فایل صوتی ندارد
As a child, you might have wondered what you would find if you could dig a hole deep into the center of the earth. Scientists have also been interested in this question, and during the 20th century they were able to learn much information about the composition of the earth. However, they did not gain this knowledge by digging a hole deep into the center of the earth, because this is an impossible task. Scientists have inferred the composition of the earth by using several sources of information. First, they have carefully recorded and measured the “seismic waves” that are released by earthquakes. Second, they have observed the composition of meteorites that have fallen to the earth’s surface from outer space.
Third, they have conducted laboratory experiments to determine the density of the earth and of different kinds of rocks. Let us now consider the structure of the earth itself. The earth is composed of three distinct layers, known as the crust, the mantle, and the core. The top layer is the crust, which is like a thin shell around the earth. The crust is composed of different kinds of rocks. Under the oceans and deep below the continents, the crust is made up of dense rocks, such as basalt, but the crust of the continents themselves is made up of lighter rocks, such as granite. The crust is only a few kilometres thick under the oceans, but can be 30 to 90 kilometres thick under the continents. Below the crust, the composition of the earth changes sharply at the point where the next layer, the mantle, begins.
This large layer is almost 3000 kilometres thick, and it makes up about two-thirds of the earth’s mass. The mantle consists of rocky materials such as silicon dioxide, magnesium oxide, and iron oxide. Even though temperatures are very high in the mantle, the rock is under such high pressure that it is unable to melt. However, the upper part of the mantle is almost in liquid form. Beneath the mantle, the core of the earth extends right to the very center of our planet, nearly 6400 kilometres below the surface. The core is itself divided into two parts: the outer part is liquid, and the inner part is solid. Scientists know that the core is at least partly liquid, because earthquake waves, which cannot travel through liquid, are stopped by the earth’s core. The earth’s core is very dense, being composed of iron and nickel. These metals are responsible for the earth’s magnetic field.
Of course, the crust, mantle, and core make up the solid parts of the earth, but we should not forget about the water and air that rest on top of the earth’s surface. About 71% of the earth’s surface is covered by water. A very small part of this water is the fresh water of lakes and rivers, but nearly all of this water is the salt water of the seas and oceans. In every 100 kilograms of sea water, there are about 3.5 kilograms of salt. More than three-quarters of this salt is common table salt, or sodium chloride, but there are also large amounts of magnesium chloride and other salts. Above both the oceans and the continents is the earth’s atmosphere. The air around us is made up mainly of nitrogen (about 78%) and oxygen (about 20%), with smaller amounts of argon, water vapor, carbon dioxide, and other gases.
The atmosphere is thickest at the earth’s surface, and becomes thinner at higher altitudes. Ten kilometres above sea level, the air pressure is only about one-fifth of what is found at sea level itself. At these high altitudes, the air becomes very cold, with temperatures more than fifty degrees below zero. Above these altitudes is found the ozone layer, where ozone molecules, each consisting of three oxygen atoms, protect the earth from the sun’s ultraviolet radiation. Thanks to the work of many scientists, we have now learned a great deal about the composition of the earth. But much more knowledge about our planet remains to be learned by future scientists!