THE STRENGTH OF THE EARTH'S CRUST 



JOSEPH BARRELL 



New Haven, Connecticut 



PART VIII. PHYSICAL CONDITIONS CONTROLLING THE 

 NATURE OF LITHOSPHERE AND ASTHENOSPHERE 



Introduction and Summary 425 



SECTION A^ 



RELATIONS BETWEEN RIGIDITY, STRENGTH, AND IGNEOUS 



ACTIVITY 



Distinctions in Physical Properties Related to Strength . . 429 

 Conditions Favoring Association of High Rigidity with Low 



Elastic Limit . 432 



Analogies between Asthenospheric Rock and Glacial Ice . . 438 



Relations of Igneous Activity to Asthenosphere and Lithosphere 441 



INTRODUCTION AND SUMMARY 



The experiments of F. D. Adams have demonstrated that 

 under combined pressures and temperatures equal to those existing 

 at a depth of eleven miles granite is about seven times stronger 

 than at the surface, showing that, up to at least certain limits, the 

 strength of the crust increases downward. The measurements of 

 tidal deformation of the earth and of the variations of latitude 

 concur furthermore, in proving that the rigidity of the earth as a 

 whole is greater than that of steel. The transmission of earth- 

 quake vibrations of a transverse nature through the earth shows, 

 not only that the earth is solid and rigid throughout, but that, on the 

 whole, rigidity increases with depth. In none of these lines of 

 investigation is there any clear suggestion of the existence of a 

 thick shell of weakness — an asthenosphere. 



On the other hand, the conclusion that broad areas of the 

 crust rest in approximate isostatic equilibrium seems to imply the 



'Section B of Part VIII, on "Relations with Other Fields of Geophysics," will 

 be published in the succeeding number of this Journal. 



425 



