Solid Earth

2.1 Isostasy

Video: Isostasy

(To play the video, please click on the image above)

Photo: Iceberg in the South Atlantic, East Scotia Sea, RV Polarstern Expedition PS119, 2019

Chapter 2.1


Isostasy - basic principle of plate tectonics

Isostasy is one of the basic principles of plate tectonics that should be understood when dealing with these processes. We assume that the lithosphere is in equilibrium with the underlying asthenosphere. And this balance is disturbed by plate tectonic movements and other external influences, so that compensatory movements occur, which ultimately help shape the Earth’s surface. This is then called isostatic compensation.

Isostasy - water-wooden block model

Wood floats in water because it is a little lighter than water, and the amount of water displaced by the wooden block corresponds exactly to the weight of the entire wooden block – Archimedes already knew this, who discovered the specific weight, so to speak. When such a wooden block floats in water, it is in equilibrium. This means that the water displaced by the wooden block has the same weight as the wooden block including the part that protrudes above the water surface (Fig. 2.1.1).

If a second wooden block of the same size is stacked on top of the first, the picture changes. The larger block of wood sticks out higher, but it also goes deeper into the water. The balance with the water is then restored. The first wooden block is 6 cm thick and protrudes 1 cm from the water, correspondingly it is immersed by 5 cm. Stacked on top of each other, two blocks are immersed in the water by 10 cm and protrude 2 cm. This reflects the isostatic compensation, because the balance is restored by immersing deeper and at the same time protruding higher. The up and down movement required for this is called isostatic compensation. In this model, the movement is of course very fast because water flows very easily and can be displaced. However, geological periods of time, i.e. tens of thousands to millions of years, are necessary for the balance between the lithosphere and asthenosphere.

Equilibrium at the compensation level

Fig. 2.1.2: Columnar illustration of the wooden block model above the compensation level (Meschede, unpubl., 2021).

The columns show the three situations seen in the model: water alone, the thin wooden block 6 cm thick and the water remaining underneath and, correspondingly, the 12 cm thick wooden block with the water underneath (Fig. 2.1.2). All three columns have the same weight, although they are different in height. This reflects the lower specific weight of the wood. The columns all exert the same pressure on the compensation level, which in this model corresponds to the bottom of the water tank.

Isostasy at different densities

Regarding the lithosphere and uppermost mantle, we consider the asthenosphere to be a fluid mass analogous to water in the wooden block model. The two blocks, shown in different colors in Fig. 2.1.3, represent continental and oceanic crust with their different densities. The density of the flowable asthenosphere is greater than that of the blocks of lithosphere (= lithospheric mantle and crust – continental or oceanic). The density of a material is expressed using the Greek letter Rho (ρ).

Oceanic crust is generally heavier than continental crust. Nevertheless, there is the same pressure everywhere on the isostatic compensation level, regardless of what material it is and how high the column is above the compensation level. By the way, it doesn’t matter where the compensation surface is in the flowable mass, you could also set it lower or higher. Equilibrium would exist everywhere because the fluid mass compensates for changes. The compensation level must therefore not go through rigid areas that are not flowable, because then compensation can no longer take place.

Models explaining isostasy

The principle of isostasy was already described in the 19th century to explain the different heights of landscapes on Earth (Fig. 2.1.4). John Pratt (1855) assumed that continental crust has different densities, but that they all lie on a common subsurface – corresponding to the isostatic compensation level. George Airy (1855), on the other hand, assumed that the continental crust has the same density everywhere and, according to the water-wooden block model, penetrates the mantle to different depths.

Felix Vening- Meinesz (1931) combined the two isostasy models and explained the different altitudes with regional compensation. The oceans are still missing in his explanation because there was no reliable data on them at the beginning of the 20th century.

The actual model for isostasy of the lithosphere

The lower part of the lithospheric mantle is made up of the same rock as the asthenosphere. The only difference between them is that the asthenosphere contains a small amount of molten rock – a content of 2-5% is assumed at the lower boundary of the lithosphere, but still so much that it is no longer solid, but plastically deformable and therefore flowable. This means that the conditions for isostatic compensation of the lithosphere are met, because similar to the water in the wooden block model, the asthenosphere can flow or escape – just much more slowly.

The lithosphere with lithospheric mantle and crust is solid throughout and not as hot as the asthenophere. As a result, the asthenosphere is slightly less dense than the lithosphere (Fig. 2.1.5). This phenomenon, for example, can be used to conclusively explain the sinking of the oceanic lithosphere with increasing age. This will be discussed in a separate video/website.

In terms of its material composition, the lithospheric mantle belongs to the upper mantle. However, due to its strength, it reacts in a similar way to the Earth’s crust and plays an important role in plate tectonic movements. Because of these physical properties, it can also be counted as part of the lithosphere, i.e. the outer solid shell of the Earth.