We have just discussed the inner structure and composition of earth which consists of three layers : the top layer called “crust” being about 35 to 60 kilometres thick; the middle layer called “mantle” being about 2900 kilometres thick; and the third layer “core” being about 3400 kilometres in radius. Now, the deepest well drilled into the earth so far is around 13 kilometres deep which is in the Kola peninsula in USSR. Since it is not possible to dig thousands of kilometres into the earth to study its structure and composition, therefore, an important question now arises : How do scientists study the structure and composition of inner parts of the earth, without actually reaching these inner parts ? We shall now try to answer this question.
The scientists study the structure and composition of the inner parts of earth by indirect methods. The two important methods which are used for this purpose are : (i) study of waves produced during the earthquakes on earth, and (ii) study of composition of meteorites which fall on the earth. We shall discuss both these methods one by one.
Study of Earthquake Waves :
One of the most important methods to find out the structure and composition of inner parts of earth is to study the waves produced by earthquakes. Earthquakes are caused by fracturing and slipping of rocks inside the earth. When earthquakes occur, they shake the whole body of the earth and hence produce waves or vibrations. The waves produced by earthquakes pass through the various regions of earth and emerge at its surface with valuable information about the physical properties of the inner regions of the earth through which they pass.
The strength and speed of the earthquake waves depend on the nature, density, and physical state (solid or liquid) of the regions of earth through which they pass. The recording and analysis of the earthquake waves or vibrations is done at various “seismic stations” which have been established in different places of the world. The study of earthquake waves tells us that as we go down the earth, the density increases gradually and reaches its maximum value in the core. The study of earthquake vibrations also tells us that the core of the earth is mainly made up of a liquid (which we know to be molten iron).
Study of Composition of Meteorites :
Another idea about the composition of earth comes from a study of the composition of meteorites. Meteorites are the chunks of matter which some time fall to the earth from outer space, and are supposed to be made up of the same matter as that of other planets (including earth). There are two types of meteorites : one made of silicate stones and the other made up of iron. Since the earth is supposed to have the same composition as meteorites, therefore, we conclude that just as the meteorites are made up of two types of materials, silicate stones and iron, in the same way, the earth is made up of same two materials : the mantle being made up of silicate rocks and the core of earth made up of iron.
In fact, since the meteorites are supposed to contain different elements in the same proportion as the planets, so a study of the composition of meteorites actually tells us indirectly as to how much of each element should be present on earth. Knowing the composition of surface rocks of the earth (which can be found out by analysis), and the complete composition of whole earth (as disclosed by meteorites), we can easily find out the composition of deep, inaccessible regions of the earth.