The document discusses the geological concept of isostasy. Isostasy refers to the principle of buoyancy where land masses float on the denser underlying mantle material. It explains that mountains create indentations in the earth's crust similar to placing a heavy object on a rubber ball. It also describes early theories on isostasy from Clarence Dutton, who coined the term, and Sir George Airy, who proposed that land masses float with varying thickness but uniform density. The concept was later refined by A. Pratt to propose uniform depth but varying density between land masses.

1. • If you were to take a large rubber ball and
place something heavy on top of it, such as
a bowling ball, what do you suppose would
happen to the rubber ball?
• The weight of the bowling ball will push the
skin of the rubber ball inward, creating a
dent. Scientists observe the same
phenomena with the Earth’s crust. When the
Earth’s crust is pushed down, creating a
small dent, we refer to this as isostasy.

3. • In the simplest example, isostasy is the principle of
buoyancy (In physics, buoyancy is a force exerted
by a fluid that opposes an object's weight). In a
column of fluid, pressure increases with depth as a
result of the weight of the overlying fluid. where an
object immersed in a liquid is buoyed with a force
equal to the weight of the displaced liquid.
• On a geological scale, isostasy can be observed
where the Earth's strong lithosphere exerts stress
on the weaker asthenosphere which, over
geological time flows laterally such that the load of
the lithosphere is accommodated by height
adjustments.
• The general term 'isostasy' was coined in 1889 by
the American geologist Clarence Dutton.

5. • Isostasy simply means a mechanical stability between the
upstanding parts and lowlying basins on a rotating earth.
• The state of balance which he (Dutton) thought must exist
between large upstanding areas of the earth’s surface,
mountain ranges and plateaus, and contiguous lowlands.
• The concept of Sir George Airy
• According to Airy, the crust of relatively lighter material is
floating in the substratum of denser material- sial is
floating in sima.
• According to him, the great mass of Himalayas was not
only a surface phenomenon: the lighter rocks of which
they are composed do not merely rest on a level surface
of denser material beneath, but, as boat in water, sink into
the denser material.
• The Himalayas are floating in the denser magma with their
maximum portion sunk in the magma in the same way as
boat floats in water with its maximum part sunk in water.

7. • Law of floatation. 1:9 –For 8848 m height of the Himalaya there must be a
root, 9 times more in length than the height of the Himalaya, in the
substratum. Thus, for 8848 m part of the Himalaya above, there must be
downward projection of lighter material beneath the mountain reaching a
depth of 79,632 m.
• If the large column above the substratum is larger, its greater part would
be submerged in the substratum and if the land column is lower, its
smaller part would be submerged in the substratum.
• Uniform density with varying thickness. This means that the continents are
made of rocks having density but their thickness or length varies from
place to place.
• It would be wrong to assume that the Himalaya would have a downward
projection of root of lighter material beneath the mountain reaching such a
great depth of 80,000 m…if accepted, would melt due to very high
temperature prevailing there, as temperature increases with depth at the
rate of 1degree C per 32 m.
• The concept of A. Pratt
• He studied the rocks and their densities of the Himalaya and neighbouring
plains and found that the density of each higher part is less than a lower
part. The density of mountain is less than the density of plateaus, that of
plateau is less than the density of plain and the density of plain is less
than the density of oceanic floor and so on.

8. • Oceanic crust (sima-Si+Mg), (depth of 0-10 kilometres)
the floor of the deep oceans, is thin, about 7km thick, and made of relatively
dense rocks like basalt. average density is between 2.9 and 4.7.
• Continental crust (sial-Si+Al) (depth of 0-75 kilometres)
is much thicker, averaging 33km, and is composed of relatively light material
such as granite. Average density is 2.9.

9. • According to him, there is a level of compensation above
which there is variation in the density of different columns
of 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.
• Uniform depth with varying density.
• There is inverse relationship between the height of the
reliefs and their respective densities.—bigger the column,
lesser the density and smaller the column, greater the
density.
• Law of compensation.
• The fundamental difference between Airy’s and Pratt:
• Uniform density with varying thickness and Uniform depth
with varying density

10. • According to him, there is a level of compensation above
which there is variation in the density of different columns
of 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.
• Uniform depth with varying density.
• There is inverse relationship between the height of the
reliefs and their respective densities.—bigger the column,
lesser the density and smaller the column, greater the
density.
• Law of compensation.
• The fundamental difference between Airy’s and Pratt:
• Uniform density with varying thickness and Uniform depth
with varying density