I thought all those rocks went round in a nice cycle of igneous to sedimentarty to metaphoric to igneo....... So you could say that volcanoes come from volcanoes maybe. I think there's a bit more than just crust in it as well, though neither of these issues has absolute answers.

Mind you my geology is a bit outdated, but that is the first of this model I have seen. It would seem that it is true to only that small region in direct relaton to active Vol.

The whole concept of Plate Tectonics is a global thing. It incorporates such phenomena as sea-floor spreading and subduction zones, mountain building etc. 'Plates' are distinct chunks of the earths surface that slip and slide about. They bump into each other, sometimes ride over/under each other on a geological time scale. The Pacific 'Ring of Fire' where many volcanoes are found is a direct manifestation of all this stuff. The sea-floor spreading bit is one piece of evidence for the Earths magnetic reversals. Compare the Western edge of N+S America to Europe and Africa - it's no coincidence that they are similar. Google any of the terms I used here and you will find more. The way in which heat convects throughout the Earth and it's effect on Earths magnetic field is much more chaotic, and more of a recent topic. In essence we live in a bi-stable system. Every once in a while it flips, but in the intermediate period regions of magnetic strength swirl around. I think I'm correct in saying that that total magnetism is the same, but it's local intensity changes. europium is probably much more qualified to explain this than I am.