Isostasy, Isostatic Adjustment and Methods of Isostatic Adjustment (Geology, Geophysics)

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Last Updated on February 13, 2023 by Okoro Daniel O.

Isostasy is the balance or equilibrium of adjacent blocks of the lithosphere (earth crust) resting, or moving on the upper mantle.

The crustal rocks are less in weight than the mantle rocks, therefore, they rest on the denser mantle in the form of rocks floating on top of the water.

Geologically, the lithosphere gives a balance with mantle rocks. This balance is called ISOSTATIC ADJUSTMENT.

Hypothetically, Isostatic Adjustment is the balance between lithosphere and mantle rocks.

Isostatic Adjustment is very important in the study of Geology (especially in Geophysics – where Earth’s interior and processes that formed it are analyzed using methods of Physics).

Methods of Isostatic Adjustment

Isostatic Adjustment can be done in three (3) ways;

  1. The Crustal Rebound Theory.
  2. Adjustment due to erosion and deposition of sediments.
  3. Subsident due to weight and heavy uplift in a subduction zone.


This refers to the rise of the crust after the removal of ice. In this method of Isostatic Adjustment (the Crustal Rebound Theory), the earth crust is viewed as an elastic ellipsoid which can expand, and as it expands, it tends to form a balance with the surface and the underlying crustal rocks.
This kind of Isostatic Adjustment occurs most in the subduction zone.

Ellipsoid“: (meaning) A surface used as a model of the shape of the earth. Shaped like an ellipse; elliptical.


This is the gradual wearing away of high density areas and the penultimate, or the subsequent deposition of such into the lowland area, causing a gradual balance of isostasy.
It is necessary to state that in this kind of Isostatic Adjustment, natural process of erosion process of erosion to cause a balance between the lithosphere and the mantle.

Penultimate“: (meaning) Next to last, second to last; immediately preceding the end of a sequence, list, etc. OR; Relating to or denoting an element of a related collection of curves that is arbitrarily close to a degenerate form.


This is an Isostatic balance which is caused by tectonic processes which induces subsidence of high density areas with a subsequent uplift. This, therefore, causes a balance between the lithosphere and the mantle.

Tectonics (or Plate Tectonics)“: (meaning) Is the chemical composition of the crust and the lithosphere.

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