Introduction
Oceans cover about 70% of the Earth’s surface. The oceans contain roughly 97% of the Earth’s water supply.
The oceans of Earth are unique in our Solar System. No other planet in our Solar System has liquid water (although recent finds on Mars indicate that Mars may have had some liquid water in the recent past). Life on Earth originated in the seas, and the oceans continue to be home to an incredibly diverse web of life.
The oceans of Earth serve many functions, especially affecting the weather and temperature. They moderate the Earth’s temperature by absorbing incoming solar radiation (stored as heat energy). The always-moving ocean currents distribute this heat energy around the globe. This heats the land and air during winter and cools it during summer.
Why are the oceans salty?
As water flows in rivers, it picks up small amounts of mineral salts from the rocks and soil of the river beds. This very-slightly salty water flows into the oceans and seas. The water in the oceans only leaves by evaporating (and the freezing of polar ice), but the salt remains dissolved in the ocean - it does not evaporate. So the remaining water gets saltier and saltier as time passes.
Salinity
| Sea salts | Parts per Thousand |
|---|---|
| chloride | 19.3 o/oo |
| sodium | 10.7 o/oo |
| sulfate | 2.7 o/oo |
| magnesium | 1.3 o/oo |
| calcium | 0.4 o/oo |
| potassium | 0.4 o/oo |
| bicarbonate | 0.15 o/oo |
| bromide | 0.07 o/oo |
| other | 0.06 o/oo |
| Total Salinity | 35.08 o/oo |
The salinity (salt content) of ocean water varies. The oceans and seas contain roughly 5 x 10 16 tons of salts. One cubic foot of average sea water contains 2.2 pounds of salt.
The oceans are about 3.5% salt (by weight). Salinity is generally reported in terms of parts per thousand (abbreviated o/oo), the number of pounds of salt per 1,000 pounds of water; the average ocean salinity is 35 o/oo.
The saltiest water is in the Red Sea and in the Persian Gulf, which have a salinity of about 40 o/oo (due to very high evaporation rates and low fresh water influx). The least salty seas are in the polar regions, where both melting polar ice and a lot of rain dilute the salinity.
The oceans
The Earth’s oceans are all connected to one another. There are five oceans: the Pacific, Atlantic, Indian, Southern, and Arctic. There are also many seas (smaller branches of an ocean); seas are often partly enclosed by land. The largest seas are the South China Sea, the Caribbean Sea, and the Mediterranean Sea.
| Ocean | Area (square miles) | Average Depth (ft) | Deepest depth (ft) |
|---|---|---|---|
| Pacific Ocean | 64,186,000 | 15,215 | Mariana Trench, 36,200 ft deep |
| Atlantic Ocean | 33,420,000 | 12,881 | Puerto Rico Trench, 28,231 ft deep |
| Indian Ocean | 28,350,000 | 13,002 | Java Trench, 25,344 ft deep |
| Southern Ocean | 7,848,300 | 13,100 - 16,400 | the southern end of the South Sandwich Trench, 23,736 ft (7,235 m) deep |
| Arctic Ocean | 5,106,000 | 3,953 | Eurasia Basin, 17,881 ft deep |
What causes ocean waves?
The winds cause waves on the surface of the ocean (and on lakes). The wind transfers some of its energy to the water, through friction between the air molecules and the water molecules. Stronger winds (like storm surges) cause larger waves. You can make your own miniature waves by blowing across the surface of a pan of water.
Waves of water do not move horizontally, they only move up and down (a wave does not represent a flow of water). You can see a demonstration of this by watching a floating buoy bob up and down with a wave; it does not, however, move horizontally with the wave.
Tsunamis (sometimes called tidal waves) are different from surface waves; they are usually caused by underwater earthquakes, volcanic eruptions, or landslides.
What causes the tides?
Tides are periodic rises and falls of large bodies of water. Tides are caused by the gravitational interaction between the Earth and the Moon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. 
Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Since the earth is rotating while this is happening, two tides occur each day. Isaac Newton was the first person to explain tides scientifically.
For information on spring and neap tides, click here.
Why is the ocean blue?
Sunlight is made up of all the colors of the rainbow: red, orange, yellow, green, blue, and violet. Some of the sunlight is reflected off the surface of the water, reflecting the color of the sky. Some of the sunlight penetrates the water and is scattered by ripples and particles in the water (this tinges the appearance of the ocean with the color of the particles). In deep water, much of the sunlight is scattered by the oxygen in the water, and this scatters more of the blue light.
Water absorbs more of the red light in sunlight; the water also enhances the scattering of blue light. Sir Chandrasekhar Venkata Raman (an Indian physicist) won the Nobel prize in 1930 for his work on light.
Some oddly-colored seas:
- The Red Sea often looks red because of red algae that live in this sea.
- The Black Sea looks almost black because it has a high concentration of hydrogen sulfide (which appears black).
Links and activities
All About Oceans and Seas - with extensive information on the Earth’s oceans
Ocean Animal Printouts
What Causes Tides?