The asthenosphere is a region in the upper mantle of the earth's interior, characterized by low-density, semiplastic (or partially molten) rock material chemically similar to the overlying lithosphere.
The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the earth's crust move around.
The asthenosphere is generally located between 45–155 miles (72–250 km) beneath the earth's surface, though under the oceans it is usually much nearer the surface and at mid-ocean ridges rises to within a few miles or kilometers of the ocean floor.
2. A portion of the upper mantle just below the lithosphere which is involved in plate movements and isostatic adjustments.
In spite of its heat, pressures keep it plastic, and it has a relatively low density.
Seismic waves pass relatively slowly through the asthenosphere, compared to the overlying lithospheric mantle; therefore, it has been called the low-velocity zone.
This was the observation that originally alerted seismologists to its presence and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity.
3. The upper part of the earth's mantle, extending from a depth of about 75 km (46.5 mi) to about 200 km (124 mi).The asthenosphere lies beneath the lithosphere and consists of partially molten rock. Seismic waves passing through this layer are significantly slowed.
Isostatic adjustments (the depression or uplift of continents by buoyancy) take place in the asthenosphere, and the magma is believed to be generated there.
Lying above the lithosphere is
- The liquid hydrosphere, comprising 71% of the earth's surface.
- The still lighter gaseous atmosphere, both of which were ultimately derived from the accretion of comets.