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Figure 4.4 Isostatic equilibrium. (a) Equilibrium in ice floating on water. (b) Disequilibrium with ice and water. (c) Disequilibrium at a planetary surface. (d) Equilibrium at a planetary surface.

Figure 4.4 Isostatic equilibrium. (a) Equilibrium in ice floating on water. (b) Disequilibrium with ice and water. (c) Disequilibrium at a planetary surface. (d) Equilibrium at a planetary surface.

A planetary body typically has a crust of less dense solids of non-uniform thickness on top of a mantle of more dense solids. For the crust to be in isostatic equilibrium the underlying mantle has to be able to flow in response to departures from isostasy, and the upper layer must be able to deform to take up the equilibrium shape. This is illustrated in Figure 4.4(c) and (d). Given sufficient time, isostasy will be achieved. The greater the plasticity of the interior and the greater the flexibility of the surface layer, the shorter the time required. The plasticity of the interior increases with temperature and pressure, and depends on composition. The flexibility of the surface layer also increases with temperature and depends on composition, and it also increases as its thickness decreases. It is quite possible for the adjustment time to be many thousands of years, or longer, and so departures from isostasy are to be expected, and are observed.

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