THE STRENGTH OF THE EARTH'S CRUST 



68 1 



compensation point to a zone of weakness much deeper and thicker 

 than the figures which have customarily been taken as a probable 

 depth of origin of magmas. The latter however rests upon uncer- 

 tain extrapolation, whereas the figures for the limits of the astheno- 

 sphere, although of no exactness and perhaps 20 or 50 per cent 

 from limits which finally may be chosen, have at least been deter- 

 mined by more direct evidence. In such a thick shell of weakness, 

 the readjustment, after an erosion cycle, of a continental interior 

 to isostatic equihbrium would require but very little viscous shear 

 and but little lateral movement. 



Lil"ho3phere 



Asfhenosphere 



CenlTospfiere 



Fig. 14. — Diagrammatic vertical section of the crust, to show nature of undertow 

 in the asthenosphere necessary to restore isostatic equilibrium in a positive interior 

 continental area after a cycle of erosion. Effects of a vertical movement of o. 5 km. 

 exaggerated 60 times. Asthenosphere grades into contiguous spheres and best limita- 

 tions in depth are not known. 



To give quantitative visualization to this conclusion Fig. 14 is 

 drawn. Suppose a plateau area 1,000 km. wide in a continental 

 interior to be separated from the region of sedimentary deposit 

 by an intermediate region 1,000 km. across. Take a section 

 I km. wide through these regions. Let an erosion cycle cause the 

 removal on the average of 0.5 km. of rock from this area to be 

 deposited over an equal area of sea-bottom. Then, during an 

 epoch of diastrophism, assume complete recovery of isostatic 

 equilibrium by undertow in a sublithospheric zone of weakness 

 600 km. thick. The vertical section of rock eroded is 500 sq. km. 

 in area. As we have chosen a width of section of i km. we may 

 also speak of this as the volume, 500 cu. km. To restore the mass 

 of this column, 500 cu. km. must be added to it and flow past the 

 vertical line which bounds it on the seaward side. As this zone of 



