asthenosphere

asthenosphere The athenosphere (derived from the Greek for ‘weak sphere’) is the relatively weak, ductile layer in the upper mantle immediately underlying the lithosphere. Although solid at normal strain rates, like the rest of the non-lithospheric mantle it can deform slowly by solid-state creep. It has an effective viscosity of between about 10¹⁹ and 10²¹ Pa s (Pascal seconds), compared with an average of about 10²¹ for the mantle as a whole and 10⁻³ Pa s for water. The lowest viscosities tend to occur under volcanically active areas, and the highest under cratonic areas (which are stable regions within continents).

The top of the asthenosphere is a gradational boundary with the overlying lithosphere, and its depth varies according to the age and temperature of the lithosphere (see lithosphere). The base of the asthenosphere is also gradational, and is not well defined. Some consider that the base occurs around 300–400 km, whereas others consider it to extend to the base of the upper mantle at 670 km.

Evidence for the local existence of an asthenosphere is derived from long-term, regional changes in elevation, such as the postglacial uplift of Fennoscandia. There, historical uplift of about 10 mm per year is occurring as a result of isostatic rebound of the lithosphere following the removal (by melting) of the load represented by the glacial ice cap. When the ice cap first extended over Fennoscandia, its weight depressed the lithosphere, and the underlying asthenosphere gradually flowed away to the sides. As the load is released, the elastically flexed lithosphere ‘rebounds’ to its pre-deformation level, and the asthenosphere flows back into its former position. The rate of rebound is limited by the viscosity of the athenosphere, and modelling such movements provides estimates of the viscosity.

Although such local studies are important, it is seismology that has demonstrated the global presence of the asthenosphere. This is inferred partly from the almost ubiquitous ‘low-velocity zone’ for P- and especially S-waves that occurs between about 100 and 300 km below the Earth's surface, and partly from studies of the attenuation of seismic energy. The low-velocity zone occurs almost everywhere, except perhaps under cratonic areas. It consists of a 0.5–1 km s⁻¹ reduction in seismic velocity between about 100 and 300 km depth. This velocity reduction is best explained by the close approach of the mantle at this depth to its melting temperature, although the actual degree of melt in the asthenosphere is probably very low except under volcanically active areas such as mid-ocean ridge axes, hot spots, and the mantle above subducting slabs. Seismic energy is strongly attenuated in the asthenosphere, in contrast to the lithosphere and the rest of the mantle, further attesting to the near-melting temperature there.

The presence of a worldwide asthenosphere is an important part of the concept of plate tectonics (see plate tectonics), allowing the detailed movements of plates to be decoupled from the convection in the underlying mantle. This allows plates to move around relatively freely, interacting only at their edges. This is in strong contrast to pre-plate-tectonic ideas on continental drift, the acceptance of which was hampered by the apparent difficulty of moving continents through a rigid upper mantle.

Roger Searle

Bibliography

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