Nyergh... must comment
'The Earth is a big molten ball with scum floating on it, some parts thickier than others, as slow magma currents cause it to crinkle, not unlike the skin on boiled milk.'
As S-waves show, the Mantle is solid. If you drill a hole down through the Crust from almost anywhere on the Earth's surface the first liquid you will hit is in the Outer Core - and then you'll hit a geyser of molten iron.
It's much better to think of the Earth as being similar to a cold Mars Bar. The outside chocolate shell (the Crust) is brittle, below that is a caramel which is technically solid yet highly plastic layer (the Asthenosphere), below that is a solid layer of fudge which is still plastic (the Mantle proper). Bend the Mars Bar slowly and the Crust cracks, but the Asthenosphere and Mantle bend gradually. Hit it hard and they fracture.
Likewise the Mantle undergoes flow over the long term, but remains entirely solid.
Where this theory breaks down is that the Earth is much less delicious than a Mars Bar.
If you really want to see isostatic rebound in action, drive along Route 1 in Southern Iceland between Hveragerði and Vík í Mýrdal. The enormous cliffs on the left of the car are the old sea cliffs from about 10kya; the strip of land you're driving on is the old beach and the sea is anything up to 10km away on the right. Towards Reykjavik the calculations are that rebounds were anything up to 7cm PER YEAR in the immediate postglacial period which would put them amongst the fastest known.