Figure 1.2. James Hutton (1726-1797), the father of Geology and proponent of internal heat as the driving force for Earth's evolution.

The idea that flow in the Earth's interior is a form of thermal convection developed slowly. Recognition of the significance of thermal convection as a primary fluid mechanical phenomenon in nature came from physicists. Count Rumford is usually given credit for recognizing the phenomenon around 1797 (Brown, 1957), although the term convection (derived from convectio, to carry) was first used by Prout (1834) to distinguish it from the other known heat transfer mechanisms, conduction and radiation. Subcrustal convection in the Earth was first suggested by W. Hopkins in 1839 and the first interpretations of geological observations using convection were made by Osmond Fisher (1881). Both of these presumed a fluid interior, so when the solidity of the mantle was established, these ideas fell out of favor.

The earliest experiments on convection in a layer of fluid heated from below and cooled from above were reported by J. Thompson (1882), who observed a "tesselated structure" in the liquid when its excess temperature, compared to that of the overlying air, was sufficiently large. But the name most closely associated with convection is Henri Benard (Figure 1.3). Benard (1900, 1901) reported the first quantitative experiments on the onset of convection, including the role of viscosity, the cellular planform, and the relationship between cell size and fluid layer depth. Benard produced striking photographs of the convective planform in thin layers of viscous fluids heated from below (Figure 1.4). The regular, periodic, hexagonal cells in his photographs are still referred to as Benard cells. Since Benard used fluid layers in contact with air, surface tension effects were surely present in his experiments, as he himself recognized. It has since been shown that Benard's cells were driven as much by surface tension gradients as by gradients in buoyancy. Still, he correctly identified the essentials of thermal convection, and in doing so, opened a whole new field of fluid mechanics. Motivated by the "interesting results obtained by Benard's careful and skillful experiments," Lord Rayleigh (1916; Figure 1.5) developed the linear stability theory for the onset of convection

Figure 1.3. Henri Benard (1880-1939) (on the left) made the first quantitative experiments on cellular convection in viscous liquids. The picture was taken in Paris about 1920 with Reabouchansky on the right.

Figure 1.4. Photograph of hexagonal convection cells in a viscous fluid layer heated from below, taken by Benard (1901).

in a horizontally infinite fluid layer between parallel surfaces heated uniformly from below and cooled uniformly from above, and isolated the governing dimensionless parameter that now bears his name. It was unfortunate that these developments in fluid mechanics were not followed more widely in Earth Science, for they might have removed a stumbling block to acceptance of the milestone concept of continental drift.

Figure 1.5. Lord Rayleigh (1842-1919) developed the theory of convective instability in fluids heated from below.

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