Isostasy

The isostasy (, tie ', from Greek ἴσος (ISOs ), equal to', and στάσις ( stasis ), stand ' ) is the geological state of equilibrium between the masses of the earth's crust and the mantle underneath.

History

In the 18th century the French surveyor Pierre Bouguer found in measurements near the Andes, that the expected, turned out local variations in gravity by the mass of the Andes is less than predicted. He concluded that there had to be below the Andes a mass deficit.

Explanations

To explain the findings Bouguers, George Airy and John Henry Pratt independently developed mid-19th century, two different models. These explanations can see the effect but both not entirely explain nowadays approach uses complex models back as the Felix Andries Vening - Meinesz.

Airy model

Airy's model based on the fact that the brittle lithosphere lies on the relatively ductile asthenosphere of the upper mantle according to the Archimedes principle. This is similar to an iceberg floating in the water. Increases the vertical mass of the lithosphere, for example by glaciation or mountain formation, its pressure rises to the asthenosphere, which gives this and the lithospheric plate sinks deeper. Since the lithosphere but a smaller density than the asthenosphere, the density drops below the mountain relative to the density below the plains.

This model by high mountains thus have a relatively deep root.

Pratt model

Unlike Airy Pratt did not think that the masses of different dive deep into the asthenosphere, but that the immersion depth for all masses ( flat land like mountains) are the same. But for the mountains, the density of the total mass decreases. This compares with a cake that rises, thus increasing in volume, but less dense.

Vening - Meinesz model

The Vening - Meinesz model assumes that the mass of the mountains bend the flexible lithosphere, and so distribute the applied load on the environment, but without penetrating the asthenosphere.

Isostatic compensation on the example of the Fennoscandian Shield

Main article: Postglacial land uplift

To observe the effect of isostatic adjustment, so the effort to reach a state of isostasy, even today in Scandinavia in the form of an enhancement process. Scandinavia was covered in the last ice age until 10,000 years ago by an ice sheet. Through this mass Scandinavia was depressed. Since then, the ice has declined, lacks this pressure, and Scandinavia slowly rises again. The elevation is now a total of 300 m and still takes place at its center (northern Gulf of Bothnia ) at a rate of 9 mm per year.

Determination of the Moho discontinuity

The Earth's crust is in isostatic, then from the geomorphology directly on the thickness of the crust, or the depth of Mohorovičić discontinuity ( Moho ) are closed, the higher it rises a mountain range, the more powerful the earth's crust under the mountains and the more deeper is the Moho.

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