The Earth, man’s home, is a planet. The Earth has special characteristics, and these are
important to man. It is the only planet known to have the right temperature and the right atmosphere to
support the kind of environments and natural resources in which plants and man and other animals can
survive. This fact is so important to man that he has developed a special science called ecology, which
deals with the dependence of all living things will continue to survive on the planet.
Many millions of kinds of plants and animals have developed on Earth. They range in size from
microscopic plant and animals to giant trees and mammoth whales. Distinct types of plants or animals may
be common in many parts of the world or may be limited to a small area. Some kinds thrive under
conditions that are deadly for others. So some persons suggest that forms of life quite different from
those known on Earth might possibly survive on planets with conditions that are far different from
conditions on Earth.
Many persons believe that the Earth is the only planet in the solar system that can support any
kind of life. Scientists have theorized that some primitive forms of life may exist on the surface of
Mars, but evidence gathered in 1976 by unmanned probes sent to the Martian surface seems to indicate that
this is unlikely.
Scientist at one time also believed that Venus might support life. Clouds always hide the
surface of Venus, so it was thought possible that the temperature and atmosphere on the planet’s surface
might be suitable for living things. But it is now known that the surface of Venus is too hot–an
average of 800 F (425 C)–for liquid water to exist there. The life forms man is familiar with could not
possibly live on Venus.
The Earth has excellent conditions for life. The temperature is cool enough so that liquid water
can remain on Earth’s surface. In fact, oceans cover more than two thirds of the surface. But the
temperature is also warm enough so that a small fraction of this water is permanently frozen–near the
North and South Poles and on some mountain tops.
The Earth’s atmosphere is dense enough for animals to breathe easily and for plants to take up
the carbon dioxide they need for growth. But the atmosphere is not so dense that it blocks out sunlight.
Although clouds often appear in the sky, on the average enough sunlight reaches the surface of the Earth
so that plants flourish. Growing plants convert the energy of sunlight into the chemical energy of their
own bodies. This interaction between plants and the sun is the basic source of energy for virtually all
forms of life on Earth.
Extensive exploration of the sea floor since 1977, however, has uncovered the existence of
biological communities that are not based on solar energy. Active areas of sea floor spreading, such as
the centers in the eastern Pacific that lie far below the limit of light penetration, have chimney like
structures known as smokers that spew mineral-laden water at temperatures of approximately 660 F (350 C).
Observations and studies of these active and inactive hydrothermal vents have radically altered
many views of biological, geological, and geochemical processes that exist in the deep sea. One of the
most significant discoveries is that the vents and associated chemical constituents provide the energy
source for chemosynthetic bacteria. These bacteria form, in turn, the bottom of the food chain,
sustaining the lush biological communities at the hydrothermal vent sites. Chemosynthetic bacteria are
those that use energy obtained from the chemical oxidation of inorganic compounds, such as hydrogen
sulfide, for the fixation of carbon dioxide into organic matter.
Although the atmosphere allows sunlight to reach the Earth’s surface, it blocks out certain
portions of solar radiation, especially X rays and ultraviolet light. Such radiation is very harmful,
and, if the atmosphere did not filter it out, probably none of the life forms on Earth could ever have
developed. So, the necessary conditions for these life forms–water, the planet in the solar system
known to have all these “right” conditions.
THE EARTH’S PLACE IN SPACE
Despite its own special conditions, the Earth is in some ways similar to the other inner
planets–the group of planets nearer to the sun. Of these planets, Mercury is the closest to the sun;
Venus is second; the Earth is third; and Mars is forth. All of these planets, including the Earth, are
basically balls of rock. Mercury is the smallest in size. It diameter is about two thirds the greatest
width of the Atlantic Ocean. Mars is larger than Mercury, but its diameter is only a little more than
half that of the Earth. Venus, width a diameter of roughly 7 600 miles (12 000 kilometers), is almost as
large as Earth.
Four of the five outer planets are much bigger than any of the inner planets. The largest,
Jupiter, has a diameter more that 11 times as great as that of the Earth. These four outer planets are
also much less dense than the inner planets. They seem to be balls of substances that are gases on Earth
but chiefly solids at the low temperatures and high pressures that exist on the outer planets.
The exact size or mass of Pluto, the most distant planet, is not known. Its composition is also
a mystery. All that is known for sure about Pluto is its orbit . Pluto’s average distance from the sun
is almost 40 times that of the Earth.
At the outer reaches of the solar system are the comets. A comet consists of nucleus of frozen
gases called ices, water and mineral particles; and a coma of gases and dust particles. Some comets also
have tails. A comet’s tail consists of gases and particles of dust from the coma. As the comet
approaches the sun, light from the sun and the solar wind cause tails to form. For this reason the tails
point generally away from the sun.
For several hundred years almost everyone has accepted the fact that the world is round. Most
persons think of it as a sphere, somewhat like a solid ball. Actually, the diameter is nearly, but not
exactly, spherical. It has a slight bulge around the equator. Measured at sea level, the diameter of
the Earth around the equator is 7 926.7 miles (12 756.8 kilometers).
The distance from the North to the South pole, also measured at sea level, is 7 900.0 miles (12
713.8 kilometers). Compared to overall diameter, the difference seems small–only 26.7 miles (43
kilometers). But compared to the height of the Earth’s surface features, it is large. For example, the
tallest mountain, Mount Everest, juts less than 6 miles (9 kilometers) above sea level. The Earth’s
shape has another slight distortion. It seems slightly fatter around the Southern Hemisphere than around
the Northern Hemisphere. This difference is, at most, about 100 feet (30 meters).
The shape of the Earth was originally calculated from measurements made by surveyors who worked
their way mile by mile across the continents. Today, artificial satellites, then calculate the
gravitational force that the Earth exerts on the satellites. From these calculations, they can deduce
the shape of the Earth. The slight bulge around the Southern Hemisphere was discovered from calculations
made in this way.
The Earth’s Mass, Volume, and Density
The mass of the Earth has been found to be, in numerals, 6 sextillions, 595 quintillions tons.
Scientists measure the Earth’ mass by means of a very delicate laboratory experiment. They place heavy
lead weights of carefully measured mass near near other in an apparatus that measures the force of the
gravitational attraction between them.
According to Newton’s law of gravitation, the force of gravity is proportional to the products of
the two masses involved. The force of the Earth’s gravity on the experimental mass is easily measured.
It is simply the weight of the mass itself. The force of gravity between two known masses in the
laboratory can be measured in the experiment. The only missing factor is the mass of the Earth, which
can easily be determined by comparison.
Scientists can calculate the Earth’s volume because they know the shape of the Earth. They
divide the mass of the Earth by the volume, which gives the average density of the material in the Earth
as 3.2 ounces per cubic inch (5.5 grams per cubic centimeter).
This average value includes all the material from the surface of the Earth down to the center of
the Earth. But not all of the material in the Earth has the same density. Most of the material on the
continents is only about half as dense as this average value. The density of the material at the center
of the earth is still somewhat uncertain, but the best evidence available shows that it is about three
times the average density of the Earth.
The Earth’s Layers
The difference in density is not the only difference between the Earth’s surface and its center.
The kinds of materials at these two locations also seem to be quite different. In fact, the Earth
appears to be built up in a series of layers.
The Earth’s structure comprises three basic layers. The outermost layer, which covers the Earth
like a thin skin, is called the crust. Beneath that is a thick layer called the mantle. Occupying the
central region is the core. Each layer is subdivided into other, more complex, structures.
The crust of the Earth varies in thickness from place to place. The average thickness of the
crust under the ocean is 3 miles (5 kilometers), but under the continents the average thickness of the
crust is 19 miles (31 kilometers). This difference in thickness under the continents and under the
oceans is an important characteristic of the crust.
These two parts of the crust differ in other ways. Each has different kinds of rocks.
Continental rocks, such as granite, are less dense than rocks in ocean basins, such as basalt. Each part
also has a different structure. The basaltic type of rock that covers most of the ocean floors also lies
underneath the continents. It appears almost as though the lighter rocks of the continental land masses
are floating on the heavier rocks beneath.
Modern theories about the Earth’s structure suggest that this is exactly what is happening. But
to understand this theory of floating rocks, called isostasy, it is necessary to know something about the
Earth’s next deeper layer, the mantle.
The mantle has never been seen. Men have drilled deep holes, such as those for oil wells, into
the crust of the Earth both in the continents and in the ocean floor. But no hole has ever been drilled
all the way through the crust in to the mantle. All measurements, scientists can deduce many
characteristics of the mantle.
The mantle is about 1 800 miles (2 900 kilometers) thick and is divided into three regions. The
rocky mantle material is quite rigid compared to things encountered in everyday experience. But if
pressure is applied to it over a long period–perhaps millions of years–it will give a little bit. So,
if the distribution of rock in the crust changes gradually, as it does when material eroded off mountains
is deposited in the ocean, the mantle will slowly give way to make up for the change in the weight of the
rock above it.
The core extends outward from the Earth’s center to a radius of about 2 160 miles (3 480
kilometers). Obtaining information about the Earth’s interior is so difficult that may ideas about its
structure remain uncertain. Some evidence indicates that the core is divided into zones. The inner
core, which has a radius of about 780 miles (1 255 kilometers), is quite rigid, but the outer core
surrounding it is almost liquid.
scientists disagree about this description of the core because it is based on incomplete seismic
wave data. The theory suggest that the density of the inner core material is about 9 to 12 ounces per
cubic inch (16 to 20 grams per cubic centimeter). The density of the outer core material is about 6 to 7
ounces per cubic inch (11 to 12 grams per cubic centimeter).
The Earth’s Surface Areas
Much scientific study has been devoted to the thin crystal area on which man lives, and most of
its surface features are well known. The oceans occupy 70.8 percent of the surface area of the Earth,
leaving less than a third of the Earth’s surface for the continents.
Of course, not all of the Earth’s land is dry. A fraction of it is covered by lakes, streams,
and ice. Actually, the dry land portion totals less than a quarter of the Earth’s total surface area.
The Salty Oceans
The oceans are salty. Salt is a rather common mineral on the Earth and dissolves easily in
water. Small amounts of salt from land areas dissolve in the water of streams and rivers and are carried
to the sea. This salt has steadily accumulated in the oceans for billions of years.
When water evaporates from the oceans into the atmosphere, the salt is left behind. The amount
of 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
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
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 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
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
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
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’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.