Lithosphere

The lithosphere is the hard, rocky outer layer of a planet or moon. On Earth, this layer is made up of the crust and the lithospheric mantle, which is the part of the upper mantle that stays elastic over timescales of thousands of years or longer. The crust and upper mantle are separated by differences in their chemistry and mineral makeup.

Earth's lithosphere

Earth’s lithosphere is the planet’s rigid outer vertical shell. It includes the crust and the lithospheric mantle, the uppermost section of the mantle that does not participate in convection. Beneath it lies the asthenosphere, a weaker, hotter, and deeper part of the upper mantle that can flow. The boundary between the lithosphere and asthenosphere is defined by how each layer responds to stress: the lithosphere stays rigid over long geologic periods, deforming elastically or by brittle failure, while the asthenosphere deforms viscously through plastic flow.

Because of this, the lithosphere’s thickness is measured as the depth where the temperature reaches the point at which brittle behavior shifts to viscous behavior. Since olivine is usually the weakest mineral in the upper mantle, the temperature at which it becomes ductile (around 1000°C) is commonly used to mark this boundary. The lithosphere is divided horizontally into tectonic plates, which often include terranes that have been added from other plates.

History of the concept

The idea of the lithosphere as Earth’s strong outer layer was first described by English mathematician A. E. H. Love in his 1911 book “Some Problems of Geodynamics.” American geologist Joseph Barrell later expanded on this in a series of papers, coining the term “lithosphere.” He based the concept on large gravity anomalies over continental crust, which suggested a strong, solid upper layer (the lithosphere) above a weaker, flowing layer (the asthenosphere). Canadian geologist Reginald Aldworth Daly further developed these ideas in his 1940 work “Strength and Structure of the Earth.” These concepts—a strong lithosphere resting on a weak asthenosphere—are now widely accepted and are fundamental to plate tectonics.

Types

The lithosphere is split into two types: oceanic and continental. Oceanic lithosphere is linked to oceanic crust, which has an average density of about 2.9 g/cm³, and is found in ocean basins. Continental lithosphere is linked to continental crust, with an average density of about 2.7 g/cm³, and lies beneath continents and continental shelves.

Oceanic lithosphere is made mostly of mafic crust and ultramafic mantle (peridotite), making it denser than continental lithosphere. At mid-ocean ridges, young oceanic lithosphere is no thicker than the crust, but it thickens as it ages and moves away from the ridge. The oldest oceanic lithosphere is typically about 140 km thick.

This thickening happens through conductive cooling, which turns hot asthenosphere into lithospheric mantle, making the oceanic lithosphere denser and thicker over time. In fact, oceanic lithosphere acts as a thermal boundary layer for mantle convection. The thickness of its mantle part can be approximated as a thermal boundary layer that thickens with the square root of time. For a few tens of millions of years, oceanic lithosphere is less dense than the asthenosphere, but afterward it becomes increasingly denser.

Although chemically differentiated oceanic crust is lighter than the asthenosphere, thermal contraction of the mantle lithosphere makes the whole layer denser. This gravitational instability causes mature oceanic lithosphere to sink beneath overriding lithosphere at subduction zones. New oceanic lithosphere is constantly created at mid-ocean ridges and recycled into the mantle at subduction zones, so it is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of continental lithosphere are billions of years old.

Geophysical studies from the early 21st century suggest that large pieces of lithosphere have been subducted as deep as 2900 km, near the core-mantle boundary, while others float in the upper mantle. Some pieces stick down into the mantle to about 400 km but remain attached to the overlying continental plate, similar to the old concept of a “tectosphere” revisited by Jordan in 1988. Subducting lithosphere stays rigid (as shown by deep earthquakes along Wadati–Benioff zones) down to about 600 km.

Continental lithosphere varies in thickness from about 40 km to perhaps 280 km.