In Superman Returns , he lifts the new landmass because he temporarily regains enough strength from the sun, then uses the Earth rock beneath the Kryptonite layer as a buffer while he pushes it up. The scene also suggests pure determination matters: he’s weakened, but not fully stopped, so he forces one last burst of power to get the job done.

What the movie implies

  • Luthor’s island is not all Kryptonite; the top is the dangerous part, while the lower material is more like earth and stone. That gives Superman at least some insulation from the Kryptonite while he gets underneath it.
  • He first recovers after being exposed to sunlight, which restores some of his strength before the final lift.
  • Even so, the Kryptonite still poisons him, which is why he collapses afterward.

Why it works dramatically

The film is playing more like classic comic-book logic than hard physics. The idea is: Superman can do the impossible when he’s desperate enough, but the cost is that it nearly kills him.

Simple version

He was able to do it because he had a brief recharge from the sun, enough insulation from the Kryptonite, and one last huge effort of will.

TL;DR: Superman lifts the “new planet” because the movie treats him as partially recharged, somewhat shielded from the Kryptonite, and able to push past his limits for a final heroic burst.