Elementary, my dear Holmes

Mechanics in the Earth and Environmental Sciences
November 10, 1995

There has long been a division between geologists and geophysicists; geologists have been qualitative and geophysicists have been quantitative. In the middle of the 19th century the two cultures collided over the age of the earth. Lord Kelvin, a physicist, hypothesised that the earth cooled from an original hot state and from knowledge of the temperature increase with depth in the earth and the theories of heat conduction he concluded that the planet's age must be between 20 and 40 million years. He attacked the uniformitarianism advocated by leading British geologists led by Sir Charles Lyell but, they, in turn, refused to accept Kelvin's conclusions.

Other distinguished British physicists of the 19th century made notable contributions to our understanding of the earth; the seismic surface waves that devastate cities are known as Rayleigh and Love waves. In the first half of the 20th century Sir Harold Jeffreys, Cambridge, dominated geophysical thinking in the United Kingdom and the United States. He made many contributions in seismic wave propagation but strongly resisted the concept of continental drift.

Alfred Wegener, a German meteorologist, espoused continental drift in the 1920s and 1930s but his meteorological mechanism was discredited by Jeffreys. Geologists such as Alexander du Toit in South Africa provided convincing qualitative arguments for continental drift based on the geometrical and geological match between South Africa and South America but did not provide a quantitative mechanism.

A noted British geologist, Arthur Holmes, provided an answer to many of these problems. In the 1910s he demonstrated that the heating due to the radioactive decay of radioactive isotopes of uranium, thorium, and potassium heated the earth and discredited Lord Kelvin's age. Holmes's work on radioactive decay in rocks provided the basis for determining the age of the earth, 4,500 million years. Holmes accepted the concept of continental drift but rejected Wegener's mechanism, in the early 1930s he suggested that thermal convection drove continental drift and used Lord Rayleigh's theory to conclusively demonstrate the validity of this hypothesis.

Because Holmes was a geologist and because Jeffreys dominated geophysics, few accepted Holmes arguments. It was not until the late 1960s and plate tectonics that Holmes's hypothesis was generally accepted. Holmes provided the connection between quantitative geophysics and qualitative geology that is evolving today.

The gulf between geophysicists and geologists is disappearing, and this means geologists must become quantitative and learn considerable continuum physics and related mathematical concepts. It also means geophysicists must learn the complex world of real geology. The book provides an excellent introduction to the fundamentals of continuum physics required by both geologists and geophysicists in order to understand geological processes. The book covers the fundamentals of elasticity, heat transfer, fluid flow, and flow through porous media. A variety of applications to geological problems are included. The book can be recommended to both students and to others who desire to learn the foundations of today's geological science.

Donald L. Turcotte is Maxwell Upson professor of engineering, Cornell University.

Mechanics in the Earth and Environmental Sciences

Author - Gerard V. Middleton and Peter R. Wilcock
ISBN - 0 521 44124 2 and 44669 4
Publisher - Cambridge University Press
Price - £50.00 and £22.95
Pages - 459

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