by Keith Hamburger

Created on: March 28, 2008

When considering why the ocean is salty, the most basic explanation centers on the water cycle, the method by which water is continuously distributed around the Earth.

The water cycle is a never-ending process. Since we have to start somewhere, and any cycle, like a wheel really has no beginning or end, let's start at the oceans.

In the ocean water is continuously evaporating from the surface. This happens everywhere there is water, just like a puddle in the street in front of your house will evaporate over time leaving the street dry, eventually. This water vapor enters the atmosphere; everywhere in our atmosphere there is some water vapor that has come from evaporation. In some places there is a lot of water vapor, and people talk about how humid it is. Other places have little water vapor, and you hear comments about "but it's a dry heat". But, even in the driest deserts there is still some water vapor in the air.

But, water vapor doesn't stay in the air forever, eventually, it comes down.

Given the right conditions of water vapor density, temperature and airflows, and many other components, water vapor condenses into small droplets and forms clouds. If the droplets get large enough they fall as rain or snow. While rain and snow will contain some dissolved solids from the atmosphere (particles of dust are actually needed for the water vapor to condense) the concentrations of those dissolved solids are very low.

After the rain and snow falls to the ground it begins an inexorable flow downhill. Unless something temporarily stops this flow, forming a pond or a lake, the water will flow until it reaches the lowest point it can, for most of the world this is the sea. As this water flows more and more materials are dissolved or picked up in suspension. High mountain streams are usually quite clear and clean while the big, sluggish rivers at lower elevations are usually muddy and dirty. This is because as the water flows it stirs up soil and picks up what it can dissolve and carries these materials with it to the sea. The further the water has gone the more materials are carried with it. Big rivers like the Mississippi get nicknames like "Big Muddy" because of all of the sediment load carried by the river.

Even when a river enters the sea, however, most of these rivers are still relatively "fresh", or have relatively low concentrations of dissolved salts. Since water is a powerful solvent, it will dissolve almost anything to one degree or another, all water has some

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