Solid Earth

1.2 Structure of the Lithosphere

Video: Structure of the Lithosphere

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Photo: Contact peridotite-gabbro at the fossil Moho, Oman ophiolite, Wadi Al Abyad, Oman, 2015

Chapter 1.2

Structure of the Lithosphere

We call the Earth’s outer solid shell the lithosphere. Compared to the Earth’s radius, which is almost 6,400 km, it is only a very thin layer, averaging 100 km (Fig. 1.2.1). That’s less than 2%, or two hundredths of the Earth’s radius. But this lithosphere protects us from the hot and fiery magma that pushes up from the Earth’s mantle.

Without exaggeration, it is difficult to see the lithosphere and to recognize its structure.

Fig. 1.2.3: Exaggerated section through the upper asthenosphere and the lithosphere (Meschede, unpubl., 2021)

There are two different types of lithosphere: continental and oceanic lithosphere.
Continental lithosphere consists of continental crust and the so-called lithospheric mantle, oceanic lithosphere consists of oceanic crust and also lithospheric mantle. The lithospheric mantle is the same for both, but it can become thicker under continental crust than under oceanic crust.
Both the continental and oceanic lithosphere are solid throughout and this is an important prerequisite for plate tectonics. Only the asthenosphere, which lies beneath the lithosphere, contains melted rock material. However, temperatures well above 1000°C prevail here, conditions in which rock melts can occur. This is why the asthenosphere is soft and flowable, which is a prerequisite for plate tectonics to function.

Fig. 1.2.4: Examples of rocks from the Earth’s crust and the lithospheric mantle (photos: Meschede).

b) Gabbro (oceanic), Wadi Al Abyad, Oman

Oceanic crust is mainly composed of basaltic rocks. The basaltic rocks include superficially extruded basalts, a volcanic rock that partially contains volcanic glass, and gabbros that have solidified at depth. In terms of composition, basalt and gabbro are the same, except that gabbro solidifies at depth and crystallizes there, forming a coarse-grained rock. Gabbro consists primarily of the minerals feldspar, pyroxene and olivine.

d) Sandstone (continental), Triassic (Buntsandstein), Rhineland-Palatinate, Germany

A typical rock of the continental crust is granite, which, like gabbro, solidifies at depth and also forms a coarse-grained rock (Fig. 1.2.4c). Granite consists largely of feldspar, quartz and mica. It occurs very frequently in continental crust, but almost not at all in oceanic crust. In the upper crustal areas, sediments and sedimentary rocks are typical (Fig. 1.2.4d). A sandstone is shown here as a representative, but there are many other rocks, such as limestone or mudstone, etc.

The lithospheric mantle is made up of rocks called peridotites and which primarily contain olivine as a mineral. The asthenosphere is made up of exactly the same rocks. Unlike the lithospheric mantle, which is completely solid, the asthenosphere contains molten rock. This means that it is in a different physical state, which is why it does not belong to the lithosphere. The lithospheric mantle is, therefore, on the one hand, part of the Earth’s mantle because it is not materially different from it, but at the same time it is part of the lithosphere, which is divided into many lithospheric plates spread over the entire Earth. These lithospheric plates are the plates we look at in plate tectonics, which can move on the soft asthenosphere.

The 3D model of the lithospheric plates of South America and Africa shows that the plates contain both continental and oceanic lithosphere. The South American and African plates each contain half of the Atlantic Ocean. There are also plates composed primarily of oceanic lithosphere, such as the Pacific Plate, but all plates, including the Pacific, contain both continental and oceanic lithosphere. The boundary of a plate does not depend on whether it is oceanic or continental lithosphere.

Structure of the Lithosphere