Well, it is.. on geological timescales. And with sudden flips at that. The record is in the orientation of suitable partices at the divergent plate boundaries ('frozen' like that when the lava cooled), which also is an indicator for ocean floor spreading and plate tectonics. It's pretty awesome, actually. But as the sibling comment says, in addition to being slow it's far too weak as well.

edit: Here's an image, because it's so awesome: http://en.wikipedia.org/wiki/Vine%E2%80%93Matthews%E2%80%93M...