ISOSTASY AS A RESULT OF EARTH SHRINKAGE 



211 



form. In this process great heat would probably be evolved and 

 would be only in part dissipated toward the surface. The heat 

 might become so great that the temperature for forming molten 

 rock at more moderate pressures might be reached. Presently 

 the geosyncline becomes a zone of such weakness that the com- 

 pressive forces acting on the crust will produce yielding at that 

 point. There will be first an arching of the formations and thus 

 a relief of the pressure on the interior. This release may make 

 conditions at depth such that the rock will become molten. In 

 the change to the molten condition there is considerable expansion 

 and therefore the earth's crust may be somewhat elevated at this 

 point and still, because of the lightness of the magma, not show 

 positive anomalies at the surface (Fig. i). Erosion on the mountain 



d 2T751+Y 3. o 



-«r- 



FiG. I. — Diagrams showing the decrease in density in depth below a mountain 

 range due to the reducing of the overburden by support from the sides and the formation 

 of lava in place. 



surface will remove weight and allow a further uplift of the higher 

 portions without increasing the burden or the gravitative pull 

 (Fig. 2). 



Before Erosion 



After Erosion 



Fig. 2. — Diagrams illustrating how the top of a range may become elevated 

 after erosion has produced irregularities. The total weight on the base is the same in 

 each case. 



