Well, I'll take exception to the guy's bad math, as well as his attempt to bring science-fiction physics into a magical fantasy game.
A cubic yard is 27 cubic feet, so a 50th level caster can create about 1.5 cubic meters, not 15 cubic meters. So drop his maximum yield by 10.
Next, a cube is a terrible shape for this move. You want a sheet, for maximum surface exposure. The cube would have the maximum reaction against it's resting surface, since earth is much more dense than air. The initial blast would use a fraction of the mass, and launch the thing like an Atlas rocket. Presuming the cube holds together, you will have invented the intercontinental "bouncing ball of doom", as it launches itself more or less straight up, eventually falls to earth, and then repeats it's performance someplace else (Coriolis force will tend to move it in a westward direction.) Sorry, no planetary destruction, just a long range random hazard.
If it doesn't hold together then you've invented the MRV version of the same thing.
And, of course if the initial kick is enough to clear the atmosphere, then it's possible it will never land. As soon as it starts to descend it will encounter atmosphere that it can react with. That may well be enough to keep it skipping along like a stone on a lake.
In any case, the initial detonation would probably leave a crater the size of a house, and a lot of broken windows in the neighborhood.
Then, there's the basic problem that the caster is standing at ground zero. Minor oversight in the planning department.
As a DM I'd say that he'd summoned Voidstone, from the Negative Material Plane. That's D&D's closest equivalent to Antimatter, and all it will do is disintegrate normal matter that comes in contact with it. It would just destroy the surface below on a continuous basis until it was all consumed. No boom, just an interesting way to dig a new well.
