Chapter 4

A mystery in the mantle

“I think the answer to Paul Gast’s original work is quite extraordinary,” says McKenzie. “Yes the mantle is very well stirred, but it isn’t well mixed because the material doesn’t diffuse. It’s just like Brighton rock, it’s streaked. In some way that we still don’t understand, the melt can be extracted from those streaks without interacting with any of the rest of it. It can’t be described by the equations I derived, which were essentially for porous flow through material, with chemical interaction. It must be something else. Presumably there’s some instability from that porous flow which concentrates the melt into channels which don’t have to be very big, just a centimetre or so across, which then allows the melt to move through large vertical distances without interaction. It’s still to me anyway a complete mystery as to how that works. And I don’t believe anyone understands that.”