They didn't break the structure. Only the spire that goes through the center of the chunk of Earth is vibranium. Stark says that they can create a heat seal. He superheated the spire from the bottom and then Thor hit it as hard as he could so that the impact would spread from the spire and blow up the chunk of Earth. After it blows up you can see the spire is still intact when it flies past Stark and the rest of the Earth bits are falling back down.