The layer of the mantle between the lithosphere
and mesosphere. Its depth in the Earth ranges from
about 155 miles (250 km) to 0 miles below the mid-ocean
ridges, and 31–62 miles (50–100 km) below different parts of
the continents and oceans. Some old continental cratons have
deep roots that extend deeper into the asthenosphere. It is
characterized by small amounts (1–10 percent) of partial melt
that greatly reduces the strength of the layer and is thought to
be the layer across which much of the movement of the plates
and vertical isostatic motions are accommodated. The name
derives from the Greek for “weak sphere.” The asthenosphere
is clearly demarcated by S-wave seismic velocities,
which show a dramatic drop through the asthenosphere
because of the partial melt present in this zone. The asthenosphere
is also known therefore as the low velocity zone, and
it shows the greatest attenuation of seismic waves anywhere
in the Earth.
The asthenosphere is composed of the rock type peridotite,
consisting primarily of olivine, with smaller amounts
of orthopyroxene, clinopyroxene, and other minerals including
spinels such as chromite. The asthenosphere is flowing in
response to heat loss in the deep Earth, and there is a current
debate on the relative coupling between the flowing asthenosphere
and the overlying lithosphere. In some models, the
convection in the asthenosphere exerts a considerable mantle
drag force on the base of the lithosphere and significantly
influences plate motions. In other models, the lithosphere and
asthenosphere are thought to be largely uncoupled, with the
driving forces for plate tectonics being more related to the
balance between the gravitational ridge push force, slab pull
force, the slab drag force, transform resistance force, and the
subduction resistance force. There is also a current debate on
the relationship between upper mantle (asthenosphere) convection,
and convection in the mesosphere. Some models
have double or several layers of convection, whereas other
models purport that the entire mantle is convecting as a single
Post a Comment

Post a Comment