It is known that a viscous material will yield easily to a comparatively slight pressure exerted over a long period of time, even though it may act as a solid when subjected to a sudden pressure, such as an earthquake wave. The rock at that depth is supposed to have high viscosity that is, it is fluid but very stiff, as tar may be. Moreover, it is thought that pressure at that depth renders the rock extremely plastic, so that it will yield easily to pressures. Immediately under the crust is a layer that is thought to be extremely weak, because it is, presumably, too hot to crystallize. The crust is made of comparatively rigid, crystalline rock, but it is fractured in many places, and does not have great strength.
Estimates of its thickness range from a minimum of about twenty to a maximum of about forty miles.
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