The continents are still on the move, though. The world will have a different map in the distant future. In the deepest parts of the ocean there are huge cracks between the plates of the crust. Hot lava comes up through these cracks, forming long ridges of jumbled, rocky mountains deep under the sea. As lava continues to come up, the new rock pushes the older, cooler rock away.
New rock is formed, and the slowly moving rocks make the ocean wider.
This is called sea-floor spreading . Plate boundaries in which the plates move away from each other are called divergent boundaries . The earth is a sphere, and the rock cannot just keep forming and moving at divergent boundaries. Eventually it has to go somewhere. As it goes, it carries the continental plates along with it, and the continents ride on top.