Thank you for that comment. I've been taught that Northern Europe was depressed by the weight of the ice sheets during the Ice Age and is still rebounding now that the ice sheets have gone and the weight is off.

I was imagining this as like compressing a sponge and never quite believed that re-expanding would take thousands of years.

But an enormous raft of granite, floating on an extremely viscous sea of even denser hot rock? At least I can see clearly that I have no useful intuitions about this. Does the raft bob up and down on a ten thousand year time scale? (Probably too heavily damped to oscillate) But the raft metaphor implies some lateral flow of the "sea" as the raft rises, so that is something to think about.