This document discusses the geological concept of isostasy, which refers to the equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density. It presents several theories of isostasy, including Airy's theory which proposes that thicker crustal areas sink deeper into the mantle, and Pratt's theory which suggests areas of lower crustal density project higher. The document also discusses isostatic effects from processes like deposition, erosion, and past ice sheets, as well as concepts like phase changes and Heiskanen's modification of Airy's theory.

1. Department of Geology
M.Sc. I Semester
Session 2019-20
Guided By Presented By
Prof . Lalsingh Solanki Ranjana Mujalda
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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
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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.
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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
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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.
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6. Fig. no. 1
Airy’s theory of isostasy
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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
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15. THANK YOU
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