Fig. 2.2.1: The currently valid model for crustal thickness in equilibrium with the asthenosphere. The compensation level is the lower edge of the picture (adopted from Meschede, 2021). Numerical values: density g/cm³.

Isostasy is one of the basic principles of plate tectonics. The lithosphere is in equilibrium with the underlying asthenosphere, which is disturbed by plate tectonic movements and other external influences such as erosion. This leads to isostatic compensatory movements.

For the conceptual model discussed here, the following average density values ​​are used: 3.0 g/cm³ for the oceanic crust, 2.8 g/cm³ for the continental crust, 3.3 g/cm³ for the lithospheric mantle and 3.25 g/cm³ for the asthenosphere, which is slightly lighter than the lithospheric mantle due to its higher temperature.

Starting position (Fig. 2.2.2):

A column of 200 km height has a base area of ​​1 cm². This column is composed of 30 km of continental crust, with 30 km being the average thickness of continental crust. Below is the lithospheric mantle with a thickness of 70 km. The asthenosphere follows down to the compensation level with 100 km. Depending on the respective densities, there are different weights for the individual column sections: 8,400 kg for the continental crust, 23,100 kg for the lithospheric mantle and 32,500 kg for the asthenosphere. Together, this results in a total weight of the column of 64,000 kg, which is placed on the compensation level. This value is the equilibrium value that must be achieved again and again in this coneptual model for all different columns.

Scenario 1 (Fig. 2.2.3):

The continental crust is doubled, as it can happen when two continents collide. The lithospheric mantle is pulled into the mantle and therefore does not contribute to the thickening.

By doubling the continental crust, the sum of the 200 km long column changes and becomes smaller because the crust has a lower density than the asthenosphere. With a total length of 200 km, 1,350 kg are missing from the balance of 64,000 kg and this missing portion must be compensated by the flowable asthenosphere. At a density of 3.25 g/cm³, 1,350 kg corresponds to a column of asthenospheric material of 4.154 km height. The amount of just over 4 km corresponds in magnitude to the actual peak heights in mountains such as the Alps. In the Himalayas it is significantly more, but this is also a result of isostatic compensation. The topic of “erosion and protrusion” is discussed separately in a later video.

Scenario 2 (Fig. 2.2.4):

The asthenosphere extends to the lower limit of the continental crust. This is what happens when a continent breaks up and a rift system develops with a graben structure, such as in the East African Rift. Since the asthenosphere is slightly lighter than the lithospheric mantle, the column under the rift system also becomes lighter overall in this scenario, i.e. here too a portion has to be compensated from below by the flowable asthenosphere. Here too the result is uplift with an amount of 1.077 km, which corresponds to a weight of 350 kg of asthenosphere.

This scenario is also realized in nature with similar uplift amounts of 1-2 km. A bulge initially forms over a future rift valley, in the middle of which a graben collapses. This graben is also known as the median rift graben.

Scenario 3 (Fig. 2.2.5):

The oceanic crust is significantly heavier than the continental crust, but it is also much thinner. In addition, the water column must also be taken into account, which is significantly lighter. For the entire column, with the same thickness of the lithospheric mantle, a significantly higher column of asthenosphere must be assumed in order to get back to the 64,000 kg total weight above the compensation level.