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Palaeomagnetism and plate tectonics
395
BOOK REVIEWS
Palaeornagnetism and Plate Tectonics. M.W. McElhinny. Cambridge University
Press, London,
1973,357
pp., & 8.50.
Irving’s book Palaeomagnetism and its Application to Geological and Geophysical Problems (Wiley, 1964) has been the main textbook magnetism
during the recent revolution
on the geological aspects of palaeo-
in the Earth Sciences, but this was written before
the idea of sea-floor spreading really got underway. Subsequently Strangway’s History of the Earth’s Magnetic Field (McGraw-Hill, 1970) and Tarling’s Principles and Applications of Palaeomagnetism (Chapman and Hall, 1971) presented books on palaeomagnetism, mostly for undergraduate and first-year postgraduate students. This new book is designed as a basis for an extended lecture course on palaeomagnetism and as an up-to-date review of the state of knowledge of the subject. The contents are logically laid out, well referenced and indexed. It commences with the main features of the present earth’s magnetic field and its long-term variation. The second chapter considers the minerals involved and the physical basis underlying the way these minerals acquire a remanent magnetization. The treatment in this chapter is inadequate for those interested in rock magnetism, but, as the book is designed to be readable for non-physicists, the coverage is sufficient
as an introduction
and for adequate comprehension
of the subsequent
text. The third chapter covers the collection, measurement and laboratory analysis of rock samples. This is probably the least satisfactory chapter, particularly the statistical analyses which only comprise six pages of statements tests are-calculated,
of the ways some statistical parameters
but no outline is given of the assumptions
and
implicit in using these tests,
the sorts of values which are considered acceptable, the errors associated with calculations using the low numbers involved in palaeomagnetic studies, and whether the calculations should be calculated using locality, site, sample or specimen values, etc. This poor treatment of statistics is not uncommon today in any of the sciences, as statistics are often used as magic formulae without sufficient consideration of the physical assumptions which underlie the equations. It is therefore important that these aspects are brought out in any textbook
at an undergraduate
In contrast,
level.
the following chapter on reversals is extremely
slightly marred by minor internal consistencies,
well presented
and only
such as stating that the geomagnetic
field
decreases to zero during a polarity transition (p. 136) and following this by a good discussion of the fact that the field in fact decreases to about 20% at these times (p. 137-138). The fifth chapter is on sea-floor spreading and plate tectonics, but is essentially concerned with the magnetic-anomaly pattern of the igneous ocean floors. As plate tectonics forms a part of the title, it is extremely disappointing to find, for example, only 3?4 pages on mountain belts and geosynclines, without reference to any of W.R. Dickinson’s papers. If plate tectonics did not form part of the title, this chapter would certainly be acceptable, but the title certainly leads one to expect much more on plate tectonics. As only the barest concepts of sea-floor spreading are outlined, mostly up to 1968, many major out-
BOOK REVIEWS
396
standing, tectonic problems are not mentioned
or discussed, such as the width of injection
zones at ridge axes, the rigidity (or rather rigid-body motion) vertical motions,
the mechanisms
sometimes termed mechanisms),
driving them (although
of plates in relation to
descriptions
of features are
whether sediments can be carried down at subduction
zones, etc. Even magnetic problems are scantily discussed; Irving’s explanation
of the decay
of remanence away from a ridge axis is given, however the real problem is not the decay process, as several are possible, but the origin of the abnormally high initial remanence. The final two chapters and the appendix form just over one third of the book and comprise a review of selected palaeomagnetic data up to the end of 1970, which affords a good basis for a discussion of several problems and a background against which the reader can evaluate later data. There is possibly a tendency towards a ready acceptance, rather than critical appraisal, of some concepts, such as Gondwanaland reconstructions, and apparently major anomalies are sometimes omitted; the Late Mesozoic polar wandering curves of, for example, Europe and North America (p. 254-255) indicate that the Atlantic was closed by some 2,500 km more than the Bullard fit for Jurassic and Cretaceous times, yet this is, of course, merely a reflection
of there being no reliable Jurassic
or Cretaceous data from extra-Alpine Europe. It is unfortunate that this review should cover the same data as Creer’s less critical review * , but this does not detract from its usefulness at the moment, although obviously some 1970 data are already supplanted. There are, of course, minor reservations but, on the whole, McElhinny’s is a welcome replacement
for Irving’s book, which is a high compliment,
priced, as prices go nowadays.
The standard of production
though only a few are original, have been well reproduced,
and, at s 8.50, it is reasonably is high and the diagrams, alexcept the photograph
of the
sun compass for which the paper quality was too poor and the twigs, although giving authenticity,
could confuse! The inclusion
of a colour plate showing different
oxidation
states in titanomagnetites is useful. It is a pity that the printers, presumably, were unwilling to put symbols in the figure legends of chapter 6, but there are virtually no printing errors, although one, isoporic foci having a life time “of 100 years” instead of “100’s of years”, is unfortunate.
The brevity of the discussion of plate tectonics and statistics, together with
the concentration on magnetic directions to the exclusion of intensities (palaeo-intensities are mentioned, but the reader is referred elsewhere for how they are determined) detract only slightly from an otherwise welcome addition to any Earth Science reading list.
D.H. TARLING (Newcastle upon Tyne)
* Creer, K.M., 1970. A review of palaeomagnetism.
Earth-&i. Rev., 6: 369-466.
BOOK REVIEWS
Palaeornagnetism and Plate Tectonics. M.W. McElhinny. Cambridge University
Press, London,
1973,357
pp., & 8.50.
Irving’s book Palaeomagnetism and its Application to Geological and Geophysical Problems (Wiley, 1964) has been the main textbook magnetism
during the recent revolution
on the geological aspects of palaeo-
in the Earth Sciences, but this was written before
the idea of sea-floor spreading really got underway. Subsequently Strangway’s History of the Earth’s Magnetic Field (McGraw-Hill, 1970) and Tarling’s Principles and Applications of Palaeomagnetism (Chapman and Hall, 1971) presented books on palaeomagnetism, mostly for undergraduate and first-year postgraduate students. This new book is designed as a basis for an extended lecture course on palaeomagnetism and as an up-to-date review of the state of knowledge of the subject. The contents are logically laid out, well referenced and indexed. It commences with the main features of the present earth’s magnetic field and its long-term variation. The second chapter considers the minerals involved and the physical basis underlying the way these minerals acquire a remanent magnetization. The treatment in this chapter is inadequate for those interested in rock magnetism, but, as the book is designed to be readable for non-physicists, the coverage is sufficient
as an introduction
and for adequate comprehension
of the subsequent
text. The third chapter covers the collection, measurement and laboratory analysis of rock samples. This is probably the least satisfactory chapter, particularly the statistical analyses which only comprise six pages of statements tests are-calculated,
of the ways some statistical parameters
but no outline is given of the assumptions
and
implicit in using these tests,
the sorts of values which are considered acceptable, the errors associated with calculations using the low numbers involved in palaeomagnetic studies, and whether the calculations should be calculated using locality, site, sample or specimen values, etc. This poor treatment of statistics is not uncommon today in any of the sciences, as statistics are often used as magic formulae without sufficient consideration of the physical assumptions which underlie the equations. It is therefore important that these aspects are brought out in any textbook
at an undergraduate
In contrast,
level.
the following chapter on reversals is extremely
slightly marred by minor internal consistencies,
well presented
and only
such as stating that the geomagnetic
field
decreases to zero during a polarity transition (p. 136) and following this by a good discussion of the fact that the field in fact decreases to about 20% at these times (p. 137-138). The fifth chapter is on sea-floor spreading and plate tectonics, but is essentially concerned with the magnetic-anomaly pattern of the igneous ocean floors. As plate tectonics forms a part of the title, it is extremely disappointing to find, for example, only 3?4 pages on mountain belts and geosynclines, without reference to any of W.R. Dickinson’s papers. If plate tectonics did not form part of the title, this chapter would certainly be acceptable, but the title certainly leads one to expect much more on plate tectonics. As only the barest concepts of sea-floor spreading are outlined, mostly up to 1968, many major out-
BOOK REVIEWS
396
standing, tectonic problems are not mentioned
or discussed, such as the width of injection
zones at ridge axes, the rigidity (or rather rigid-body motion) vertical motions,
the mechanisms
sometimes termed mechanisms),
driving them (although
of plates in relation to
descriptions
of features are
whether sediments can be carried down at subduction
zones, etc. Even magnetic problems are scantily discussed; Irving’s explanation
of the decay
of remanence away from a ridge axis is given, however the real problem is not the decay process, as several are possible, but the origin of the abnormally high initial remanence. The final two chapters and the appendix form just over one third of the book and comprise a review of selected palaeomagnetic data up to the end of 1970, which affords a good basis for a discussion of several problems and a background against which the reader can evaluate later data. There is possibly a tendency towards a ready acceptance, rather than critical appraisal, of some concepts, such as Gondwanaland reconstructions, and apparently major anomalies are sometimes omitted; the Late Mesozoic polar wandering curves of, for example, Europe and North America (p. 254-255) indicate that the Atlantic was closed by some 2,500 km more than the Bullard fit for Jurassic and Cretaceous times, yet this is, of course, merely a reflection
of there being no reliable Jurassic
or Cretaceous data from extra-Alpine Europe. It is unfortunate that this review should cover the same data as Creer’s less critical review * , but this does not detract from its usefulness at the moment, although obviously some 1970 data are already supplanted. There are, of course, minor reservations but, on the whole, McElhinny’s is a welcome replacement
for Irving’s book, which is a high compliment,
priced, as prices go nowadays.
The standard of production
though only a few are original, have been well reproduced,
and, at s 8.50, it is reasonably is high and the diagrams, alexcept the photograph
of the
sun compass for which the paper quality was too poor and the twigs, although giving authenticity,
could confuse! The inclusion
of a colour plate showing different
oxidation
states in titanomagnetites is useful. It is a pity that the printers, presumably, were unwilling to put symbols in the figure legends of chapter 6, but there are virtually no printing errors, although one, isoporic foci having a life time “of 100 years” instead of “100’s of years”, is unfortunate.
The brevity of the discussion of plate tectonics and statistics, together with
the concentration on magnetic directions to the exclusion of intensities (palaeo-intensities are mentioned, but the reader is referred elsewhere for how they are determined) detract only slightly from an otherwise welcome addition to any Earth Science reading list.
D.H. TARLING (Newcastle upon Tyne)
* Creer, K.M., 1970. A review of palaeomagnetism.
Earth-&i. Rev., 6: 369-466.