The gravitative stratification of the outer crust of the earth has probably come into existence mainly by crystallization differentiation and squeezing out of the residual magma on the one hand, and partial re-fusion and squeezing from older rocks on the other hand. Granitic magmas might be, to use a metaphor that may sound somewhat vulgar, characterized as the sweat that oozes out from the body of mother earth during the convulsions of orogeny. The sial crust has thus grown gradually thicker during the geological ages and is still growing.
The apparent downward increase of the granites depends upon three different circumstances:
The granitic magmas are formed and have mostly solidified at great depths in the mountain zones;
much basic material (like the ophiolites or the spilites) is brought up in the upper zones of mountains (in the orogens) either as surface flows or overthrust nappes;
large scale extrusions of basaltic lavas in the fault zones of the continents and oceans (in the kratogens) add to the basic material occurring at the earth’s surface.