Figure 3

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.

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.

a mile – 5,2800 feet – 1,609 kilometres

7.4 Say whether the following statements are true or false

1. 1. The earth is a spheroid flattened at the poles.

2. 2. The axis on which the earth rotates is longer by 27 miles than the equatorial diameter.

3. 3. The hydrosphere includes all bodies of water in liquid state.

4. 4. The lower boundary of the crust is 20-25 miles beneath the ocean and only about 4 miles beneath the surface of the continents.

5. 5. Materials the mantle composed of, are much denser than those making up the crust.

6. 6. The temperature of the outer core may be as high as that at the surface of the sun.

7. 7. All naturally radioactive elements are known to change finally into lead.

7.5 Fill in the blanks with the suitable words from the text

1. ………. makes up only 0.02 per cent of the mass of the earth.

2. ………. is composed of materials much denser than those making up the crust.

3. The atmosphere, the hydrosphere and the lithosphere are the ………. layers of the earth.

4. The earth is not a true ………. but a ………. .

5. The discovery of ………. gave the scientists a new means of estimating ………. of the earth.

6. ………. includes the rocks of the continents as well as the rocks beneath the ocean floor.

7. The continents are made up of ………. covered by a thin layer of ……, and underlain by ………. .

7.6 Complete the following sentences

1. 1. Compared with quite an ordinary star, like the sun, the earth ………. .

2. 2. By measuring the amount of lead present in rocks containing uranium we can ………. .

3. 3. The atmosphere, the outermost layer, is ………. .

4. 4. The floor of the ocean basins is mostly underlain by………. .

5. 5. The mantle is believed to consist of ………. .

6. 6. The oldest rock so far discovered has been dated at ………. .

7. 7. One gram of uranium will yield ………. .