The surface of Earth is constantly changing. On small scales, the changes are caused by wind, rain, and flowing water. On a large scale, though, they’re caused by the motions of Earth’s crust. New crust is created on the ocean floor, where molten rock pushes up from the mantle, the layer below the crust. And old crust disappears as one plate plunges below another, pushing its rock back into the mantle. A similar process may be at work on Europa, one of the big moons of Jupiter. In that case, though, the plates are made of ice.