(Lansing State Journal, May 26, 1993)
Question submitted by Scott Dennison.
As you know, sea water is a solution of many different salts. Salts are ionic compounds that often dissolve readily in water. In the oceans, sodium and chloride ions, which form ordinary table salt, are the most abundant. The large majority of solutes come from the mineral constituents of the planet.
While pure table salt dissolves quickly in water, most rocks do not. However, given enough time and energy, even stone will wear down and in doing so release soluble constituents. This is called erosion. When rain falls, it may percolate through the ground or run off into streams and rivers. In both cases, the physical energy of flow and the chemical nature of the water itself contributes to dissolving mineral salts. These salts can be carried by rivers into the sea. Volcanic eruptions also release a lot of matter in the form of ash and dust. Much of this settles into the oceans, where it can either dissolve or settle to the bottom. The formation of the oceans, early in our planet's history, also involved an energetic mixing of materials and probably contributed substantially to the constitution of sea water.
The ratio of solutes in the oceans is essentially constant all over the world, because they are so large and mixing is so complete over time. This also means than the total concentration of salts can change only very slowly.