and quite correctly uses the nomenclature of 1 pT, i.e. one microtesla, which does not sound like a very significant anomaly. But on the following page in the caption for Fig. 71, he reverts to using 1300 nT (i.e. 1.3 pT) in describing the largest anomaly in an aeromagnetic survey over the West Australian goldfields. It is clear that the Tesla is much too big a unit for geomagnetism and the use of different

prefixes

is confusing

in practical

use.

However, these minor criticisms aside, the textbook is well written and is generally very readable except for the mathematical derivations. The illustrations, except graphic mostly textbook

for several errors. derive

magnetograms,

The examples, from

Australia.

are of excellent as would

quality

be expected

It is anticipated

that

and there are few typo-

from

the author’s

domicile.

it will be well utilized

as a

for courses in geomagnetism. PETER

HOOD

(Ottawa.

Ont.. Canada)