No, I think we have to assume it had plates similar to those on Earth. I just mentioned the conditions across the planet to give a sense of distribution.
They don't crumble for the same reason that they don't crumble for everyone:
1) Suspension of disbelief
2) Tactile TK (something Reed mentioned once when Sue asked him why a building Gladiator was holding hadn't fallen apart)
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Tactile TK is the common answer, and IMHO it applies here as well.
Earth has 12 major tectonic plates. To move a tectonic plate requires far more force than the equivilant to pushing a boat floating on water, as some posters here think.
Tectonic plates in order to move have to basically crush another tectonic plate underneath it's edge and force it to go into the mantle of the planet. You can't just push one and only push the weight of that plate; by pushing one, you are pushing it against all of the other plates on the planet.
Therefore if something was pushing tectonic plates away from each other with enough strength to separate them, you are talking literally billions of tons of force to do so at the bare minimum, and that would be constantly applied pressure over millions of years. To do the same over the coarse of a few hours would take exponentially more force.
Hulk, in order to keep the plates from separating, had to struggle against the force. The only plausible explanation is tacticle TK, and it would also explain why Hulk has a long history of being able to affect pure energy with his hands (examples are ripping through the skin of a forcefield, tossing a being made of electricity, etc..) It's not just once or twice that Hulk has been able to grab pure energy with his hands and affect it either, it has been repeated time and time again over many years of his history.
Therefore Hulk's tacticle TK was strong enough to literally hold billions of tons of force together.
In the worlds strongman competition, they actually have a contest of something similar:
While it's true that it's not the same as lifting the plates, when you look at tectonic force, you have billions of tons of pressure applied at a constant rate for hundreds of millions of years to make changes to the continental structures of the Earth, so holding that level of force is an amazing feat, in fact if you look at the physics involved it puts his bracing against the mountain range in Secret Wars look pathetic in comparison.
Cool beans, but the questions thread would be better for this since this is very specific in its focus...or even Hulk's respect thread, for discussion of his possible limitations.