Strength of the Earth

Among the most perplexing problems in geologic science has been the persistent one of how an earth whose exterior is composed of hard rocks can have undergone repeated deformations as if composed of very weak and plastic materials. Tidal and seismological evidence indicates that the earth as a whole reacts to stresses of brief duration with a rigidity of the order of that of steel, while orogenic and gravitational phenomena suggest that the earth is non-rigid and very weak. In consequence of this apparently conflicting evidence recent views regarding the earth have ranged between the extremes of great rigidity and high fluidity. It is here shown that this conflict of data is more apparent than real. By means of the principles of physical similarity it is possible to translate geological phenomena whose length and time scales are outside the domain of our direct sensory perceptions into physically similar systems within that domain. When we do this we discover that the behavior, in response to gravitational stresses in geologic space and time, of an earth composed of hard rocks and possessing a viscosity appropriate to the post-glacial uplifts in Europe and North America would be very similar to that of the weak plastic materials and viscous liquids of our everyday experience.