Question

how is the lithosphere formed?​

Answer 1

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Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age.

Answer 2

Some physical problems related to modeling the conditions of the formation and evolution of the lithosphere are discussed. It is shown that if we take into account both the effects of thermal expansion and compressibility we could receive results with no change or even an increase of density under the P-T conditions within the lithosphere. During planetary accretion and differentiation of Earth, the planet could have been entirely molten and at some point of its evolution was entirely covered by a magma ocean. The formation and composition of the early lithosphere were mostly related to processes of the differentiation of matter and the rate of cooling of the magma ocean. The process of the differentiation of the magma ocean would begin during its formation and continue until its solidification. This stratification of the composition of Earth caused an initial state of separation of rocks and minerals into slabs within the upper mantle and crust, which was strictly regulated by their density, whether solid or melted. The difference in density between felsic, intermediate, mafic, and ultramafic magmatic slabs within the magma ocean was enough to prevent the exchange of matter between them, and therefore mantle-wide convection could not have taken place. The solidification of the magma ocean, upon which the process of the formation of the lithosphere is dependent, most likely began with the formation of a forsterite (or forsterite-rich peridotite) slab at a depth of about 100 km followed by the solidification of Earth’s surface, cooled by heat radiation from the surface and the cooling effect of the early atmosphere. It is shown that under the thermal conditions of the magma ocean, carbonate rocks were unstable and decomposed, releasing carbon dioxide into the atmosphere. Water could also not exist in its liquid state at the time of the magma ocean, and together with the carbon dioxide would form a thick and dense early atmosphere.