The ocean makes up more than two-thirds of the earth's surface. When the ocean was first forming, the rainwater washed minerals from the land into the ocean. Seawater contains many different dissolved salts and chemicals species, but is dominated by chloride, sodium, sulfate, magnesium, calcium and potassium. Sodium, potassium and calcium are among the elements eroded from rocks and minerals that wash into the oceans, which explains why the ocean contains dissolved salts or saltwater. The temperature of the ocean depends upon the intensity of the sun. It is hot in the tropics and freezing at the poles. It is cooler at deeper depths where the sun cannot penetrate than at the water's surface, which is constantly being warmed by the sun.
The ocean's waters are constantly moving and changing. As the wind blows, it puts the surface of the ocean into motion making waves, the steady up and down movement. The winds can also help to direct the currents that circulate on the water's surface. The direction of currents is influenced by the shorelines of continents and by the rotation of the Earth. Currents stir up the ocean's waters helping to supply heat, oxygen and food.