WATER

Planet Earth has a total surface area of close to 197 million square miles. Most of this surface is covered by water. Most of this water is salty or in the form of ice and thus is not directly usable by humans. About 97% of the total water on earth is in the oceans or salt lakes. Another 2% of the total amount is locked up in the form of ice. It is estimated that at any given time only about 1/3 of 1% of the total water on earth can be used by humans for agriculture or human consumption.

Oceans cover close to 140 million square miles, or about 71% of the earth's surface. Over 1/5 of the land on the continents, somewhere between 11 and 12 million square miles, is also covered by water. Most of this water is in the form of ice. The 5.1 million square miles of Antarctica, the northern regions of North America, Europe and Asia, and the highest mountains all around the globe, are all under ice. The ice cover is quite thick in some places. Parts of Antarctica are covered by masses of ice whose depth reaches two miles or more. In addition to being covered partly by ice, many areas of the continents are under smaller bodies of liquid water, lakes, rivers, streams and the like. In all, these smaller bodies of water cover about one million square miles of the land. In sum, a total of about 151 million square miles, or about 77% of the entire globe, is under water or ice.

The volume of the water at the earth's surface is immense as the water coverage reaches great depths in very large parts of the ocean. The ocean floor at the margins of the continents is not all that deep, usually less than 700 feet in their deepest parts. However, these continental shelves, as they are called, comprise less than 8% of the entire ocean floor. The continental shelves end abruptly, and the sea floor descends rapidly to great depths. There are two major deep zones. The one nearest the continental shelves, known as the continental rises, constitutes about 15% of the area of all the oceans. The largest parts of the ocean floor, which comprise about 76% of the entire ocean area, are generally very large flat areas that lie at a uniform depth below the sea, between 16 to 20 thousand feet, depending on the ocean. There are deeper areas yet. But these comprise only a very small part of the ocean, a little more than 1%. They are known as trenches and reach depths of over 36 thousand feet, or close to 7 miles. The highest point above the surface of the earth is not nearly as severe. Mt. Everest is only about 30 thousand feet high. The combined volume of all the water in the oceans and on the continents is about 340 million cubic miles.

This amounts to a fairly large proportion of the entire volume of the earth. The earth's volume is about 260 billion cubic miles, only about 760 times as great as the volume of the water. If all the water on earth were separated from the planet, it would form a liquid planet of its own whose size would be 870 miles in diameter.