This also allows a qualitative assessment of the devision of the Earth into lithosphere and asthenosphere . The asthenosphere begins where the geotherm intersects the melting point curve, and initiates partial melting which lowers the shear strength and allows deformation to occur. The adiabatic gradient is the key concept in understanding convection and temperature. Adiabatic means without heat loss and the gradient must be exceeded for thermal convection to occur. The adiabatic gradient is that change in temperature with depth that occurs due to adiabatic heating when pressure increase heats a material.
Thunderstorms occur because the gradient is in excess of adiabatic.
Imagine a parcel of material that is raised an amount h in a liquid lower pressure means it expands, and when it expands it cools. The presence of allowing materials in a mixture can drastically lower the melting temperature. We think this is what is going on in the core, where the outer core contains materials that produce a lower melting temperature, and the inner core is a more pure iron form that has precipitated out. The system of interest is the system Fe-S, which can form a solid solution at core pressures and temperatures. The Eutectic mixture, producing a minimum core melting point allows an explanation for how the inner core can be solid and the outer core liquid, while still maintaining a super adiabatic gradient in the outer core.