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Landforms on the earth’s surface are closely related to the internal structure of the earth. Although the study of the internal structure of the Earth is primarily a subject of geology, it is also studied in geography for the analysis of terrestrial landforms. Since the inner part is not visible to humans, information about it was obtained mainly from indirect sources. Scientists have created information about the internal structure of the Earth based on the following sources.
Artificial Sources
1. Density
The average density of the earth is 5.5, while the density of the earth’s crust is about 3.0. Therefore, it is concluded that the average density of the earth’s core (innermost) would be more than 5.5. Generally, the density of the core is 11, which is 7 to 8 times heavier than water. It is clear from this that there will be more density in the inner parts. Various pieces of evidence related to density show that the core of the earth has the highest density.
2. Pressure
Concerning the high density of the core, one can refer to the weight and pressure of the rocks. Increasing pressure does increase density, but each rock has its limit beyond which it cannot become denser, no matter how much the pressure increases. This means that the rocks in the interior are made up of heavy metals with a higher density.
3. Temperature
Generally, the temperature increases by 1 °C for every 32 meters of depth, but the rate of temperature increase also decreases with increasing depth. Up to a depth of 100 km, there is an increase of 12°C per km. After that, up to 300 km, the temperature rises at a rate of 2 °C per km and then 1 °C per km. In tectonically active areas, the temperature is relatively high. There is a constant flow of heat from the interior of the Earth to the outside in the form of thermal convection waves. This is made even more clear by the theory of plate tectonics.
Natural Sources
1. Volcanic action
Based on the study of hot and liquid magma emanating from a volcanic eruption, it can be said that somewhere deep in the earth there must be a layer that is in a liquid or semi-liquid state. This is called a magma chamber. However, no concrete information about the internal structure of the Earth from a volcanic eruption has been found.
2. Evidence from seismology
Seismology is the science in which the study of seismic waves is done by marking them with a seismograph. It is such a direct means that sufficient information is available about the internal structure of the Earth.
3. Evidence from meteorites
Meteoroids are solid structures floating freely in space. Sometimes these bodies collide with the Earth when they enter the Earth’s gravitational field. Due to excessive friction of the atmosphere when falling to the ground, the outer layer of meteorites is destroyed, and the inner part is exposed. The internal structure of the Earth was estimated based on the composition of these heavy substances found in their central part.
Evidence of theories related to the origin of the earth
The planetary hypothesis considers the Earth’s core to be solid, while in the tidal and nebular hypothesis it is liquid. Thus, there are only two possibilities regarding the interior of the earth, that either it can be solid or liquid.
Earth’s chemical structure and different layers
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Based on the research of the International Union of Geodesy and Geophysics (IUGG), the interior of the Earth has been divided into three major concentric zone or layers, which are as follows-
1. Crust
The IUGG considers its average thickness to be 30 km. Although according to other sources, the thickness of the crust is 100 km. Based on the difference in the speed of seismic waves, the earth’s crust is divided into two subdivisions – Upper crust and Lower crust. According to the IUGG, the speed of the “P” wave in the upper part of the crust is 6.1 km per second and 6.9 km per second in the lower part. The average density of the upper crust is 2.7 and 3.0 for the lower crust. This difference in density is believed to be due to pressure. The discontinuity in density between the upper and lower crust is called the Conard discontinuity. The crust is mainly composed of silica and aluminum. Therefore, it is also called the Sial layer.
2. Mantle
In the lower part of the crust, there is a sudden increase in the speed of seismic waves, and it increases from 7.9 to 8.1 km per second. A discontinuity is formed between the lower crust and the upper mantle, indicating a change in rock density. This discontinuity was discovered in 1909 AD by the Russian scientist A.K. Mohor Vicic (A. Mohor Vicic). That is why it is also called Moho-Discontinuity. The mantle extends to a depth of 2,900 km from the Moho-discontinuity. Its volume is about 83% of Earth’s total volume and about 68% of its mass. The mantle is composed mainly of silicon dioxide and magnesium. That is why it is also called the SiMa layer.
The mantle is again divided into three parts by IUGG based on the speed of seismic waves-
- From Moho discontinuity to 200 km.
- 200 km. To 700 km
- 700 km to 2,900 km.
The speed of seismic waves slows down (7.8 km per second) at a depth of 100 to 200 km in the upper mantle. That is why this part is also called the low-velocity zone. This density discontinuity between the upper mantle and the lower mantle is called the Repeti discontinuity.
The asthenosphere is also called a magma chamber because it supplies magma to the volcanoes that formed on Earth. The lithosphere (3.09 g/cm3) floats on top of the asthenosphere because the density of the asthenosphere is 4.5 g/cm3. It is also called the low-velocity zone. This is where the deceleration in seismic waves occurs.
3. Core
In the lower mantle, there is a sudden change in the “P” wave, and it increases to 13.6 km. in a second. It refers to a sudden change in rock density that causes a discontinuity called the Gutenberg-Wishart discontinuity. Apart from Gutenberg – discontinuity to 6,371 km. is called the Core. It is also divided into two parts – 2,900 to 5,150 km. and 5,150-6,371 km. These are called the outer and innermost cores respectively. The density-related discontinuity found between the outer and innermost core is called the Lehmann discontinuity. The core has a density of 10 in the uppermost part, which increases to 12 to 13 in the interior and 13.6 in the innermost parts. The density of the core is therefore more than twice the density of the mantle.
Since there is relatively more fluid, the speed of P waves becomes 11.23 km. per a second. Although the core should remain in a molten state due to the high temperature, but it remains in a semi-liquid or plastic state due to the high pressure. The volume of the core is only 16% of the entire earth, but its mass is about 32% of the total mass of the earth. Although there is some silicon in the inner parts of the core, it is made of nickel and iron. Therefore, it is also called the NiFe layer. Currently, the amount of silicon in the outer crust is 20 percent, and the amount of iron and nickel is 80 percent.
Table 1 General Elements in Earth’s Crust
| Elements | Weight (in Per cent) |
| Oxygen (O) | 46.60 |
| Silicon (Si) | 27.72 |
| Aluminum (Al) | 8.13 |
| Iron (Fe) | 5.00 |
| Calcium (Ca) | 3.63 |
| Sodium (Na) | 2.83 |
| Potassium (K) | 2.59 |
| Magnesium (Mg) | 2.09 |
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