Regional
Institute of
Education,
Bhopal
Geography
assignment
“Biodiversity”
Submitted to : dr. Soyhunlo Sebu
Submitted by : Jaya Nehru
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The document explains what isostasy is, it origin , causes and its implications in our day to day life
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Regional
Institute of
Education,
Bhopal
Geography
assignment
“Biodiversity”
Submitted to : dr. Soyhunlo Sebu Submitted by : Jaya Nehru
: B.A.B.Ed. iv sem.
∆ What is Isostasy?
Isostasy is the state of gravitational equilibrium between Earth's crust (lithosphere) and mantle (asthenosphere) such that the crust "floats" at an elevation that depends on its thickness and density. The word isostasy is derived from a Greek word isostasios meaning ‘in equipoise’ or 'in balance'. It was first coined by the American geologist Clarence Dutton in 1859 and later confirmed by Pratt. The concept of isostasy is based on the principle of buoyancy first outlined by Archimedes. Buoyancy is the ability of an object to float in a fluid by displacing a
Huge plates of crustal material (lithosphere) “float” on denser, plastically flowing rocks of the asthenosphere. The “depth” to which an object sinks depends on its weight and varies as the weight changes. This equilibrium or balance between blocks of crust and the underlying mantle is called isostasy. The heavier a block of crust is (such as a mountainous region), the deeper it penetrates into the mantle because of its greater mass and weight. Isostasy occurs when each block settles into equilibrium with the underlying mantle.
∆ Origin of the idea of Isostasy
The first hint of isostasy was obtained during middle of the eighteenth century, when the length of a degree of latitude at the equator was measured. This exercise was part of a major project carried out to determine the shape of the Earth. Initially, it was believed that the
Earth is a perfect sphere. But subsequent experiments suggested that it could be a prolate or oblate spheroid. If the Earth was a prolate spheroid, a degree of latitude would be shorter at poles than at equator. But for an oblate spheroid, a degree of latitude would be greater at poles. The initial experiments conducted in Peru (now Ecuador) in South America showed that the Earth is oblate. During these investigations, latitudes measured based on the methods like triangulation method and astronomical method showed a difference. This observation was explained as an effect of Newtonian gravitational attraction between the plumb bob and the Andes mountain. On closer examination, this explanation didn’t quite work as the calculated deflection of the plumb bob was too large. In other Words, the calculated Newtonian attraction of the mountains was too large. So during the expedition to Andes in 1755, Pierry Bouguer ,a French photometrist and the first to measure the horizontal gravitational pull of mountains, found that the peak of Chimborazo was not attracting the plumb line as much as it should have. He then realized that the gravitational attraction
Astronomical method result = 5° 23’ 37.058” Difference = 5.236” Thus, the biggest question that resulted in the postulation of the concept of isostasy was “ Why these mountains are not exerting force according to their size? ” Various geologists gave various explanations as given below : The Himalayas are hollow and are composed of bubbles, not rocks. Due to this fact, the weight and density of the Himalayas would be low and thus their gravitational force would also below. This explanation cannot be accepted because such a high mountain if composed of bubbles, cannot stand on the earth’s surface. The rocks of the Himalayas are of low density and thus their attraction is also low. Many theories were also postulated to explain the reason behind this , one of which is the Airy's theory of Isostasy.
∆ Airy’s Theory of Isostasy
George B. Airy (1855), the Astronomer Royal, presented a paper arguing that the discrepancy could be explained if the excess weight of the mountains is balanced by the lighter material below. According to Sir Airy the crust composed of lighter material is floating in the substratum of denser material i.e. SIAL is ,floating over SIMA. The Himalayas are floating in the denser magma with their maximum portion sunk in the magma in the same way as the boat floats in water with its maximum part sunk in the water. If we assume the average density of the crust and the substratum to be 2.67 and 3.0, for every one part of the crust to remain above the substratum, then 9 part of the crust must be in the substratum. In another words, the law of floatation demands ‘the ratio of free board to draught is 1:. Uniform Density Varying Depth
And due to this reason , on oceanic column, the value of g >9.8 (positive gravitational anomaly) , on plains g =9.8 ,on plateaus g <9.8 and on mountains g <<9. (negative gravitational anomaly). The concept of gravitational anomalies also helped us to know the presence of oil in the oceans. Oil is found in dome like structures in the ocean floor .As we know that oil is lighter than SIAL ,the amount of SIMA displaced by these domes is even lesser than the surrounding SIAL. As a result the value of g will be much greater than 9.8 above this oil dome.
∆ Isostatic effects
Ice sheets – Isostatic Effects of Ice Sheets
The formation of ice sheets can cause the Earth’s surface to sink. Conversely, isostatic post-glacial rebound is observed in areas once covered by ice sheets that have now melted, such as around the Baltic Sea and Hudson Bay. As the ice retreats, the load on the lithosphere and asthenosphere gets reduced and they rebound back towards their equilibrium levels. In this way, it is possible to find former sea cliffs and associated wave-cut platforms hundreds of meters above present-day sea level. The rebound movements are so slow that the uplift caused by the ending of the last glacial period is still continuing.
Rising of Himalayas
Due to continuous weathering and erosion over a long period of time ,the weight of the crust becomes less .With less weight, the continental crust makes an ‘isostatic adjustment’ causing it to rise vertically (float higher) in the mantle due to the principle of buoyancy. But the roots are in the contact of the viscous SIMA which do not allow them to rise until
∆ Limitation of Airy’s theory of
Isostasy
According to Airy, every upstanding part must have a root below in accordance with its height in the ratio of 1:9. Then the Himalayas would have 8848 X 9 = 79,632 meters deep roots. It would be wrong to assume that the Himalayas would have a downward projection of the root of lighter material reaching such a great depth. Because even if accepted such a long root would melt due to the very high temperature prevailing there. The temperature increases with increasing depth at the rate of 1- degree centigrade per 32 meters. So at this depth, the temperature would be 2700 degrees Celsius, more than the temperature at the surface.
∆ Bibliography
- Physical geography by Majid Hussain https://www.britannica.com/science/isostasy- geology http://egyankosh.ac.in/bitstream/ 123456789/57473/1/Unit4.pdf