Solid Earth

3.2 Formation of the Lithosphere

Video: Formation of the Lithosphere

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

Photo: Lherzolite (mantle rock), Hokkaido, Japan, 2009

Chapter 3.2

Formation of the Lithosphere

Fig. 3.2.1: Non-exaggerated section through the upper mantle and lithosphere (Meschede, unpubl., 2021)

The formation of oceanic crust occurs at the constantly opening fissures in a spreading zone. Fig. 3.2.1 shows the upper mantle in the area of ​​the spreading zone without exaggeration. In order to be able to view the processes taking place at the spreading center, the area marked here with a red frame must be exaggerated.

Fig. 3.2.2: Strongly exaggerated section through the lithosphere and asthenosphere at a spreading zone (Meschede, unpubl., 2021; modified and completed after Frisch & Meschede, 2021)

The formation of oceanic crust occurs in the uppermost kilometers, where a crust with an average thickness of 5 to 8 km is formed. At the opening gap, the oceanic crust is built up to its final thickness and composition. In Fig. 3.2.2 you can see that the thickness of the crust no longer changes towards the sides. This is one of the most important differences to the formation of the lithospheric mantle.

The lithospheric mantle has the same material structure as the asthenosphere; the only difference is the partial melting of the asthenosphere, which makes it flowable. The two small illustrations on the right (Fig. 3.2.2) indicate how you can imagine partial melting. The rectangles show highly enlarged sections of the asthenosphere at the points where the small rectangles in the asthenosphere can be seen beneath the spreading center. The edge length of such a rectangle would be a few millimeters or centimeters. The light areas consist of molten rock, while the darker areas are in a crystalline state and solid.

The amount of partial melting depends on the depth at which the asthenosphere is located. The pressure decreases upwards because the overlay becomes smaller. This decrease in pressure plays a crucial role because it contributes significantly to the fact that the melt proportion increases significantly towards the top. This is called a decompression melt. The melting process is caused solely by the pressure relief, the decompression, when the mantle material rises; no additional heat or other processes that support melting are required. In some cases, the partial melting rate at shallow depths in the area directly beneath the spreading zone can increase to over 25%. These partial melts ultimately feed the magma chambers beneath the spreading zone, from which the oceanic crust is then formed.

The lithospheric mantle forms beneath the oceanic crust not from the magma chamber from which the oceanic crust is formed. As the oceanic crust drifts away from the spreading center, the oceanic crust and the underlying asthenosphere gradually cool. However, when it cools, the asthenosphere turns into lithospheric mantle, because when the molten parts are crystallized, the rock is solid and is then part of the lithospheric mantle. As a result of this cooling, the lithospheric mantle continuously thickens downward the further it drifts away from the spreading center. That is why the lower boundary of the lithosphere is not as sharp as the boundary between the crust and mantle, where there is a clear change in material. The cooling process initially occurs relatively quickly, but then slows down and the thickening of the lithosphere continues over a long period of time, which is assumed to be around 80 to 100 million years under the oceanic crust. After that, there is no longer any noticeable thickening.

Fig. 3.2.3: Age structure  of the lithosphere (Meschede, unpubl., 2021)

The age structure of the lithosphere (Fig. 3.2.3) shows the difference between oceanic crust getting older away from the spreading center, while the lithospheric mantle ages upward and simultaneously to the side.