The condition that balances the upper and lower parts of the Earth’s crust is called “geoequilibrium” or isostatic condition. The term isostasy was first used by American geologist Dutton in 1889. Due to isostasy, mountains, plateaus, plains, and seas exist side by side. Exogenous forces continuously accumulate soil in the plains and seas by cutting it from the mountains, resulting in more weight on the plains and the bottom of the sea and less weight on the mountains. The pressure of the Earth’s crust on the Earth is not equal due to the lack or excess of load.
Homeostasis helps reduce pressure deficit or excess. The height of mountains on the Earth’s crust decreases due to denudation, and they rise again through equilibrium. In this way, the balance of both mountains, plains, and oceans remains the same. The structure of the Earth and the changes taking place in it can be explained based on the homeostasis theory.
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This principle means that wherever there is equilibrium on the Earth’s surface, there will be a proportional land area below the surface as per the parts raised on the surface. A continent that is, on average, 1600 meters above the mean depth of the sea must be made of a different material than the bottom of the sea, and it will have considerable extension down towards the Earth. Only in this condition can the continent and the ocean remain in permanent equilibrium. The general meaning of balance is “the state of physical or mechanical stability in the raised and lowered areas of the rotating Earth.”
According to J.A. Steers, “Wherever equilibrium is established on the surface of the Earth, there are equal amounts of matter under the same surface areas.”
Arthur Holmes, while explaining land balance, said that “the condition of balance found on the Earth’s surface between mountain ranges, plateaus, plains, or sea bottoms of different heights is called weight or land balance.”
The density of different parts of the Earth is different, as proven by the study of seismic waves, which has been confirmed by research done by the Survey of India. In 1859 AD, two methods were used to find the latitude in the middle of the Ganga-Indus plain: the Plumb method and the Triangulation method. The results obtained by both methods differed significantly. The plumb line was attracted towards the Himalayan Mountains, but it was not attracted as much as it should have been, considering the vastness of the Himalayan site. The attraction of the Himalayas on the plumb line was not for 15 seconds but only for 5 seconds. In 1855, it was proved that the density of the rocks below the Himalayas is not high but is less than the average density of the Earth (2.7).
In short, it can be said that there is an empty or ineffective space within the Himalayan Mountains. It was also proved that the density of the sea floor is higher than the average density of the Earth. The important conclusions drawn from the above-mentioned research are: (1) low-density regions tend to sink downwards, and (2) water is attracted from the part of less density to the part of higher density.
Pratt’s Theory:
According to Ghat, the founder of the homeostatic theory, the homeostatic force is active at all times. Due to erosion on the surface, the load becomes lighter, while deposition in the lower parts of the Earth’s crust keeps increasing, and the soft rocks of the Earth become mobile and move from the denser part towards the region of lower density. As a result, the Earth’s surface sinks in the high-density area and rises in the low-density area. In other words, to maintain the balance between different landforms and to balance the local excess attraction, there must be a low density of the expanded substances below the surface.
Pratt has tried to explain this situation by proposing a theory. They state that the density of the terrestrial pillars is relatively high for those with greater height. Thus, the higher columns are less similar due to their higher density compared to the smaller columns. For this reason, the pressure of the upper and lower parts is the same on the equilibrium line. The lighter parts of the crust can be compared to pieces of zinc and silver, while the heavier parts can be compared to heavier objects such as gold and lead. If these pieces of metals of different weights are allowed to float in a vessel filled with mercury in such a way that they are immersed in the mercury to the same depth, then each piece will have to be smaller or bigger to bring the same weight. The bigger piece would have more mercury than the smaller one.
Many scholars have opposed Pratt’s opinion and have said that the divisions of the Earth’s crust do not show a distribution of metals in separate sections, as in the example of different metals.
Airy’s Theory:
Airy presented his opinion in 1859 AD. According to him, every part of the Earth’s crust is equal, and the higher the part, the same proportion of its sufficient part is entered into the ground, and the less high part, its less part is sunk into the ground. Airy compared the different landforms of the Earth to a floating iceberg. As much as part of the iceberg remains above the water, about eight times its part remains below. According to Archimedes, any floating object displaces a liquid equal to its volume. Nineteen parts of the ice (ice) remain above the water, and the remaining 89 parts are immersed in the water. For every elevated part of the surface, eight times its part lies below the surface of the substratum.
To prove this idea, Airy compared different parts of the Earth’s crust with small pieces of iron metal. If pieces of iron of different dimensions are immersed in mercury, then the larger pieces will be immersed more than the smaller pieces to keep the balance. Airy said that due to the high standing of the mountains, their roots have to be immersed more deeply in the Earth, which keeps them in balance.