It was at this point in the planet’s history that the earth’s eggshell-like crust, which was slowly forming on the surface from this cooling scum, began to stop doing what up to that point it was prone to do, and that is to keep on remelting itself. For eons it kept sinking back into the mantle just a few millennia after it had formed, utterly wrecking itself in the process — and then it would pop up out of the molten ocean of lava and be reborn in a totally different guise. Instead, all of a sudden, large chunks of crust were staying afloat, more or less permanently. In cooling, the crust was forming itself into rocks that would themselves be permanent — if only the external forces permitted them to remain at the surface and did not try to drag them or push them down toward the heat again.
As they slowly cooled, some of these rocks-to-be separated themselves out, according to perfectly understandable laws of physics: The lighter materials of the scum rose to the surface, the heavier ones passed downward in one enormous fractionating column — a little like the Skaergaard, though over infinitely longer periods of time and under very different physical conditions. The lighter materials generally formed themselves into those rocks we now call granites – the coarse-grained rocks that tend to be prettily light in color as well as in consititution. The heavier fractions created layers of rocks like basalt and diorite and gabbro, which were darker and tended to sag downward under the force of gravity, forming sloughs, whereas the granites tended to form uplands. The darker and heavier slabs lay sluglike and low on the earth’s surface, and in time they began both to accumulate and to accommodate water that fell from the skies; over many millions of years, this resulted in the creation of oceans. Dark rocks underlay the seas; granites made up the new continents. And this law of basic igneous geology has remained a verifiable truth ever since.
From page 77 of A Crack in the Edge of the World