Isostasy - Meaning and different models of isostasy
- 1. Isostasy By Prof. Amandeep Singh Assistant Professor in Geography Contact no. – 88476-82257 Email- akhalsa18@gmail.com 5/6/2023
- 3. 5/6/2023 Isostasy is derived from the Greek language isos (equal ) + stasis ( stand still) Thus isostasy means a state of motion without movement gravitational equilibrium is found between the Earth's crust and the mantle in such a way that the crust floats on the mantle at a certain height and this particular height is judged by the thickness and density of the crust. The continents are light because they are made of light matter the oceanic power is heavy because they are made of heavy matter. This depends on the principle of buoyancy Bounce is the ability of a substance to float in a liquid, which is caused by the fatigue of a substance equal to its own weight.
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- 6. 5/6/2023 • When tectonic plates collide, the crust is pushed up, creating high-elevation features like mountain ranges. • However, the formation of mountains also increases the weight of the crust in those areas, which can cause the crust to sink back down over time. • This sinking process is known as isostatic readjustment. • As the crust sinks, the mantle below it flows in to fill the space, creating a process called subsidence. • This subsidence can cause the mountain range to gradually decrease in elevation over time. • Isostatic readjustment can take millions of years, and it is a key process in the long- term evolution of mountain ranges. Isostasy Adjustment
- 7. 5/6/2023 Isostasy and Mountain building
- 8. 5/6/2023 Isostasy also plays an important role in the movement of glaciers. When glaciers form, they exert a downward force on the crust, causing it to sink down. As the glaciers melt and retreat, the crust rebounds back up, a process known as glacial rebound. This process can be seen in areas that were once covered by glaciers, such as Canada and Scandinavia. These areas are still rebounding today, thousands of years after the glaciers melted away. The principle of isostasy helps us to understand the forces that drive glacial rebound and how it affects the elevation of the Earth’s crust. Isostasy and Glacial Rebound:
- 9. 5/6/2023 Isostasy and Glacial Rebound:
- 10. 5/6/2023 Development of the Isostasy Concept • concept grew out of the attraction of giant mountainous masses. • Pierre Bouguer that the towering volcanic peak of Chimborazo was not attracting the plumb line, as it should have done.He thus maintained that the gravitational attraction of the Andes ‘is much smaller than that to be expected from the mass represented by these mountains’. • Similar discrepancies were noted during the geodetic survey of the Indo- Gangetic plain for the determination of latitudes under the supervision of Sir, George Everest, the then Surveyor General of India in 1859. • The difference of latitude of Kalianpur and Kaliana (370 miles apart) was determined by both the direct triangulation method and the astronomical method.
- 11. 5/6/2023 • Kaliana was only 96 km away from the Himalayas. The difference between two results amounted to 5.23 seconds as given bellow Results obtained through triangulation= 5° 23’ 42.294”. Results obtained through astronomical method = 5° 23’37.058”. Difference= 5.236”
- 12. 5/6/2023 Concept of Sir George Airy • the inner part of the mountains cannot be hollow; rather the excess weight of the mountains is compensated (balanced) by lighter materials below. • According to him the crust of relatively lighter material is floating in the substratum of denser material. • In other words, ‘sial’ is floating over ‘sima’. • In other words, the Himalayas are floating in the denser magma with their maximum portion sunk in the magma in the same way as a boat floats in water with its maximum part sunk in the water. This concept in fact involves the principle of floatation. • the law of floatation demands that ‘the ratio of freeboard to draught is 1 to 9. • He simply maintained that the crustal parts (landmasses) were floating, like a boat, in the magma of the substratum. • if we apply the law of floatation then we have to assume that for the 8,848 meters height of the Himalaya there must be a root, 9 times more in length than the height of the Himalaya, in the substratum. Thus, downward projection of lighter material beneath the mountain reaching a depth of 79,632m
- 13. 5/6/2023 Density of earths crust is uniform Law of Floating
- 14. 5/6/2023 Concept of Archdeacon Pratt • He studied the rocks (and their densities) of the Himalayas and neighboring plains • He found that the density of each higher part is less than a lower part. • In other words, the density of mountains is less than the density of plateau, that of the plateau is less than the density of plain and the density of plain is less than the density of oceanic floor, and so on. • This means that there is an inverse relationship between the height of the reliefs and density. • According to Pratt, there is a level of compensation above which there is variation in the density of different columns of the land but there is no change in density below this level. • Density does not change within one column but it changes from one column to other columns above the level of compensation.
- 15. 5/6/2023 Varying Density Line of Compensation Explanation of concept of Pratt
- 16. 5/6/2023 Concept of Hayford and Bowie • Hayford and Bowie have propounded their concepts of isostasy almost similar to the concept of Pratt. • According to them there is a plane where there is complete compensation of the crustal parts. • Densities vary with elevations of columns of crustal parts above this plane of compensation. • The plane of compensation (level of compensation) is supposedly located at the depth of about 100 km
- 17. 5/6/2023 Concept of Jolly He disapproved the view of Hayford and Bowie about the existence of a level of compensation at the depth of about 100 km on the ground that the temperature at this depth would be so high that it would cause complete liquefaction and thus level of compensation would not be possible According to Joly, there exists a layer of 10-mile (16 km) thickness below a shell of uniform density. The density varies in this zone of 10-mile thickness. It, thus, Joly assumed the level of compensation as not a linear phenomenon but a zonal phenomenon. In other words, he did not believe in a ‘line (level) of compensation’ rather he believed in a ‘zone of compensation’ (of 10-mile thickness). his concept is closer to Airy’s concept rather than the concept of Hayford and Bowie.
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