You searched for: “asthenosphere
A soft layer of the upper mantle of the earth under the lithosphere.

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.

Word Entries at Word Info: “asthenosphere
asthenosphere (s) (noun); asthenospheres (pl)
1. The zone or layer of the earth's upper mantle that lies below the rigid, hard crust or surface of the earth and consists of several hundred kilometers of deformable rock, which is capable of plastic deformation, and in which magmas may be generated and the velocity of seismic waves reduced: "Seismologists who study earthquakes are keen to understand the asthenosphere in both the north and south hemispheres, hoping to detect signs that predict earthquakes."
2. A portion of the upper mantle just below the lithosphere which is involved in plate movements and isostatic adjustments: "In spite of asthenosphere's 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 and this was the observation that originally alerted seismologists to it's presence and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity."

"The asthenosphere lies beneath the lithosphere or 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) and consists of partially molten rock and which makes seismic waves passing through this layer significantly slower. "

"Isostatic adjustments (the depression or uplift of continents by buoyancy) take place in the asthenosphere, and the magma is believed to be generated there."

3. Etymology: asthenosphere comes from Greek asthenēs, "weak"; from Greek astheneia, "weakness"; from a-, "without" + sthenos, "strength" + -o-, "a connective vowel" + -sphere, "around, zone".

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.

"Sometimes referred to as the weak sphere, the asthenosphere is characterized by being weaker and more elastic than the surrounding mantle."

"Its lack of shear strength results from the high temperature of the rocks approaching the melting point. Since seismic waves travel more slowly in the asthenosphere; it is also referred to as the low velocity zone."

"The asthenosphere's elastic behavior and low viscosity allow the overlying plates to move laterally and also allow the overlying crust and mantle to move vertically in response to gravity to achieve isostatic equilibrium or the theoretical balance in buoyancy of all parts of the earth's crust, as though they were floating on a denser layer beneath them."

—Compiled from information located at
"asthenosphere", Scientific American Science Desk Reference;
John Wiley & Sons, Inc.; New York; pages 219 and 220.

"The asthenosphere is the mantle (layer of the earth between the crust and the core) is believed to make up eighty-four percent of the earth by volume and sixty-seven percent by mass."

"The asthenosphere is about 1,802 miles, or 2,900 kilometers, thick and consists of silica, plus iron-rich, magnesium-rich, and other metal-rich minerals."

"The hot plastic asthenosphere, part upper mantle and lower crust, separates the more brittle crust-mantle lithosphere above from the mesosphere below."

"The asthenosphere is thought to be responsible for the movement of the lithospheric plates (crustal plates) that slowly carry the continents around the planet, and the asthenosphere is about 186 miles, or 300 kilometers, thick."

"The more solid mesosphere, located below the asthenosphere, includes part of the upper and all of the lower mantle."

"Scientists theorize that convection in the upper mantle-lower crust, or asthenosphere, slowly carries the lithospheric plates around the planet; while another theory states that convection at a depth of about 375 to 435 miles, or 603 to 700 kilometers, in the part of the mantle (mesosphere) is transferred to the asthenosphere and moves the plates."

—Compiled from information located in
"Geology", The New York Public Library Science Desk Reference;
MacMillan Publisher; New York; pages 377 and 379.
This entry is located in the following unit: sphero-, spher-, -sphere- (page 1)