264 B. WILLIS DISCOID AL STRUCTURE OF THE LITHOSPHERE 



opposed. In the intensely elastic rock they are instantly active and 

 responsive to the slightest change in their relative values. The litho- 

 sphere is vibrant, more vibrant than the most sensitive musical string. 



This concept, which expresses the contest that persists in all bodies 

 subject to gravitation and heat, needs to be emphasized, because rocks 

 as we know them are dull and lifeless, and it is difficult to conceive of 

 them as constituting a sphere which is intensely alive. 



Let us now consider the effect of heat upon the strength of rock, which 

 is expressed by the curve LMNPQR of plate 8. Drawn on the assumption 

 of surface temperature, that curve exaggerates the minimum strength 

 which it is supposed to represent. The true minimum would fall within 

 it, and yet not very far within it, if we may accept the relations of com- 

 pression and expansion, as shown on plate 9, as approximately correct. 

 The effects of expansion due to normal temperatures in the isostatic shell 

 are relatively small as compared with the effects of compression. 



In this connection it is convenient to consider the effects on the strength 

 of rock of an abnormal rise of heat sufficient to produce fusion. Let us 

 assume that the rise of temperature is due to a rise of heat from below. 

 Fusion, beginning below, will at some level reduce the internal friction 

 practically to zero. Let this be represented by the point Z, plate 8, whose 

 abscissa is zero. At some point X the effect of the abnormal temperature 

 will be negligible — that is, the rock will have its normal strength. 

 Between X and Z the strength will vary as represented by the hypothetical 

 curve XYZ. 



The curve XYZ represents a transition between two states of rock, in 

 both of which rock will flow. The resistance which the rock offers to 

 deformation is, however, very different, being in the one case practically 

 zero, in the other equal to the weight of a column of rock many miles 

 high. Even though it be true, as it is, that the internal viscosity of 

 molten rock is not quite zero, it closely approaches zero as compared with 

 the enormous absolute strength of cold rock under high pressure. 



Recalling the relative weakness of cold rock in relation to vertical shear 

 at depths below that at which the factor of safety is reduced to 1, we see 

 how seriously local melting reduces the resistance of the lithosphere to 

 shear. Failure and subsidence are inevitable as melting is approached or 

 occurs, unless heated rock be hermetically sealed in. 



The two states of flow contrast in their capacity to transmit stress. 

 The molten rock, with internal friction near zero, will transmit any 

 stress, however slight, in all directions equally, according to the laws of 

 hydrostatics. The solid rock will transmit any stress in the direction in 

 which the stress is exerted. It will fail in the direction of maximum 



