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  • Institute of Lifelong Learning , University of Delhi

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    DISCIPLINE COURSE -1

    SEMESTER -1

    Paper: 101: GEOMORPHOLOGY

    Unit 1: Geomorphology and Internal Structure of

    the Earth

    1.3: Isostasy

    Lesson Developer:

    Dr. Prabuddh Kr. Mishra

    (Bhim Rao Ambedkar College)

    University of Delhi, Delhi

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    Table of Contents

    1.3: Isostasy

    1.3.1 Introduction

    1.3.2 Discovery of the concept

    1.3.3 The concept of Sit George Airy

    1.3.4 The concept of Archdeacon Pratt

    1.3.5 Conclusion

    Summery

    Glossary

    References

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    1.3.1 Introduction

    Isostasy simply means a mechanical stability between the upstanding parts and the low-

    lying basins on a rotating earth. Isostasy has been derived from a German word isostasios

    (meaning in equipoise in a state of balance). American geologist Dutton proposed the principle

    of isostasy in the paper On Some of the Greater Problems of Physical Geology (1892). He was

    the first person to propose and express his view to indicate the state of balance which he

    thought must have exist between large upstanding areas of the earths surface, mountain

    ranges, plateaus and contiguous lowlands, etc. According to Dutton, the upstanding parts of

    the earths surface (mountains, plateaus, plains and ocean basins) must be compensated by

    lighter rock materials from beneath so that the crustal reliefs should remain in mechanical

    stability. According to J.A. Steers (1961) this doctrine states that whatever equilibrium exists

    on the earths surface, equal mass must underlie equal surface area.Isostatic equilibrium is an

    ideal state where the crust and mantle would settle into in absence of disturbing forces. The

    waxing and waning of ice sheets, erosion, sedimentation, and extrusive volcanism are examples

    of processes that perturb isostasy.

    1.3.2 Discovery of the Concept

    Though the concept of isostacy came much later in the mid 19th century, however, its

    concept grew out of gradual thinking in terms of gravitational attraction of giant mountainous

    masses. Pierre Bouguer during his expedition of the Andes in 1735 found that the towering

    volcanic peak of Chimborazo (one of the ranges of Andes) was not attracting the plumb line as

    it should have done. He thus maintained that the gravitational attraction of Andes is much

    smaller than that to be expected from the mass represented from these mountains. Similar

    discrepancies were noted during the geodetic survey of the Indo-gangetic plain for the

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    determination of latitudes under the supervision of Sir George Everest, the then surveyor

    general of India in 1859. The debate on the discrepancies of the gravitational deflections of the

    plumb line and numerous discrepancies resulted into the postulation of the concept of isostacy.

    There are two important hypothesis proposed to explain the concept of Isostasy by Sir George

    Airy and Archdeacon Pratt. There are other concepts of Isostasy given by Hayford and Bowie

    and their concepts were almost similar to Pratt.

    Click on the link to watch the Video on Concept of Isostasy

    http://www.youtube.com/watch?v=G9rN7qhUQZg

    1.3.3 The concept of Sir George Airy (hypothesis proposed in 1855)

    According to Airy, the inner parts of the mountains are not hollow; rather the excess

    weight is compensated by the lighter materials below. According to him the crust of relatively

    lighter material is floating in the substratum of dense material. Therefore, the continents which

    are made of lighter sial are floating over the sub-stratum which is built of the denser sima.Thus,

    Himalaya is also floating in the denser glassy magma. He suggested that the lighter sial of the

    Himalaya is floating over the denser material of sima lying underneath. Not only this, but the

    Himalaya is floating in the denser magma with their maximum portion sunk in magma in the

    same way as a boat floats in water with its maximum parts sunk in the water.

    This concept in fact involves the principle of floatation. For example, an iceberg floats

    in water in such a way that for every one part to be above water-level nine parts has to be

    below the water. If it is assumed the average density of the crust and substratum to be 2.67

    and 3.00 respectively, then, for every one part of the crust to remain above the substratum,

    nine parts of the crust must be in the substratum. In other words, law of floatation demands

    that the ratio of freeboard to drought is 1 to 9. If the law of floatation is applied as stated

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    above, in the case of the concept of Airy, then we have to assume that for the 8,848 meters

    height of the Himalaya, there must be a root, 9 times more in length than the height of the

    Himalaya, the substratum. Thus, for 8,848 meters part of the Himalaya above, there must be

    downward projection of lighter material beneath the mountain reaching the depth of roughly

    80,000 meters.

    Thus, according to Airy, Himalaya was exerting their real attraction force because their

    existed a long root of lighter materials in the substratum which compensate the material above.

    Based on the above observation Airy postulated that if the land column above the substratum

    is larger, its greater part would be submerged in the substratum and if the land cover is lower,

    its smaller part would be submerged in the substratum. A per Airy, the density of different

    column does not change with depth, i.e., uniform density with varying depth (Fig.1).

    He showed that the landmasses could be conceived as wooden flocks of different height

    floating in water. These blocks are said to be in the state of hydrostatic balance or equilibrium.

    Similarly, land masses of similar density of rocks, penetrate deep into the substratum in

    proportion to their height. To prove the concept he took several pieces of copper of varying

    length and put them in a basin full of mercury. These pieces of copper sunk upto varying depths

    depending on their lengths. The longest block will float with greatest amount rising above the

    level of mercury surface and the shortest block has upper surface lowest. With all block now

    floating at rest, it is obvious that block rising highest also extends to grater depths.

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    Fig.1 Airys Concept of Isostasy (Image Source: Wikipedia)

    This concept of Airy also suffers from certain defects and errors. If we accept the Airys concept

    of isostasy then, every upstanding part must have a root below with its height. That means

    Himalayas have the root equivalent to 79,632 meters, which is not possible. It is wrong to

    assume that Himalaya have a downward projection of lighter material beneath the mountain,

    which will have such a great depth. Moreover, such a long root would melt due to very high

    temperature prevailing at such great depth. Though there is an error on Airys concept the

    geologists favour Airys explanation the most than others.

    1.3.4 The concept of Archdeacon Pratt (hypothesis proposed in 1859)

    According to Pratt the gravitational attraction of the Himalayas was less than the mass

    represented by these mountains because they are made up of much lighter materials, i.e. rocks

    of much lower density. According to Pratt, there is a level of compensation above which there is

    a variation in the densities of different columns of land but there is no change in density below

    that level. Thus, the central theme of the concept of Pratt on Isostasy may be expressed as

    Uniform depth with varying densities. According to Pratt equal surface area must underlie

    equal mass along the line of compensation. This statement may be explained with the help of

    an example.

    There are two coloumns A and B, along the line of compensation. Both the coloumns, A

    & B, have equal surface area but there is difference in height (B is lower in height than A). Both

    the coloumns must have equal mass along the line of compensation. So the density of coloumn

    B is more than the density of Coloumn A so that weight of both the coloumns becomes equal

    along the line of compensation. So Pratts concept of inverse relationship between the height of

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    different coloumns and their respective densities may be expressed in the following manner