# Objectives, Concept of Isostasy Part-1

In the previous chapter, we have learnt that the interior of the earth is very hot. Earthquakes and volcanoes are concentrated along a few narrow belts. The type and density of rocks of the crust are variable. The surface features are dynamic in character. This dynamism is due to the endogenetic and exogenetic forces. Endogenetic forces are caused from below the surface.

Due to this force, an area may get elevated or gets submerged. Endogenetic forces try to make the surface irregular while exogenetic forces are those which operate from above the surface. They try to eliminate the irregularities of the surface through the process of denudation.

## OBJECTIVES

The major objectives of this chapter are:

• To define isostasy

• To describe the variation in relief features on the earth’s surface

• To explain the views of Airy and Pratt on the isostatic adjustment

• To explain the concept of continental drift

• To enumerate the evidences of continental drift

• To explain the concept of plate tectonics

• To identify and locate different plates on the world map

• To explain the mechanism of plate movement

• To associate earthquakes and volcanoes with plate boundaries

## 3.1 CONCEPT of ISOSTASY

The term ‘Isostasy’ is derived from the Greek word ‘Isostasios’, meaning the state of being in balance. We have seen that the mountains have many peaks and relatively great heights. Similarly, plateau and plain have flat surfaces. They have moderate and lower height, respectively.

On the contrary, oceanic beds and trenches have greater depths. There is a great difference in height among these features. We know that the earth is rotating while keeping perfect balance among its various features. Thus, our earth is considered to be in isostatic equilibrium.

Isostatic Balance: Views of Airy: Airy considered the density of different columns i.e., plains, plateaus, mountains, etc. to be the same. Hence, he proposed the idea of uniform density with varying thickness. We know that the upper crust of the earth is made up of lighter material. In this layer, silica and aluminium are found in abundance, hence it is known as Sial. It is less dense than the lower layer. Airy assumed that the Sialic crust is floating over the Sima (silica and magnesium, the lower denser layer). Crustal layer is uniform in terms of density with varying length of columns. Therefore, those columns are projecting down into the asthenosphere depending upon the proportions of the column. It is due to this reason that the root has developed or the sima has been displaced from below.

To prove this concept, he took an example of wooden blocks of various sizes and immersed them into water. All the blocks are of same density. They get immersed differently in proportion to their sizes. In the same way, the higher features with great height seen on the surface of the earth have deeper roots whereas the features having shorter lengths have shorter roots beneath. It is the concept of root which is sustaining the higher elevation. He is of the opinion that the landmasses are floating like a boat in the substratum or the magmatic asthenosphere. According to this concept, the root beneath the Mt. Everest would be 8848X8 = 70784 metres below the sea level. But on this basis, Airy has been criticized because the root is not possible to be at such a great depth. The root material will melt due to higher temperature found at this great depth.

1. Isostatic Balance: Views of Pratt: Pratt considered the land blocks of various heights to be different in terms of their density. Taller landmass has lesser density and smaller height features are denser. In other words, there is an inverse relationship between the height and density of a landform feature. For a higher column, the density will be lesser and for a shorter column, the density will be higher. He accepted that all blocks of different heights get compensated at a certain depth into the substratum. In this way, a line is being demarcated above which there is an equal pressure with varying heights. He denounced the root concept of Airy and accepted the concept of a level of compensation. For proving this concept, he took a number of metal bars of varying density with same weight and put them into mercury. In this way, they form a line by all those bars, which he regarded to be the level of compensation.

2. Global Isostatic Adjustment: It is quite apparent that there is no complete isostatic balance over the globe. As the earth is unstable, the endogenetic forces often disturb the crustal balance. The earthquakes and volcanic eruptions along a particular belt do not signify any balance, but a sort of adjustment is needed continuously. Endogenetic forces and their tectonic effects are major causes of the imbalance on the surface. But nature always tries to make an isostatic adjustment with itself.

Exogenetic forces are trying to eliminate the differences on the surface of the earth. In this process, they are peeling off, transporting down to far flung places, and depositing them. The isostatic balance is maintained by the underneath flowage of material by subsidence at the place of deposition and upliftment at the peeling off place in their proportion to the denudation.