The PHYSICAL Ocean

Oceans cover about 70% of the earth's surface with water, to an average depth of about 2 miles (10,000 feet); the total amount of water on the earth is 300 million cubic miles of water. This huge amount of water has a big impact on the rest of the planet surface--the dry land that we live on-- in many important ways. First, let's look at the origin and composition of seawater, and then how oceans influence important processes on land.

The Origin of the Oceans

Billions of years ago when the earth was just forming, there were no oceans; the planet was much hotter than today as matter came together to form the original planet. As the earth began to cool down, water held within the rocks that formed the earth's mantle began to seep upwards. Gaseous water vapor in the primitive atmosphere condensed and fell as the first rain. You can still see this subterranean water coming to the surface in the form of geysers at places such as Yellowstone Park (picture at right). Over millions of years this water formed the oceans. Originally this water didn't have a lot of salts dissolved in it, but over time, it became the salty seawater we have today.

Why Seawater is Salty

Seawater has about 3.5% salt in it-- not a lot, but far too much for it to be used for drinking or watering your garden. Most of this salt is common table salt-- sodium chloride (NaCl), with smaller amounts of potassium, magnesium, sulfur, and calcium. Some of these elements came up from the mantle with the water during the out-gassing process that originally formed the oceans. But most of this salt accumulated over time due to weathering of rocks that make up the continents-- as rain fell on the land, it slowly dissolved parts of the rocks, and the soluble salts were washed into the sea. So, it is really the hydrologic cycle (discussed below), which creates rainfall in the first place, that makes seawater salty.

Ocean Topography

The ocean looks flat to us, but really it has the same kind of topography as the land surface underneath all that water. Near the edge of the continents are gently sloping plains called the continental shelves, where the water is fairly shallow (only hundreds of feet deep). These areas extend out from 10 to 50 miles offshore, and are very productive fishing grounds. Out farther the ocean floor drops off, with valleys, ridges, whole mountain ranges lying in water up to 7 miles (35,000 feet) deep! The following movie done by NASA shows what it would be like to cruise over the deep ocean floor in a submarine.

Click on the image at right to see the movie:

We should also remember that sea level, or the average elevation of the surface of the oceans, is not constant. The gravitational pull of the moon causes water in the ocean to "pile up" or "spread out", causing tides along coast lines. Over geologic time sea level has varied tremendously due to changes in the earth's average temperature. If the earth gets warmer, the polar ice caps melt and sea level rises, flooding more land-- and the opposite occurs during cold periods. Twenty million years ago sea level was very high, and the coast line of Georgia stretched from Columbus to Augusta! It might be that people are causing climate change through emissions of carbon dioxide, which could result in increases in mean sea level (see the section on Environmental Issues).

The Hydrologic Cycle

The oceans are a key part of the hydrologic cycle. The oceans absorb huge amounts of solar energy, which increases the water temperature and promotes evaporation (see diagram at right). This evaporated seawater (now free of salts, since they do not vaporize) is the major source of atmospheric water vapor, which eventually condenses over land as rainfall. That rainfall, obviously, supports plant life, which supports all animal life-- including us! Water that is not used by plants in evapo-transpiration eventually flows back to the sea in rivers, carrying along a load of dissolved salts.

Ocean Currents and Climate

The water near the ocean surface does not stand still, but circulates in a regular pattern, creating ocean currents (shown at left). These currents have a major effect on how different life forms live in the oceans, as well as influencing the weather on land. Currents are created by a combination of forces-- the rotation of the earth on its axis, the heating of surface water due to solar radiation, and density differences in seawater at the surface compared to deeper water.

The Gulf Stream is a famous current that flows from the equator up the eastern side of North America, across the North Atlantic to northern Europe. It is warm water (red in the picture at right) heated by the tropical sun, and it releases it heat to the atmosphere as it travels north. Thus, the northeastern U.S., as well as Ireland and England, have a much warmer, more moderate climate than they normally would. Sometimes changes in ocean currents can have major effects on climates. El Nino is an example where changes in ocean currents cause major changes in weather patterns over land, as well as affect biological productivity in parts of the ocean. Click here for a brief summary of the El Nino effect.

Ocean currents also occur vertically-- water moves up from deeper regions to the surface, due in part to surface currents and wind circulation. Deep water is colder and higher in plant nutrients than normal surface water, and where upwelling of this deep water occurs, plants and animals thrive. Coastal waters along the west coast of North and South America have upwelling zones that have huge fisheries. Part of the El Nino effect is that this upwelling is reduced, causing lower fish production.

Next-- Life in the Oceans -- or return to the Main Page....