The Earth

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The Earth
The earth is one of the nine planets, which all revolve around the Sun. Of them the earth is neither
the largest nor the smallest. The earth is constantly changing. Long, long ago, the earth was not l
ike the earth today. It was only a molten sphere. Gradually the molten sphere cooled down. At last t
he rocks and life appeared on the earth. It;s commonly believed that the earth is a huge mass of roc
k , water, and gases. The general shape of the earth is that of a flattened sphere. It is not perfec
tly round but slightly flattened at the poles. Artificial satellites have confirmed that the earth h
as a circumference of approximately 25,000 miles, and it is 7,927 miles in diameter at equator and a
bout 27 miles less, namely about 7900 miles m in diameter at the poles. This is a small difference a
mounting to only one-third of one percent when compared to the earth’s size.
The earth is usually divided by scientists into three spheres: the atmosphere, the hydrosphere and t
he lithosphere. The solid part of the earth is called the lithosphere. Actually, we know more about
the water and gases covering the earth than we know about the solid earth itself. We can see the sur
face of the earth, and mines and deep wells tell us a little of what is under the surface. However,
the deepest wells drilled by geologists from the former Soviet Union extend only about 7 miles below
the surface. Thus, compared to the almost 8,000-mile diameter of the earth, this is only a scratch
on its surface. What is below this 7-mile depth? No one knows for sure, but scientists believe the e
arth is made up of several layers.
The outermost layer, the part we see on the surface of the earth, is the crust. This is believed to
be about 7 to 30 miles thick in land areas and thinner under the oceans. Geologists have found that
the crust is about 4 to 10 miles thick under most of the oceans. The rocks of the crust actually for
m a thin shell which covers the globe.
The mantle, which is about 1,800 miles thick, lies underneath the crust. It is thought to be compose
d of heavier rocks than the material making up the crust. Because of the great pressure and heat in
this layer, the mantle is neither quite a solid nor quite a liquid . In other words, the material ma
king up the mantle is thought to be in a plastic state. The land masses making up the crust are thou
ght to “float” on this plastic layer.
The next layer is the outer core, nearly 1,400 miles thick. It is believed to be composed mostly of
iron and nickel in a molten state at a very high temperature. Evidence indicates that the outer core
is about twice as dense as the material in the mantle.
The innermost layer, the inner core, extends about 800 miles to the earth’s center. It also is proba
bly composed of iron and nickel. Because the pressure is nearly 60,000,000 pounds per square inch, t
he inner core is more like a true solid. The material making up the inner core is thought to be abou
t three or four times as dense as the material making up the mantle. You might thick of the earth as
being constructed much like a baseball wit a two-layered core, a thick layer being around the core(
mantle), and a thin skin on the surface(crust).
The other two of the three spheres will be introduced in the following. The hydrosphere is the water
layer covering the earth. Water and air have been the most important factors in changing the surfac
e of the earth throughout its history. Most of the earth’s water is in the oceans, but rivers and la
kes are also a part of the hydrosphere. How much water is there on the earth? Scientists have estima
ted that if all the mountains were leveled off and sea bottom were raised, there would be enough wat
er to cover the earth to a depth of a mile and a half.
The atmosphere surrounds the lithosphere and the hydrosphere. The atmosphere is made up of gases, ch
iefly nitrogen and oxygen, surrounding the solid and liquid parts of the earth. In the study of the
earth, only a few of these gases will be mainly concerned about. Oxygen is important because it is c
ombined with many other minerals in the earth’s crust. Carbon dioxide, in water solution, also has h
elped to form many rocks and minerals. At present, the composition of dry air is well-known as nitro
gen, oxygen, argon, carbon dioxide and rare gases.
The water vapor in the earth’s atmosphere is responsible for many geological changes. It is one of t
he most essential agents by which the earth’s surface has been changed over many, many millions of y
ears.
The earth is a complex, dynamic planet that has changed continually since its origin some 4.6 billio
n years ago. These changes and the present-day features we observe are the result of interactions be
tween the various interrelated internal and external Earth subsystems and cycles. In fact, the earth
is unique among the planets of our solar system in that it supports life and has oceans of water, a
hospitable atmosphere, and a variety of climates. It is ideally suited for life as we know because
of a combination of factors, including its distance from the Sun and the evolution of its interior,
crust, oceans, and atmosphere. Over time, changes in the earth’s atmosphere, oceans, and to some ext
ent, its crust have been influenced by life processes. In turn, these physical changes have affected
the evolution of life.
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