Concept of isostatic adjustment and isostatic models
- 1. Department of Geology M.Sc. I Semester Session 2019-20 Guided By Presented By Prof . Lalsingh Solanki Ranjana Mujalda 01-12-2021 1
- 2. CONTENT Introduction Airy’s theory Pratt’s Theory Heiskanen’s Theory Phase Change Isostatic Effect Of Deposition And Erosion Isostatic Effect Of Ice Sheets Denudation And Isotastic 01-12-2021 2
- 3. Introduction Isostasy (Greek ísos "equal", stasis "standstill") is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust“ floats" at an elevation that depends on its thickness and density. This concept is invoked to explain how different topographic heights can exist at Earth's surface. When a certain area of Earth's crust reaches the state of isostasy it is said to be in isostatic equilibrium. Isostasy does not upset equilibrium but instead restores it (a negative feedback). It is generally accepted that Earth is a dynamic system that responds to loads in many different ways. 01-12-2021 3
- 4. However, isostasy provides an important 'view' of the processes that are happening in areas that are experiencing vertical movement. Certain areas (such as the Himalayas) are not in isostatic equilibrium, which has forced researchers to identify other reasons to explain their topographic heights (in the case of the Himalayas, which are still rising, by proposing that their elevation is being supported by the force of the impacting Indian Plate the Basin and Range Province of the Western US is another example of a region not in isostatic equilibrium.) Although originally defined in terms of continental crust and mantle, it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes such as the Hawaiian Islands. In the simplest example, isostasy is the principle of buoyancy wherein an object immersed in a fluid is buoyed with a force equal to the weight of the displaced fluid. On a geological scale, isostasy can be observed where Earth's strong crust or lithosphere exerts stress on the weaker mantle or asthenosphere, which, over geological time, flows laterally such that the load is accommodated by height adjustments. The general term 'isostasy' was coined in 1882 by the American geologist 01-12-2021 4
- 5. Airy’s Theory The Airy hypothesis says that Earth’s crust is a more rigid shell floating on a more liquid substratum of greater density. Sir George Biddell Airy, an English mathematician and astronomer, assumed that the crust has a uniform density throughout. The thickness of the crustal layer is not uniform, however, and so this theory supposes that the thicker parts of the crust sink deeper into the substratum, while the thinner parts are buoyed up by it. According to this hypothesis, mountains have roots below the surface that are much larger than their surface expression. This is anologous to an iceberg floating on water, in which the greater part of the iceberg is underwater. 01-12-2021 5
- 6. Fig. no. 1 Airy’s theory of isostasy 01-12-2021 6
- 7. Pratt’s Theory The Pratt hypothesis, developed by John Henry Pratt, English mathematician and Anglican missionary, supposes that Earth’s crust has a uniform thickness below sea level with its base everywhere supporting an equal weight per unit area at a depth of compensation. In essence, this says that areas of the Earth of lesser density, such as mountain ranges, project higher above sea level than do those of greater density. The explanation for this was that the mountains resulted from the upward expansion of locally heated crustal material, which had a larger volume but a lower density after it had cooled. Fig. no. 2 01-12-2021 7
- 8. Geographical Comparison Between Airy and Pratt Airy Pratt Sial is floating on sima. There is uniform density for all the columns The columns are of varying depth The line of compensation is a jig saw line because of different depth of columns Sial is sinking in sima The different material are all of varying density. The depths of columns are uniform. Line of compensation is as straight line. 01-12-2021 8
- 9. Heiskanen’s theory Heiskanen, the French scientist, contributed significantly to the theory of isostasy. This scientist modified Airy’s concept and showed variation of density within the sialic columns themselves. He accepted ‘the root concept’ of Airy, but he also showed that density may vary horizontally from column to column. In other words, there is horizontal as well as vertical variation in the density of rocks of the earth’s crust. This scientist for the first time made an extensive application of Airy’s concept and called his modified concept as Heiskanen hypothesis. 01-12-2021 9
- 10. Phase Change The altera phasenative explanaation of vertical movement is phase change. Some material when subject to considerable pressure, change their atomic and crystal structure and compact themselves into a smaller space i.e they undergo phase change olivine, which forms a large propotion of mantle material, behaves in this way .thus,at a certain level in the mantle.where ceitical pressure is reached. A phase change occurs and a phase change boundary can be drawn in. 01-12-2021 10
- 11. Isostatic Effects Of Deposition And Erosion The deposition of sediments causes the Earth's crust to sink because sediments have more than twice the density of water and ice. Crustal isostatic adjustment to an increased sedimentary load occurs onshore below local base levels in lakes and reservoirs, offshore on and adjacent to the continental shelf and in shallow oceanic areas affected by volcanism and the growth of coral reefs. Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate. Therefore, as a mountain range is eroded, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata eroded away and the lower layers rebounded upwards. 01-12-2021 11
- 12. Isostatic Effect Of Ice Sheets The formation of ice sheets can cause 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 is 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 metres 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. 01-12-2021 12
- 13. Isostasy And Regional Landscapes Regional landscapes are shaped to a large extend by various earth movements. Isostasy determines the relative upliftment and subsidence of regional surfaces. Isostasy is active on continental plains, where erosion by wind,water,ice removes material, and adjustment is required; The weight of dams require crustale equilibrium. The weight of damned up water can cause isostatic sinking of the crust below it. 01-12-2021 13
- 14. Denudation and isostasy Isostatic rebound can be calculated thus: Since the rate of denudation is functionally related to relief and the latter is reduced through time, it is possible to calculate ,on the basis of the above relationship ,that after 11 month .year . landmass would be reduced to 10% 0f its origional relief and after 22 month.year .it should be reduced to 1% as the rate of denudation declines both with time and relief . H=Br/A H=isostatic compensation, B=specific gravity of surface rocks removed, A=specific gravity of material replacing at depth. R=thickness of surface layer removed . Assuming A=3.4 and B=3.6 then H=0.76 r 01-12-2021 14