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7.3 Read and translate the text
The Earth
Our planet on which we live, the earth seems to be a very large body. But
compared with quite an ordinary star, like the sun the earth
is only a tiny speck in the
universe. The earth is not a true sphere but a spheroid flattened at the poles so that the
axis on which it rotates is shorter by 27 miles than the equatorial diameter. The outer
layers of the earth are the atmosphere, the hydrosphere, and the lithosphere. The
atmosphere, the outermost layer, is the envelope of gases that surrounds the earth.
The hydrosphere includes all the bodies of
water on the earth in gaseous, liquid, and
solid states. It makes up only 0.02 per cent of the mass of the earth.
The three major parts of the earth are the crust,
the mantle, and the core. The outermost more or
less rigid shell covering the earth constitutes the
earth’s crust. It includes the rocks of the
continents as well as the rocks beneath the ocean
floor. The lower boundary of the crust is 20 to 25
miles beneath the surface
of the continents and
only about 4 miles beneath the ocean. The
continents are made up of a rock type known as
granite covered by a thin layer of sedimentary
rocks, and underlain by basalt, while the floor of
all ocean basins is mostly underlain by basalt with
a thin layer of sedimentary rocks above.
The mantle composed
of materials much denser
than those making up the crust extends to a depth of 1800 miles, its density
increasing with depth. It is believed to consist of dunite or some closely allied
ultrabasic silicate rock.
Figure
The core of the earth seems to consist of two parts having different properties.
The outer core, more than 1,000 miles thick below the mantle behaves like a liquid.
The inner core, 18 times as dense as water, behaves like a solid, although its
temperature may be as high as that at the surface of the sun.
How old is the earth? The discovery of radioactivity around 1900 gave the
scientists a new means of estimating the age of the earth. All naturally radioactive
elements are known to change finally into lead. One gram of uranium will yield
1/7 000 000 000 of a gram of lead a year. Thus by measuring
the amount of lead
present in rocks containing uranium we can estimate how much radioactive change
occurred and what length of time needed for this change. The oldest rock so far
discovered has been dated at about 3.7 billion years old. We know the earth was
formed earlier than that and astronomers believe it to have been approximately 4.5
billion years ego.
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