Lithosfeer

Article

October 17, 2021

The lithosphere or rock shell is the outermost part of the solid Earth and is about 80 km thick (depending on whether it is oceanic or continental lithosphere). The lithosphere differs from the underlying asthenosphere in that the rock is colder, stronger and more rigid. In addition to the crust, the lithosphere comprises the hard upper part of the mantle.

Composition

The lithosphere consists of the Earth's crust and the upper part of the Earth's mantle. The crust consists of rocks with the average composition of basalt (oceanic crust) or andesite (continental crust). The mantle consists of mantle rock, which has the composition of peridotite. Between the crust and the mantle there is a sudden transition in chemical composition called the Mohorovičić discontinuity (Moho for short). The boundary with the asthenosphere is usually defined by the temperature, where the transition between the two layers can be regarded as an isotherm. Due to the high temperature and pressure, the asthenosphere, in contrast to the lithosphere, only deforms plastically. Heat flow in the asthenosphere therefore also takes place by convection, the lithosphere can only lose its heat by conduction.

Plate tectonics

The lithosphere does not stand still. Due to the phenomenon of plate tectonics, the Earth is constantly changing in appearance. For example, millions of years ago all continents were one; the supercontinent Pangea. New lithosphere is formed at a mid-ocean ridge or a rift. There, two tectonic plates move apart and magma from the Earth's mantle comes into contact with the cool surface. This magma then solidifies into the lithosphere. As a plate moves away from the mid-ocean ridge, it will continue to grow over time. A sediment layer will slowly but surely form on top of the plate. A plate sinks into a subduction zone, where the plate can grow to a thickness of 100 km. In such a subduction zone, the lithosphere disappears into the mantle. Subduction usually does not occur at continental lithosphere. Continental lithosphere moves with the plates, but does not sink when it collides with another plate. In a collision with oceanic lithosphere, the oceanic plate will sink (this is the case in the Andes, for example). When colliding with another continental lithosphere, the plates will eventually be pressed against each other with such a great force that mountain formation takes place.

Flow behavior and deformation

When mechanical stress occurs in a rock, this can lead to deformation. A distinction can be made here between ductile deformation, in which the material deforms plastically without breaking, and brittle deformation, in which a fracture occurs in the material after elastic deformation. The description of the flow behavior of the rocks that make up the lithosphere is also referred to as rheology of the lithosphere.

Strength model for continental lithosphere

In the late 1970s, a simple rheological model for the continental lithosphere was drawn up by Brace and Goetze, among others. This simple model assumes a crust with the properties of quartz, while the underlying mantle is assumed to consist of olivine. Furthermore, a "normal" geothermal gradient is assumed, where the temperature increases almost linearly with depth. In this rheological model a theoretical strength profile of the lithosphere can be calculated, as shown in the adjacent diagram. The left diagram shows the theoretical strength curves as a function of depth. The brittle strength of a rock is mainly dependent on the total pressure caused by the overlying rock. With increasing ambient pressure, and thus with increasing depth, the brittle strength increases linearly, as shown by the black

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