lithosphere Plate tectonics relies on the concept of a rigid lithosphere. Although the term ‘crust’ was used originally in geology to denote such a rigid layer, the modern concept of the lithosphere generally includes the crust and the uppermost part of the upper mantle (Fig. 1). Whereas the base of the crust is essentially a compositional boundary between mafic and ultramafic rocks, characterized by a rapid increase in seismic P-wave velocity to more than 8 km s⁻¹ (the Mohorovicicć discontinuity), the lithosphere is defined rheologically as the strong, elastic layer at the surface of the Earth, overlying the weak, ductile asthenosphere. The base of the lithosphere is marked by ťhe transition from brittle to ductile deformation, and is probably gradational. The lithosphere, as defined in this way, is sometimes referred to as a ‘mechanical boundary layer’.

Because the lithosphere is rigid, it cannot convect as the deeper mantle does. Heat therefore passes through it mainly by conduction, which is a less effective mechanism than convection. In consequence, the geothermal gradient in the lithosphere is considerably higher than that in the rest of the mantle, and there is a large temperature difference across it. One can therefore also think of the lithosphere as a thermal boundary layer.

There are several ways of estimating the thickness of the lithosphere. Because it is a rheological boundary, and rheological properties depend, inter alia, on strain rate, the thickness also depends on strain rate.

One of the most common ways of estimating lithospheric thickness is to use seismology. Since this depends on the elastic deformation of rocks at frequencies of about 0.01–1 Hz, it is effectively a very high strain-rate method. Seismology shows that the velocity of seismic waves generally increases with depth in the upper mantle. However, in most places there is a small decrease in velocity starting at depths of around 100 km, which is thought to mark the transition from lithosphere to asthenosphere. The thickness of the lithosphere defined on this basis varies between different geological provinces. It is greatest under the cratonic centres of continents, where it may possibly reach around 300 km, and it is least, probably about 4 km, under mid-ocean ridges where new lithosphere is formed.

Lithospheric thickness generally increases with age since its last major tectonic reactivation. The lithosphere–asthenosphere transition should roughly follow the isotherm corresponding to the brittle–ductile transition. For oceanic lithosphere the depths of isotherms are determined by conductive cooling since its creation at a mid-ocean ridge. These depths can be computed and are approximately proportional to the square root of the age, as therefore is the lithospheric thickness. The seismically defined oceanic lithosphere increases in thickness from just a few kilometres at zero age to about 100 km at ages of over 100 Ma (million years). These depths correspond approximately to the 1000 °C isotherm.

Another method of estimating lithospheric thickness is by measuring how much it flexes under applied loads. Geophysicists have shown that the lithosphere can be modelled successfully as a thin elastic plate overlying a viscous substratum. The magnitude and wavelength of the flexure depend on the applied load and on the thickness of the plate. A variety of loads can be modelled, including sea mounts, ocean islands, subducting slabs, and the isostatic forces acting on the dipping boundaries of fault blocks. Such studies also show an increase of lithospheric thickness with age, but the thicknesses inferred are approximately one-third of those determined seismically. This is because flexural adjustments take place over long timescales of around 10⁵ years or more, and at these low strain rates the brittle–ductile transition occurs at lower temperatures. In fact, the elastic thickness estimates from flexural studies fall between the 350 °C and 650 °C isotherms (Fig. 2).

The fact that the lithosphere comprises a significant part of the upper mantle has an important effect on its rheology. Oceanic lithosphere is typically over 90 per cent mantle; its strength, dominated by the mineral olivine, is high, and its density is also relatively high. Continental lithosphere may be only about 60 per cent mantle, and with significant quantities of typically crustal minerals such as feldspar and quartz it is much weaker, and relatively light. Thus, if two lithospheric plates are in collision and one is oceanic, the oceanic one can resist major deformation and be relatively easily overridden to form a subduction zone. However, if two continental plates are in collision, both are too light to sink easily, but they are also weak enough to be relatively easily folded or faulted. Thus continental collision zones tend to be wide and complex, containing many individual fault blocks as well as large fold mountains.

Roger Searle

Bibliography

Kearey, P. and and Vine, F. J. (1996) Global tectonics. Blackwell Science, Oxford.

Lithosphere

The word lithosphere is derived from the word sphere, combined with the Greek word lithos, meaning rock . The lithosphere is the solid outer section of Earth, which includes Earth's crust (the "skin" of rock on the outer layer of planet Earth), as well as the underlying cool, dense, and rigid upper part of the upper mantle. The lithosphere extends from the surface of Earth to a depth of about 44–62 mi (70–100 km). This relatively cool and rigid section of Earth is believed to "float" on top of the warmer, non-rigid, and partially melted material directly below.

Earth is made up of several layers. The outermost layer is called Earth's crust. The thickness of the crust varies. Under the oceans , the crust is only about 3–5 mi (5–10 km) thick. Under the continents, however, the crust thickens to about 22 mi (35 km) and reaches depths of up to 37 mi (60 km) under some mountain ranges. Beneath the crust is a layer of rock material that is also solid, rigid, and relatively cool, but is assumed to be made up of denser material. This layer is called the upper part of the upper mantle, and varies in depth from about 31–62 mi (50–100 km) below Earth's surface. The combination of the crust and this upper part of the upper mantle, which are both comprised of relatively cool and rigid rock material, is called the lithosphere.

Below the lithosphere, the temperature is believed to reach 1,832°F (1,000°C), which is warm enough to allow rock material to flow if pressurized. Seismic evidence suggests that there is also some molten material at this depth (perhaps about 10%). This zone which lies directly below the lithosphere is called the asthenosphere , from the Greek word asthenes, meaning weak. The lithosphere, including both the solid portion of the upper mantle and Earth's crust, is carried "piggyback" on top of the weaker, less rigid asthenosphere, which seems to be in continual motion. This motion creates stress in the rigid rock layers above it, forcing the slabs or plates of the lithosphere to jostle against each other, much like ice cubes floating in a bowl of swirling water . This motion of the lithospheric plates is known as plate tectonics , and is responsible for many of the movements seen on Earth's surface today including earthquakes, certain types of volcanic activity, and continental drift.

See also Continental drift theory; Earth (planet); Earth, interior structure

lithosphere The upper (oceanic and continental) layer of the solid Earth, comprising all crustal rocks and the brittle part of the uppermost mantle. It is generally considered to deform by brittle fracture and if subjected to stresses of the order of 100 MPa. It comprises numerous blocks, known as tectonic plates, which have differential motions giving rise to plate tectonics. The concept was originally based on the requirement for a rigid upper layer to account for isostasy. Its rigidity is variable, but much greater than 10²¹P, which corresponds with the underlying asthenosphere. Its thickness is variable, from 1–2 km at mid-oceanic ridge crests, but generally increasing from 60 km near the ridge to 120–140 km beneath older oceanic crust. The thickness beneath continental crust is uncertain, probably some 300 km beneath the cratonic (see CRATON) parts of the continental crust, but the absence of the asthenosphere in these regions makes definition difficult. Compare ATMOSPHERE; and HYDROSPHERE.

lithosphere , brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle . The crust and mantle are separated by the Moho or Mohorovicic discontinuity (see earth and seismology ). The thickness of the lithosphere varies from to around 1 mi (1.6 km) at the mid-ocean ridges to approximately 80 mi (130 km) beneath older oceanic crust. The thickness of the continental lithospheric plates is probably around 185 mi (300 km) but is uncertain due to the irregular presence of the Moho discontinuity. The lithosphere rests on a soft layer called the asthenosphere , over which the plates of the lithosphere glide. See plate tectonics .

lithosphere The upper (oceanic and continental) layer of the solid Earth, comprising all crustal rocks and the brittle part of the uppermost mantle. Its thickness is variable, from 1–2 km at mid-oceanic ridge crests, but generally increasing from 60 km near the ridge to 120–140 km beneath older oceanic crust. The thickness beneath continental crust is uncertain, but is probably some 300 km in places.

lithosphere The outer rigid shell of a planetary body, including the crust and part of the upper mantle. On the Earth it is distinguished from the asthenosphere beneath, which is a weaker, deformable part of a planetary body. The Earth's lithosphere, which includes the crust and the upper mantle, is about 150 km deep beneath the continents, and 80 km beneath the oceans.

lith·o·sphere / ˈli[unvoicedth]əˌsfi(ə)r/ • n. Geol. the rigid outer part of the earth, consisting of the crust and upper mantle. DERIVATIVES: lith·o·spher·ic / ˌli[unvoicedth]əˈsferik; -ˈsfi(ə)r-/ adj.

lithosphere Solid, upper layer of the Earth which includes the crust and the uppermost mantle. Its thickness varies, but is c.60km (40mi); it extends down to a depth of c.200km (125mi). It is made up of a number of tectonic plates that move independently, giving rise to plate tectonics.