The origin of mountain ranges, volcanoes, earthquakes, the ocean basins, the continents, and the very nature of the Earth's interior are questions as old as science itself. In the development of modern science, virtually every famous Natural Philosopher has conjectured on the state of the deep interior and its relationship to the Earth's surface. And every one of these thinkers has come to essentially the same general conclusion: despite the obvious solidity of the Earth beneath our feet, the interior must have flowed, in order to create the complex surface geology we see today. Although we can trace this idea as far back as written scientific record permits, it nevertheless remained a strictly qualitative hypothesis until the early part of the twentieth century. Then several timely developments in physics, fluid mechanics, geophysics, and geology finally established a true physical paradigm for the Earth's interior, mantle convection.

In reviewing the development of the concept of mantle convection, we find it is impossible to identify one particular time or event, or one particular individual, as being decisive in either its construction or its acceptance. Instead, the subject's progress has followed a meandering course, assisted along by the contributions of many. Still, there are a few scientific pioneers whose insights were crucial at certain times. These insights deserve special recognition and, when put together, provide some historical context with which future progress can be measured.

The idea of flow in the Earth's interior was popular among the early Natural Philosophers, as it was commonly assumed that only the outermost portion of the Earth was solid. Descartes imagined the Earth to consist of essentially sedimentary rocks (the crust) lying over a shell of denser rocks (the mantle) with a metallic center (the core). Leibniz proposed that the Earth cooled from an initially molten state and that the deep interior remained molten, a relic of its formation. Edmond Halley argued that the flow of liquids in a network of subsurface channels would explain his discovery of the secular variation of the geomagnetic field. Both Newton and Laplace interpreted the equatorial bulge of the Earth to be a consequence of a fluid-like response to its rotation. The idea that the Earth's interior included fluid channels and extensive molten regions, as expressed in the artist's drawing in Figure 1.1, was the dominant one until the late nineteenth century, when developments in the theory of elasticity and G. H. Darwin's (1898) investigation of the tides indicated that the Earth was not only solid to great depths, but also "more rigid than steel."

In the eighteenth and early nineteenth centuries, resolution of an old controversy led geologists to accept the idea of a hot and mobile Earth interior. This controversy centered on

Figure 1.1. An early depiction of the Earth's interior, showing channels of fluid connected to a molten central core. This was the prevailing view prior to the nineteenth century.

the origin of rocks and pitted the so-called Neptunists, led by Abraham Werner, who thought all rocks were derived by precipitation from a primeval ocean, against others who held that igneous rocks crystallized from melts and were to be distinguished from sedimentary rocks formed by surficial processes. Their most influential member was an amateur scientist, James Hutton (Figure 1.2), who advanced the concept of uniformitarianism, that the processes evident today were those that shaped the Earth in the past. He also held to the idea of a molten, flowing interior, exerting forces on the solid crust to form mountain ranges, close to the modern view based on mantle convection. Ultimately Hutton's view prevailed, and with it, an emphasis on the idea that the fundamental physical process behind all major geological events is heat transfer from the deep interior to the surface. Thus, geologists were receptive to the idea of a hot, mobile Earth interior. However, most of the geological and geophysical evidence obtained from the continental crust seemed to demand vertical motions, rather than horizontal motions. For example, in the mid-nineteenth century it was discovered that mountain ranges did not have the expected positive gravity anomaly. This was explained by low-density continental roots, which floated on the denser mantle like blocks of wood in water, according to the principle of hydrostatic equilibrium. This implied, in turn, that the mantle behaved like a fluid, allowing vertical adjustment. However, the notion that the crust experiences far larger horizontal displacements was less well supported by evidence and was not widely held.

1.1 Introduction

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