Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere. The age is often equal to L/V, where L is the distance from the spreading centre of mid-oceanic ridge, and V is velocity of the lithospheric plate. H ∼ 2 κ t is the age of the given part of the lithosphere. These concepts of a strong lithosphere resting on a weak asthenosphere are essential to the theory of plate tectonics. These ideas were expanded by Reginald Aldworth Daly in 1940 with his seminal work 'Strength and Structure of the Earth.' They have been broadly accepted by geologists and geophysicists. The concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must exist a strong, solid upper layer (which he called the lithosphere) above a weaker layer which could flow (which he called the asthenosphere). Love in his 1911 monograph 'Some problems of Geodynamics' and further developed by Joseph Barrell, who wrote a series of papers about the concept and introduced the term 'lithosphere'.

The concept of the lithosphere as Earth's strong outer layer was described by A.E.H. The lithosphere is subdivided horizontally into tectonic plates, which often include terranes accreted from other plates. The temperature at which olivine becomes ductile (~1,000 ☌ or 1,830 ☏) is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle. The thickness of the lithosphere is thus considered to be the depth to the isotherm associated with the transition between brittle and viscous behavior. The Lithosphere-Asthenosphere boundary is defined by a difference in response to stress: the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation.

The lithosphere is underlain by the asthenosphere which is the weaker, hotter, and deeper part of the upper mantle.

Earth's lithosphere, which constitutes the hard and rigid outer vertical layer of the Earth, includes the crust and the uppermost mantle.

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