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."
"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."