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.

It 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.

Thought to be responsible for the movement of the lithospheric plates (crustal plates) that slowly "carry" the continents around the planet, 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.

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.