Convection is the process by which heat is transferred by the motion of water, air, or some other substance, like magma, or molten rock. Processes such as volcanism, earthquakes, and plate tectonics are due to convection of magma within the earth. Air temperatures and climate are influenced by ocean temperatures as heat is distributed by convection and other processes as water moves throughout the ocean and heats or cools the air with which it has contact. Convection of magma within the earth is driven by heat and density: hotter, less dense molten rock rises toward earth"s surface. Convection in the ocean is driven by water density. Water is most dense when it is coldest: colder water is more dense than warm water. Water is also more dense when it is saltier: sea water is more dense than fresh water. Cold, salty, dense water in the ocean sinks to the ocean floor, and comes back to the surface through various mechanisms.