Introduction
Though the study of constitution of the interior of the earth is out side the domain of geography but its elementary knowledge is necessary for the geographers because the nature and configuration of the reliefs of the earth's surface largely depend on the nature, mechanism and magnitude of the endogenetic forces which originate from within the earth. It is decidedly true that it is very difficult task to have accurate knowledge of the constitution of the earth's interior because it is beyond the range of direct observation by man but recently seismology has helped to have some authenticated knowledge about the mystery of the earth's interior. The sources which provide knowledge about the interior of the earth may be classified into 3 groups.
- Artificial sources
- Evidences from the theories of the origin of the earth
- Natural sources
e.g. volcanic eruption, earthquakes and seismology.
Artificial Sources
Numerous inferences can be drawn about the constitution of the interior of the earth on the basis of density of rocks, pressure of superincumbent load (weight of overlying rocks) and increasing trend of temperature with increasing depth inside the earth.
It is commonly believed that the outer thinner part of the earth is composed of sedimentary rocks the thickness of which ranges between half a mile to one mile (0.8 km to 1.6 km.) Just below this sedimentary layer there is the second layer of crystalline rocks, the density of which ranges between 3.0 and 3.5 at different places. The average density of the whole earth is about 5.5. Thus, it appears that the density of the core of the earth will be, wihtout doubt, more than 5.5. Generally, the density of the core of the earth is around 11.0. Cavendish attempted to calculate the average density of the earth in 1798 on the basis of the Newton's gravitational law. According to him the average density of the earth is 5.48.
Poynting calculated the average density of the earth as 5.49 g cm in the year 1878. Since 1950 several attempts are being made to calculate the density of the earth on the basis of satellites. The satellite studies have revealed the following results about the density of various parts of the earth-average density of the earth = 5.517 g cm (-cube) average density of the earth’s surface = 2.6 to 3.3 g cm (-cube) , the average density of earth’s core = 11g cm (-cube)
Pressure
Now question arises, what is the reason for very high density of the core? Previously it was believed that very high density of the core was because of heavy pressure of overlaying rocks. It is common principle that pressure increases the density of rocks. Since the weight and pressure of rocks increase with increasing depth and hence the density of rocks also increases with increasing depth. Thus, it is proved that (ii) very high density of the core of the earth is due to very high pressure prevailing there because of superincumbent load. This inference is proved wrong on the ground that there is a criticial limit in each rock beyond which the density of that rock cannot be increased inspite of increasing pressure therein. It may be, thus, forwarded that (iii) very high density of the core of the earth is not because of very high pressure prevailing there. If the high density of the core of the earth is not because of high pressure of overlying rocks then (iv) the core must be composed of intrinsically heavy metallic materials of high density. The experiments have revealed that the core of the earth is made of the mixture of iron and nickel. This inference is also validated on the basis of geocentric magnetic field.The metallic core is surrounded by a zone of such rock materials, the upper part of which is composed of crystalline rocks.
Temperature
It is evident on the basis of information available from the findings of bore holes and deep mining that temperature increases from the surface of the earth downward at the rate of 2° to 3°C for 100 metres. It may be pointed out that it becomes very difficult to find out the rate of increase of temperature beyond the depth of 8 km. The rate of increase of temperature in the continental crust has been calculated based on geothermal graphs and the following generalization has been made. In the tectonically active areas (like the Basin and Range Province of the USA) temperature remains 1000'C at the depth of 43 km from the surface of the earth while the temperature remains only 500°C at the depth of 40 km from the surface in tectonically stable areas. This information provides significant knowledge about the nature and behaviour of the continental crust. It is evident that high temperature of 1000°C at the depth of 43 km in the tectonically active areas is nearer to the initial melting point of the rocks of lower crust and mantle mainly basalt and peridotite.
This fact explains the situation of high temperature in the continental crust as described above because disintegration and decay of radioactive minerals generate more heat in the crustal areas. It, thus, appears that the rate of increase of temperature downwards decreases with increasing depth. The following facts may be presented about the thermal condition of the interior of the earth
- The asthenosphere is partially molten. The temperature is around 1100°C at the depth of 100 km which is nearer to initial melting point.
- The temperature at the depths of 400 km and 700 km (from the earth's surface) has been estimated to be 1,500°C and 1,900°C respectively.
- The temperature at the junction of mantle and outer molten core standing at the depth of 2,900 km is about 3700°C.
- The temperature at the junction of outer molten core and inner solid core standing at the depth of 5,100 km is 4,300°C
Evidences from the theories of the origin of the earth
Various exponents of different hypotheses and theories of the origin of the earth have assumed the original form of the earth to be solid or liquid or gaseous. According to the 'Planetesimal Hypothesis' the earth was originated due to accretion and aggregation of solid dust particles known as ‘planetesimals'. Based on this corollary the core of the earth should be in solid state. According to the ‘Tidal Hypothesis' the core of the earth should be in liquid state because the earth has been taken to have been formed, according to this hypothesis, from the tidal materials ejected from the primitive sun. Read More about the origin of the earth . According to the 'Nabular Hypothesis' of Laplace the core of the earth should be in liquid state. Zoeppritz and Ritter have opined that the core of the earth is made of gases but this concept may not be accepted because if we assume the core of the earth in gaseous state many more problems will emerge. There may be only two possibilities viz. either the core may be in solid state or liquid state. This problem would be dealt with while dealing with the evidences of seis-mology.
Important Definitions
Barysphere: represents the innermost zone of the interior of the earth and extends from 2800 km. depth to the nucleus of the core. The average density ranges between 8 and 11.
Core: is the deepest and most inaccessible zone of the interior of the earth and extends from the lower boundary of the mantle at the depth of 2900 km. to the center of the earth i.e. upto 6371 km. The density increases from 10 at the mantle-core boundary downword to 11.6 at the center of the core.The core has two subzones i.e. outer core (from 2900 km. to 5150 km. depth) and inner core (from 5150 to 6371 km. depth).
Crust: is the outermost layer (zone) of the earth with average density of 2.8 to 30 and average thickness of 30 km.
Density : refers to the amount of mass per unit volume of substance, usually measured in gram per cubic centimeter (g/cm3)
Lithosphere : literally means rocksphere (lithos means rock) which represents the solid portion of the continents having a thickness or about 100 km. and average density of 3.5. It is composed of mostly silicate minerals.