The sea floor is about 4.6 billion years old and covers most of the Earth's surface. It is composed of ocean basins like the Pacific, Atlantic, Indian, and Arctic oceans. The sea floor is constantly changing through the process of plate tectonics, where lithospheric plates move across Earth's surface. New sea floor is created at mid-ocean ridges through the process of sea floor spreading, while ocean trenches form as plates subduct under each other. The structure and composition of the sea floor provides clues about Earth's geological history.