Isostasy: Meaning and Theories

The term "Isostasy" is derived from the Greek word "Isostasios", which means the state of being in balance. (equipoise). This doctrine states that the less dense materials of the Earth surface (SIAL) must float over the denser magma (SIMA) of the Earth's interior.

Historical Background

This phenomenon was initially observed by Pierre Bouguer during his expedition to the Andes (1735-45). He wondered that the mountain has a large mass but, comparatively, has less gravitational attraction. About a century later, similar discrepancies were observed by Sir George Everest (then Surveyor General of India) in the survey of the Great Himalayas. He indicated a lack of compensating mass beneath the visible mountain ranges. During the mid-19th century, the term Isostasy was first proposed by Clarence Dutton, an American Geologist.

Many geographers and scientists have developed hypotheses on the basis of Isostasy. Let us discuss a few major researcher’s concepts.

Concept of George Airy

Sir George Biddell Airy was an English Mathematician and an astronomer.

For his hypothesis, he assumed the following −

• The crust has a uniform density throughout.

• The inner part of the mountain cannot be hallowed; rather the excess weight is compensated for by lighter materials below.

• SIAL (lighter material) is floating over SIMA (relatively denser material).

Thus, the Great Himalayas is floating in the denser magma like a boat floating in water with its maximum part sunk in water.

Principle of floatation

An iceberg floats in water in such a way that every part is above sea level, but nine parts of it remain below water level. i.e., the ratio of freeboard to draught is

He opined that, if the land column was larger, its great part would be submerged on substratum. In other words, "Uniform density with varying thickness" is the central theme of his concept. His theory is known to be a better explanation of mountains within continental regions.

Criticism

If we apply his concept to the Great Himalayas (8848m), there must be 9 times more length (79,632m) in the substratum. It would be impossible since the compensated mass would have been melted away due to high the temperature prevailing there.

Concept of Archdeacon Pratt

John Henry Pratt was an English mathematician who studied the rocks of the Great Himalayas and neighboring plains and found that the density of the higher part is less than that of the lower part. In other words, the density of mountains is less than that of plateaus; the density of plateaus is less than that of the plains, which is less than that of the oceanic floor and so on. This means that there is an inverse relationship between the height of the relief and density.

According to him, there is a line of compensation above which the density of different columns of land varies, but below which there is no such change in density. The central theme is "Uniform depth with varying density".

His theory essentially explains the difference between continents and oceans, since the continental crust is largely of granitic composition, which is less dense than the basaltic ocean basin.

Concept of Hayford and Bowie

They propounded this concept along the lines of Pratt. According to them, there is an inverse relationship between the height of the columns of the crust and their respective densities (as assumed by Pratt) about the line of compensation. They assumed that the plane of compensation is supposedly located at a depth of 100 km. The columns with lower density stand higher than the columns with higher density.

The assumption is that 'the varying volume of matter in the several columns is compensated by their density, in such a fashion that they exert equal downward pressure at the level of compensation and thus balance one another’.

Criticism

Their concept is not tenable since they take relief in the form of vertical columns, but the crustal features are found in the form of horizontal layers.

Concept of Joly

He disapproved of the concept of Hayford and Bowie, noting the following

• The level of compensation cannot be at a depth of 100 km on the ground that the prevailing high temperature would cause liquefaction.

• There would be practical geological disturbances below the level of compensation such that density could not remain uniform below the level of compensation as mentioned by Hayford and Bowie.

According to Joly, there exist a layer of a 10-mile thickness below a shell of uniform density. The density varies in this zone. Thus, he assumed the level of compensation was not linear but a Zonal phenomenon. He believed in the 'Zone of compensation'. It is interpreted that his concept is in close agreement with floatation the idea. Hence, his concept is closer to Airy's concept, than Hayford and Bowie's concept.

Global Isostatic Adjustment

There is no complete isostatic adjustment over the globe because of the fact that the Earth is so unresting due to endogenetic and exogenetic forces. Nature always tends towards isostatic adjustment. For example, a newly formed mountain undergoes severe denudation due to tectonic activity. This leads to the continuous lowering of the height of the mountain, which is balanced by sediments deposited in the oceanic areas. Due to this mechanism, the mountainous areas gradually become lighter and the oceanic floor become heavier.

The following are examples of the phenomenon of Isostatic recovery across the world.

• North America and Eurasia were subsided due to enormous accumulation of ice sheets during Pleistocene glaciation, but the land mass began to uplift suddenly due to deglaciation and thus the isostatic balance was disturbed.

• Glacial adjustment in Scandinavian countries after the Pleistocene great ice age.

• Raised beaches in Finland of about 250m over the last 8000 years under the process of Isostatic recovery.

Conclusion

Understanding the dynamics of Isostasy helps us get knowledge of more complex phenomena such as Mountain building, Sedimentary basin formation, Break-up of continents, Formation of new ocean basins, Glacial adjustment and so on.

Frequently Asked Questions

What is Isostasy?

Isostasy is the mechanical stability between the earth's crust and mantle on the rotating Earth. It maintains an equilibrium between buoyancy force and gravitational force.

What is Principle of floatation?

An iceberg floats in water in such a way that every part is above sea level, but nine parts of it remain below water level. i.e., the ratio of freeboard to draught is 1: 9.

What are the examples of Isostatic recovery across the world?

Some examples of isostatic recovery are −

• North America and Eurasia were subsided due to enormous accumulation of ice sheets during Pleistocene glaciation, but the land mass began to uplift suddenly due to deglaciation and thus the isostatic balance was disturbed.

• Glacial adjustment in Scandinavian countries after the Pleistocene great ice age.

• Raised beaches in Finland of about 250m over the last 8000 years under the process of Isostatic recovery.