Definition of Asthenosphere

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

layer.

See also CONVECTION AND THE EARTH’S MANTLE; MANTLE;

PLATE TECTONICS.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar