How elastic plates can keep the mountains up

Isostasy and Flexure of the Lithosphere
June 21, 2002

The study of processes that support mountain chains, such as isostasy and lithospheric flexure, is a discipline that is of primary importance to earth scientists. However, very few books have been dedicated to the subject and there is not one that reflects the advances made since the development of the theory of plate tectonics in the 1960s. Therefore, Anthony Watts's book will be welcomed by researchers in earth sciences and particularly by graduate students.

The concepts of isostasy and flexure are familiar to students of geology. Isostasy is often likened to the process that keeps the tops of icebergs floating above the sea surface. Mountains are part of a layer of light material, the crust, that lies on top of the denser mantle. If isostasy is operating then the excess mass of mountains at the surface is supported by a mass deficiency at depth. Lithospheric flexure is a quite different mechanism of support for mountains. The idea in this case is that the outer layers of the earth can behave like an elastic plate so that loads, such as mountains, are supported by bending of the plate. Unfortunately the possibility of confusion arises when one attempts to relate the concepts of isostasy or flexure to detailed geophysical or geological observations, or to understand how the forces involved may interact with other geological processes. The careful description of theory and observations given in Watts's book should allow the reader to avoid such confusion.

The opening two chapters are an addictive introduction to the history of isostasy. Watts quickens the pulse and opens the mind with his well-balanced descriptions of spectacular geology, perilous expeditions and scholarly Victorian disputes. But his real strength here is in the manner with which he binds the current theory of isostasy to the development of the theory. This link should give students of isostasy the confidence that comes from being able to place one's studies in their proper context.

The book then turns to drier matters in a chapter on the theory of elastic plates. It is at this point that the author reveals most clearly his intention for the book to form part of a toolkit for graduate students. He takes the reader from the definitions of stress and strain with careful steps to a number of mathematical expressions that have proved to be useful in flexural studies. These steps and the accompanying figures make the arguments clear enough for determined geologists to follow. In a number of cases this clarity is at risk from small typographical errors, which are too common in the book. Watts also has the handy idea of supplying a number of the equations in electronic form. Unfortunately, the files are only available in a format that requires an expensive piece of software that is not ubiquitous in earth-sciences institutions.

The second half is devoted to the past 30 years of development of the subject. Many of the arguments are unresolved, and Watts, who has played an active role in several of these debates, faced a tough choice. Should he present only his own views with strong, coherent arguments uncluttered by the protests of other researchers? Or should he admit the opinions of these workers and risk weakening the clarity of his message? In the main, Watts takes the latter approach, which makes the reading more tedious and less clear than that of the opening chapters. However, the reviews of literature and theory that he presents will be a valuable guide to many who are beginning their research in this field.

John Maclennan is a research fellow, Institut de Physique du Globe de Paris, France.

Isostasy and Flexure of the Lithosphere

Author - A. B. Watts
ISBN - 0 521 622 7 and 00600 7
Publisher - Cambridge University Press
Price - £75.00 and £29.95
Pages - 458

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