.. salt dissolved in the oceans is, on the average, 34.5 percent by weight. About the same percentage can be obtained if three quarters of a teaspoon of salt is dissolved in eight ounces of water. Water Supply for the Earth Water that evaporates from the surface of the oceans into the atmosphere provides most of the rain that falls on the continents. Steadily moving air currents in the Earth’s atmosphere carry the moist air inland. When the air cools, the vapour condenses to form water droplets.
These are seen most commonly as clouds. Often the droplets come together to form raindrops. If the atmosphere is cold enough, snowflakes form instead of raindrops. In either case, water that has traveled from an ocean hundreds of even thousands of miles away falls to the Earth’s surface. There, except for what evaporates immediately, it gathers into streams or soaks into the ground and begins its journey back to the sea.
Much of the Earth’s water moves underground, supplying trees and other plants with the moisture they need to live. Most ground water, like surface water, moves toward the sea, but it moves more slowly. The Balance of Moisture and Temperature The movement of water in a cycle, from the oceans to the atmosphere to the land and then back to the oceans, is called the hydrologic cycle. The oceans have a strong balancing force on this cycle. They interact with the atmosphere to maintain an almost constant average value of water vapour in the atmosphere.
Without the balancing effect of the oceans, whole continents could be totally dry at some times and completely flooded at others. The oceans also act as a reservoir of heat. When the atmosphere above an ocean is cold, heat from the ocean warms it. When the atmosphere is warmer than the ocean, the ocean cools it. Without it, the differences between winter and summer temperatures, and even between those of day and night, probably would be greater. The Food and Water Supply All of man’s food comes from the earth.
Very little comes from the sea. Almost all of it comes from farms on the continents. But man can use only a small portion of the continents for farming . About 7 percent of the Earth’s land is considered arable, or suitable for farming. The rest is taken up by the swamps and jungles near the equator, the millions of square miles of desert, the rugged mountains, and–mostly in the Far North–the frozen tundra.
Man has been searching for ways to produce more food to supply the demands of the Earth’s continually increasing population. Many persons have suggested that the oceans might supply more food. They point out that the oceans cover more than 70 percent of the Earth’s surface and absorb about 70 percent of sunlight. Since sunlight is a basic requirement for agriculture, it seems reasonable that the oceans could supply a great deal of food. But what seems reasonable is not always so.
Almost all the plants that live in the oceans and absorb sunlight as they grow are algae. Algae do not make very tasty dish for man, but they are an important part of the food pyramid of the oceans. In this pyramid the algae are eaten by small sea creatures. These, in turn, are eaten by larger and larger ones. Man now enters the pyramid when he catches fish, but the fish he catches are near the top of the pyramid.
All the steps between are very inefficient. It takes about a thousand pounds of algae to produce a pound of codfish, less than a day’s supply of food for a man. To feed the growing population of the world, man must find an efficient way to farm the sea. He cannot depend simply on catching fish. Much of the Earth’s land area is unusable for agriculture because of the lack of adequate water.
Millions of acres of land have been converted into farmland by damming rives to obtain water for irrigation. Some scientists have estimated that if all the rivers of the world were used efficiently, the amount of land suitable for farming might increase by about 10 percent. Another way to increase the water supply would be to convert ocean water into fresh water. Man has known how to this for more than 2 000 years. But the process has been slow, and even with modern equipment it is costly. The distillation plant for the United States navel base at Guantanamo, Cuba, produces more than 2 million gallons of water a day, but at a cost of $1.25 for every thousand gallons.
In New York City, where fresh water is available, the cost is about 20 cents per thousand gallons. Scientists have investigated the use of nuclear-powered distillation plants. One plant would produce 150 million gallons of water daily at a cost of 35 to 40 cents per thousand gallons. It also would provide nearly 2 million kilowatts of electricity. The Atmosphere The Earth’s structure consists of the crust, the mantle, and the core. Another way of defining the Earth’s regions, especially those near the surface, makes it easier to understand important interactions that take place.
In this definition, the regions are called the lithosphere, the hydrosphere, and the atmosphere. The lithosphere includes all the solid material of the Earth. Litho refers to stone, and the lithosphere is made up of all the stone, rock, and the whole interior of the planet Earth. The hydrosphere includes all the water on the Earth’s surface. Hydro means water, and the hydrosphere is made up of all the liquid water in the crust–the oceans, streams, lakes, and groundwater–as well as the frozen water in glaciers, on mountains, and in the Arctic and Antarctic ice sheets.
The atmosphere includes all the gases above the Earth to the beginning of interplanetary space. Atmo means gas or vapour. The atmosphere extends to a few hundred miles above the surface, but it has no sharp boundary. At high altitudes it simply gets thinner and thinner until it becomes impossible to tell where the gas of interplanetary space begins. The atmosphere contains water vapour and a number of other gases. Near the surface of the Earth, 78 percent of the atmosphere is nitrogen. Oxygen, vital for all animal species, including man, makes up 21 percent.
The remaining one percent is composed of a number of different gases, such as argon, carbon dioxide, helium, and neon. One of these–carbon dioxide–is a vital to plant life as oxygen is to animal life. But carbon dioxide makes up only about 0.03 percent of the atmosphere. The weight of the atmosphere as it presses on the Earth’s surface is great enough to exert an average force of about 14.7 pounds per square inch (1.03 kilograms per square centimeter) at sea level. The pressure changes slightly from place to place and develops the high and low pressure regions associated with weather patterns.
The pressure at 36 000 feet (11 000 meters)– a typical cruising altitude for commercial jet planes–is only about one fifth as great as atmospheric pressure at sea level. The temperature of the atmosphere also falls at high altitudes. At 36 000 feet (11 000 meters), the temperature averages -56 C. The average temperature remains steady at –56 C and up to an altitude of 82 000 feet (25 000 meters). Above this altitude, the temperature rises.
The atmosphere has been divided into regions. The one nearest the Earth–below 6 miles (10 kilometers)–is called the troposphere. The next higher region, where the temperature remains steady, is called the stratosphere. Above that is the mesosphere, and still higher, starting about 50 miles (80 kilometers) above the surface, is the ionosphere. In this uppermost region many of the molecules and atoms of the Earth’s atmosphere are ionized. That is, they carry either a positive or negative electrical charge.
The composition of the upper atmosphere is different from that of the atmosphere near the Earth’s surface. High in the stratosphere and upward into the mesosphere, chemical reactions take place among the various molecules. Ozone, a molecule that contains three atoms of oxygen, is formed. ( A molecule of the oxygen animals breathe has two atoms.) Other molecules have various combinations of nitrogen and oxygen. In higher regions the atmosphere is made up almost completely of nitrogen, and higher still almost completely of oxygen.
At the outer most reaches of the atmosphere, the light gases, helium and hydrogen, predominate. The Earth’s Magnetic Field Scientists explain that another boundary besides the atmosphere seems to separate the environment of the Earth from the environment of space. This boundary is known as the magnetopause. It is the boundary between that region of space dominated by the Earth’s magnetic field, called the magnetosphere, and interplanetary space, where magnetic fields are dominated primarily by the sun. The Earth has a strong magnetic field.
It is as if the Earth were a huge bar magnet. The magnetic compass used to find directions on the Earth’s surface works because of this magnetic field. This same magnetic field extends far out into space. The Earth’s magnetic field exerts a force on any electrically charged particle that moves through it. There appears to be a steady “wind” of charged particles moving outward from the sun.
This solar wind is deflected near the Earth by the Earth’s magnetic field. In this interaction, the Earth’s magnetic field is slightly squeezed in on the side that faces the sun, and pulled out into a long tail on the side away from the sun. In the magnetosphere, orbiting swarms of charged particles move in huge broad belts around the Earth. Their movement is regular because they are dominated by the comparatively constant magnetic field of the Earth. The discovery of these radiation belts by the first American satellite, Explorer 1, was one of the earliest accomplishments of the space age.
The charged particles within the radiation belts actually travel in a complex corkscrew pattern. They move back and forth from north to south while the whole group slowly drifts around the Earth. When the magnetic field of the sun is especially strong, the magnetosphere is squeezed. The belts of trapped particles are pushed nearer to the Earth. Scientists are not certain what causes the famous aurora borealis, or northern lights, and the aurora australis, or southern lights. According to one explanation, when the trapped particles are forced down into the Earth’s atmosphere, they collide with particles there and a great deal of energy is exchanged. This energy is changed into light, and the spectacular auroras result. The Earth Through Time The Earth’s crust formed about 4.5 billion years ago.
Since then the surface features of the land have been shaped, destroyed, and reshaped, and even the positions of the continents have changed. Over the years, various kinds of plants and animals have developed. Some thrived for a time and then died off: others adapted to new conditions and survived. All these events are recorded in the Earth’s rocks, but the record is not continuous in any region. Geologists can sometimes fill in the gaps by studying sequences of rocks in various regions of the Earth.
The Earth’s Motion and Time The Earth makes one rotation on its axis every 24 hours with reference to the sun. It is 24 hours from high noon on one day to high noon on the next. It takes 365.25 days–one year–from the Earth to travel once around the sun. Calendars mark 365 days for most years, but every fourth year–leap year–has 366 days. When observed from over the North Pole, the Earth rotates and revolves in a counterclockwise direction.
When observed from the South Pole, the Earth rotates and revolves in a clockwise direction. The Changing Earth The great features of the Earth seem permanent and unchanging. The giant mountain ranges, the long river valleys, and the broad plains have been known throughout recorded history. All appear changeless, but changes occur steadily. Small ones can be seen almost any day. The rivulets of mud that form on the side of a hill during a rainstorm move soil from one place to another.
Sudden gusts of wind blow dust and sand around, redistributing these materials. Occasionally, spectacular changes take place. A volcano erupts and spreads lava over the surrounding landscape, burying it under a thick layer of fresh rock. Earthquakes break the Earth’s crust, causing portions of it to slide and move into new positions. In the lifetime of one man, or even in the generations of recorded history, these changes have been small compared to the changes that created mountains or the vast expense of the prairie. But the recorded history of man covers only a short period of the Earth’s history. Scientists believe that the Earth has existed for about 4.5 billion years.
Man’s recorded history extends back only about 6 000 years, or 0.0000013 percent of the Earth’s age. There is ample evidence that the Earth’s surface has changed greatly since its original formation.