ISOSTASY:
Source of
the isostasy concept:
The phenomenon on the surface of the earth changing from the 4.6 billion years ago
at various height layer and the formation and erosion are still continue today.
The erosion happens on the earth’s surface, it may be submerged below the sea
level. Because > 16.5 km² rocks are eroded and eroded material deposited in
the ocean bed. There may be a balance stage if not then it cannot maintain its
upstanding on the surface of the earth.
This
idea pushes the isostasy theory of gravitational equilibrium between the land
and the ocean surface.
Origin:
Geologist C.
E. Dutton has first introduced the word “isostasy” which were originated from
two German “iso” & “stasis” mean “equal standing” or “equipoise” or
equilibrium.
Definition:
Isostasy is
the gravitational equilibrium phase or stability between the earth’s crust and mantle.
The various dense rock or earth’s(landscapes) crust floating into the liquid
asthenosphere. The constructor substances of continental and oceanic crusts
float in same pressure or distress in the specific depth in everywhere at the
level of COMPENSATION (same equilibrium surface level).
Explanation
of isostasy concept:
Through this
concept, we get the basic knowledge about how the different topographical
landscape like mountains, deserts, plains and also the ocean floating into the
asthenosphere(mantle). It is varying the thickness and density of the rocks
which form the landscape. Every year the continental erosion takes place and
these carried through the river channels and deposited into the seas or oceans.
Then the weight of the continental crust will decrease but landscapes are
upstanding. heavy oceanic crust push pressure downward in the asthenosphere, to
balance equilibrium in the compensation level, the materials of the
asthenosphere move toward the continental crust and push pressure upward. This
upward pressure-formed new continental uplift crust. Through this, the isostasy
maintains an equilibrium state between the crusts.
Isostasy
theory of Airy:
The main theme of Airy’s model was “Sial is floating in the Sima”, Like less dense woods
are floating on the much dense water.
1. Airy thought that all the continental the crust is made of same dense materials which average density are 2.7gm/ and these
were light materials.
2. Other hands the oceanic crust was formed with much
denser materials, average density around 3.0gm/ cu which causes the continental
crust were floating on the oceanic crust. Like the ice-blocks are floating on
the water around 9/10 ratio.
Isostasy theory
of Pratt:
J. H. Pratt
introduce his isostasy theory in 1859, Pratt tried to focus on the density of the
different surface blocks (mountain, desert, plane, etc).
1. According to Pratt theory, there is some equilibrium
state or level where all the surface blocks are floating in the same level (level
of compensation).
2. The different
denser blocks have same downward pressure in the much denser mantle or
asthenosphere and upstanding in different vertical height due to different the density of the blocks.
3. The light the dense block will be floating on the highest position and the heavy dense blocks/columns
will float in the lower height near the asthenosphere.
Isostasy theory
of Hayford and Bowie:
Hayford and
Bowie’s theory of isostasy had similarity with Pratt’s isostasy theory. According
to them, there is a line or plane where the complete compensation of crustal the plate is situated around 100 km under the mantle.
- Density varies with the elevation of
the different crustal blocks or columns and
- It has equal downward pressure at
the line of compensation.
- The lighter density crust blocks like
mountain, deserts are standing higher than the heavy density crust blocks like
plane and ocean crust.
Concept of Joly:
Joly introduced
his theory of isostasy contradicting to the Hayford and Bowie theory. Joly rejects
the existence of the zone of compensation, rather than line or level of
compensation.
- According to him if there the line of
compensation lay in the depth of 100 km from the ground then the material will
be liquid due to the temperature, we know that the temperature increases 30°C/km.
- So, Joly assumed that the zone of compensation
may exist a layer of 16kms (10 miles) and the zone is not linear.
- It has a slight glimpse of the law of
flotation (Airy).
- In his theory, the light material
blocks are floated on the heavy materials.
The theory
of Isostasy of Heiskanen:
The Heiskanen the hypothesis is a widely appealing, or deal among Airy’s and Pratt's hypothesis.
- 1 his hypothesis says that around 66%
of the topography is reimbursed by the root course of action (the Airy model)
and 33% by Earth's structure over the point of confinement between the outside
and the substratum (the Pratt model).
- The block of rock of the crust’s columns
are varies both vertically and horizontally on the surface.
Holmes’ the theory was influenced by the Airy’s theory.
- Light dense continental crust block and heavy dense oceanic crust blocks.
- Lighter blocks submerged into the light asthenosphere to balance between the light and heavy materials blocks.
- Light blocks are submerged in the greater depth in the light asthenosphere.
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