Isostasy Theory of Aren Diacen Pratt (A. D. Pratt)

The theory of Isostasy is based on the idea that there is a balance between the high mountains and low plateaus and plains of the earth. Geologists have presented this theory of Isostasy in various ways. Geologist G. B. Airy presented a Isostasy theory about the earth's equilibrium. In 1859, geologist Aren Diacen Pratt (A. D. Pratt) presented another theory of Isostasy against this theory given by Airy. The theory of Isostasy presented by A. D. Pratt mensioned below:

A. D. Pratt's theory: According to geologist A. D. Pratt, the density of all parts of the earth is not equal. Pratt states in his theory that all parts of the Earth's solid surface are floating on the same plane above the fluid substratum inside the Earth. However, the surfaces of the earth's solid ground are located at different heights and have created equilibrium conditions. Pratt also points out that the parts of the Earth's solid surface that have the lowest density are the parts that elevated more above the Earth's surface. On the other hand, the densest parts of the Earth's solid surface are the least elevated parts. This concept presented by A. D. Pratt is also called Level of Compensation.

Fig-1: According to A. D. Pratt, Mountains are higher because they are made of lighter material than Plains.

Experiment and proof of A. D. Pratt's theory: A. D. Pratt expressed his theory by experimenting with many blocks in a container full of mercury. Pratt proved in experiments that the blocks had different sizes. But the lower part of these blocks rested on the same level in the mercury container. Based on this evidence, Pratt opined that plains and mountains are composed of the same material. He pointed out that the lower part of the blocks is on the same level and the upper part of them is different in height due to the difference in sizes. The authenticity of Pratt's concept can be proved by the following two experiments.

1st Experiment: If different parts of the earth (such as different parts from plains, mountains, plateaus and the seabed) are cut and placed on a fluid substratum, then these parts will float as shown in figure-2 below. The bottoms of all parts will lie on the same level. However, due to variation in density, their surfaces will rise to different heights and create equilibrium conditions. The conditions for this experiment are –

(1) If the cross-sections and weights of these parts are equal,

(2) If their densities are not equal and

(3) If their heights are different.

Fig-2: A. D. Pratt's First experiment on Isostasy as per Fig-1.

2nd Experiment: If many blocks (P1, P2 and P3) are placed in a container full of mercury, these blocks will float as shown in figure-3 below. The bottoms of all blocks will lie on the same level. The conditions for this experiment are –

(1) If the cross-sections and weights of these blocks are equal,

(2) If their densities are not equal and

(3) If their heights are different.

Fig-3: A. D. Pratt's Second experiment on Isostasy as per Fig-1.

As can be seen in Figure 3, the blocks (P1, P2 and P3) of equal cross-section and weight are positioned on the same level in the fluid and are balanced at different heights above. The blocks removed the same amount of fluid. So, the lower the density of the block, the higher its height at the top.

Presented by Md. Shahin Alam