Earth's Thermal Structure

Earth's interior is very hot. The temperature increase near the surface is roughly 10° per km. The planet is hot because of three heat sources

  • Heat of formation
  • Heat from "freezing" of inner core
  • Radioactive heat

We are not sure of the total contributions of these sources. We know the amount of heat escaping Earth better than we know what's going on in the deep interior. The total heat flow is about 44 TerraWatts. Heat escapes Earth's surface at a rate of about 0.086 W per square meter, which is very small compared with the heat input from the Sun, which is roughly 1,370 per square meter.

Deep in the Earth, below about 100-200 km, heat is transported by a process called convection. Convection is driven by a combination of gravitational and thermal effects that result in lighter, less-dense (hotter) parts of the mantle and core moving outward and heavier, more-dense (cooler) parts of the mantle and core moving inward. This movement transports heat from the interior to Earth's surface.

 

At best we can only crudely estimate an average temperature profile for Earth (it varies in three-dimensions), but the figure below shows one of the recent estimates. Major structural boundaries in the planet are shown with the dashed lines.

 

 

The solidus is the curve at which a material will melt. The estimated solidus for mantle and core materials is shown on the diagram. When the temperature is above the solidus, the material is melted.

You have to be careful when you think about Earth materials, their deformation characteristics depend on the composition, temperature, pressure, and the time scale of the deformation. On long time scales (thousands of years and longer) the mantle deforms like a viscous material. On short time scales associated with seismic wave deformations, the mantle deforms like a solid.

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.