Isostasy: The Gravitational Equilibrium | Principal Hypothesis of Isostasy

Isostasy is the state of gravitational equilibrium between Earth’s crust and mantle such that the lighter crust “floats” at an elevation that depends on the thickness and density of the underlying layer.

The term “Isostasy” was coined by the American geologist Clarence Dutton in 1889. It is derived from the Greek word “Isostasios”, meaning the state of being in balance.

Isostasy

The key concept of isostasy is based on Archimedes’ principle where buoyancy and gravitational force balance each other.

Consider that buoyancy force that pushes a low-density mountain upward must be balanced by the gravitational force that pulls it downward. It is the balancing act of the laws of science that restores equilibrium (negative feedback) and explains how different topographic heights like mountains, plateaus, ridges can exist on the earth’s surface.

It is same as an iceberg floating in the water. On the accumulation of more ice the iceberg sinks more in water, but if some of the ice melts or gets removed from the iceberg, then the remaining iceberg will rise.

In addition to the vertical movement of the land and sea, isostatic adjustment of the Earth also involves horizontal movements. It may even cause changes in Earth’s gravitational field and rotation rate, polar wander, and earthquakes.

Why does the lithosphere float on top of the partially molten asthenosphere?

At depths between 96 and 200 kilometres below the surface, the rocks exist in the plastic state. Here pressures due to elevated masses and depressed areas are equal. This zone is called the zone of compensation.

The Three Principal Hypothesis of Isostasy

In the 19th century, while carrying out a trigonometrical survey, it was found that the deflection of the plumb line towards the Himalayas was not proportional to the gravitational attraction of the mass of mountain range.

The value was much less than the estimated value. As a result in 1855, both Airy and Pratt proposed that the continents consisted of lighter material floating on a denser substratum.

  • Airy’s hypothesis

In this model, the crust has a uniform density but varying depth up to which the root or the crustal material reaches. The different topographic heights are accommodated by changes in crustal thickness; therefore deeper roots are present below mountains and smaller roots below plains.

Pascal’s law was the basis of this hypothesis, within a fluid in static equilibrium, the hydrostatic pressure is the same on every point at the same elevation (surface of hydrostatic compensation). In other words:

h1⋅ρ1 = h2⋅ρ2 = h3⋅ρ3 = … hn⋅ρn

  • Pratt’s hypothesis

The density of crustal material varies therefore different topographic heights are accommodated by lateral changes in rock density. No root formation takes place, but there is only a level of compensation. Today Airy’s hypothesis is generally accepted to be a better explanation of less dense mountains having deep roots extending into the mantle.

The thickness of sial under plains near sea level is only about 30 kilometre. The existence of such roots has been confirmed by gravitational and seismic data, whereas Pratt’s essentially explains the difference between the lighter granitic continental crust and the denser basaltic oceanic crust.

where the lithosphere acts as an elastic plate and its inherent rigidity distributes local topographic loads over a broad region by bending. Airy and Pratt isostasy are statements of buoyancy, whereas flexural isostasy is a statement of buoyancy when deflecting a sheet of finite elastic strength.

Heiskanen hypothesis, by Weikko Aleksanteri Heiskanen, is an intermediate hypothesis between Airy’s and Pratt’s. It says that approximately two-thirds of the topography is compensated by the root formation ( Airy’s hypothesis) and one-third by Earth’s crust above the boundary between the crust and the substratum (Pratt hypothesis).

It helped in explaining how large topographic loads like the Hawaiian Islands could be compensated by regional rather than local displacement of the lithosphere.

Evidence of Isostasy

The addition and removal of weight by waxing and waning of ice sheets, erosion, sedimentation, and extrusive volcanism effect isostasy. Where sedimentation occurs, the weight of the sediment may cause the crust below to sink.

Similarly, where erosion occurs, the crust may rebind. Isostasy also effects complex phenomena like mountain building, sedimentary basin formation, the break-up of continents and the formation of new ocean basins.

Parts of Norway and Sweden which were covered by thick Pleistocene ice sheets are now “rebounding” upwards due to the melting of ice as the load on the lithosphere is reduced. Erosion of mountaintops results in more uplift to maintain the isostatic equilibrium. The asthenosphere rises, and isostatic equilibrium leads to more mountain building in that region. This effect can be best observed in the late Cenozoic uplift of the Sierra Nevada in California.

Are the Himalayas still rising due to Isostatic Equilibrium?

After much research, it has been found that certain areas like the Himalayas and the Basin and Range Province of the Western US are not in isostatic equilibrium.

The increasing topographic heights of the Himalayas, are supported by the continued collision of the impacting Indian Plate and the Eurasian Plate.

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