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Isostasy theories:
1 = Airy, 2 = Pratt
(blocks of the earth's crust or the lithosphere as columns with their density in g / cm³, including the asthenosphere and the earth's mantle)

The Isostasy (, tie ', from Greek. Ἴσος (ISOS), equal', and στάσις (stasis), Booth ') is the geological equilibrium state between the masses of the earth's crust and the underlying mantle .


In the 18th century, the French geodesist Pierre Bouguer determined when taking measurements near the Andes that the expected local deviations in gravity due to the mass of the Andes were less than predicted. He concluded from this that there must be a mass deficit below the Andes.

Explanatory approaches

To explain Bouguer's findings, George Airy and John Henry Pratt independently developed two different models in the mid-19th century. However, neither of these explanatory approaches can fully explain the effect, nowadays more complex models such as that of Felix Andries Vening-Meinesz are used .

Airy model

Airy's model is based on the fact that, according to Archimedes' principle , the brittle lithosphere lies on the comparatively ductile asthenosphere of the upper mantle . This is similar to an iceberg floating in water.

If the vertical mass of the lithosphere increases, for example due to glaciation or mountain formation , the pressure on the asthenosphere increases, causing it to yield and the lithospheric plate to sink deeper. Since the lithosphere has a lower density than the asthenosphere, the density below the mountains also decreases relative to the density below the flatlands .

According to this model, high mountains have a comparatively deep root .

Pratt model

In contrast to Airy, Pratt did not assume that the masses immerse themselves at different depths into the asthenosphere, but that the immersion depths are the same for all masses (flatlands and mountains). For mountains, however, the density of the total mass decreases. This can be compared to a cake that rises, increases in volume, but loses density.

Vening-Meinesz model

The Vening-Meinesz model assumes that the masses of the mountains bend the flexible lithosphere and thus distribute the load on the surroundings without penetrating the asthenosphere.

Isostatic compensation using the example of the Fennoscandian shield

see main article: Postglacial Land Uplift

The effect of isostatic compensation , i.e. the effort to achieve a state of isostasis, can still be observed in Scandinavia today in the form of a lifting process . In the last glacial period , Scandinavia was covered by an ice sheet until 10,000 years ago. Scandinavia was depressed by its mass. This pressure has been absent since the ice receded, and Scandinavia is slowly rising again. The uplift is now 300 m and still takes place at a rate of 9 mm per year in its center (northern Gulf of Bothnia ).

Determination of the Moho discontinuity

If the earth's crust is in isostasis, conclusions can be drawn directly from the geomorphology about the thickness of the crust or the depth of the Mohorovičić discontinuity (Moho): the higher a mountain rises, the thicker the earth's crust below the mountain, and the deeper the Moho lies.


  • Jacobshagen, Arndt, Götze, Mertmann, Wallfass: Introduction to the geological sciences. Eugen Ulmer, Stuttgart 2000, ISBN 3-8001-2743-1

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