Earth about under the oceans to for the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are composed. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core. The Earth's inner core might rotate at a
Lithosphere-Asthenosphere boundary extent of partial melt; these are factors that affect the rheological differences in the lithosphere and asthenosphere. The lithosphere-asthenosphere boundary represents a rheological boundary. Colder temperatures at Earth's shallower depths affect the viscosity and strength of the lithosphere. Colder material in the lithosphere resists flow while the "warmer" material in the asthenosphere contributes to its lower viscosity. The increase in temperature with increasing depth is known as the geothermal gradient and is gradual within the rheological boundary layer. The lithosphere is the portion of the thermal boundary layer commonly defined by its purely conductive heat transport. Throughout the rheological boundary,