Petrogenesis and the Physics of the Earth

Abstract

It is in many ways desirable to establish the connection of igneous activity with ascertained facts regarding the nature of the earth as a whole and if possible with the early history and the ultimate origin of the earth. Any system of petrogeny must, of course, be reconcilable with geophysical facts, in so far as these are facts, but ii is a different matter to suppose that petrology must be based upon some chosen system of cosmogony. From the very nature of its subject-matter cosmogony must be ever less capable than petrology of reaching demonstrable conclusions. This is, perhaps, true of geophysics also, but in less degree. A brief survey of the data and of some present-day conclusions in geophysical matters may be desirable, together with some suggestion as to their connection with the advocated system of petrogenesis (Bowen, 1928, p. 303). This statement was Bowen's opening paragraph for Chapter 17, written with his customary style and clarity. Many parts of the chapter remain valid today, but knowledge of the physics of the earth has increased enormously since 1928. A brief survey ... of some present-day conclusions in geophysical matters was desirable fifty years ago. Today, a survey is essential, because geophysics has become a starting point for many aspects of petrogenesis. Similarly, geophysicists need the data of petrology for characterization of the earth materials whose properties they measure. Cosmogony, invigorated by the space program, has now become a fruitful source of geochemical data with relevance to petrogenesis, and the intensive study of the rock samples returned from the Moon has provided many insights into magmatic processes. New apparatus for laboratory measurements ranging from the physical properties of minerals, rocks, and melts to the phase equilibrium relationships of minerals and rocks at high pressures has added much useful data for the interpretation of geophysical measurements and magmatic processes. The theoretical approaches of thermodynamics and geophysical fluid dynamics have been applied successfully to the earth's interior. The theories of sea-floor spreading and plate tectonics, defining large-scale tectonic environments on a global scale, and involving the movement of large assemblages of rock across and within the earth in horizontal and vertical directions, have added dynamism to petrology since the mid-1960s. As a result of movements from physical processes, pressure and temperature change, and rocks undergo phase transitions, which include melting, the first stage of magmatism. Petrogenesis and the physics of the earth are now simply two overlapping parts of the interdisciplinary earth sciences.

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