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The Earth has a crust, mantle, outer core and the inner core with each one getting hotter than the next. How come, over millions and millions of years, the heat that is at the center of the Earth hasn't conducted throughout the planet's material so that the entire planet is one even temperature?

This always bothered me because we all learn that temperature diffuses from high areas to low areas, yet the Earth's center is super hot while if you dig a one foot hole, the ground feels quite cold. I never understood this. Thoughts?

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  • $\begingroup$ Dig a deeper hole... $\endgroup$
    – DJohnM
    Commented Oct 21, 2016 at 18:49

4 Answers 4

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It's a bit like when you put a thick jumper on. The inside of your clothing ends up being warmer than the outside of your clothing.

Most of the heat within the earth can be attributed to radioactive decay (of long lived isotopes like potassium). This heat is constantly being conducted out to the surface. (Yes, if you go down into a deep mine, you will get hotter.) It turns out that kilometres of rock works as a reasonably good insulator.

Remember that the difference in temperature affects how quickly heat is transferred. If the surface was nearly as hot as the interior (like when it originally formed) then the surface would radiate heat into the cold night sky much faster, and the crust would conduct internal heat away from the core to the surface even slower, and this imbalance would cause the surface to lose net thermal energy and cool down (while the core heats up even further); this process continues until an equilibrium is reached (where each layer of the earth has its own roughly stable temperature, and each layer is getting rid of excess thermal energy at just the same rate as it acquires it).

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  • $\begingroup$ A steady state is not necessarily an equilibrium state and I believe you misused the word equilibrium, while you meant steady state instead. $\endgroup$
    – untreated_paramediensis_karnik
    Commented May 6, 2020 at 17:08
  • $\begingroup$ Yep, used "equilibrium" in the colloquial sense meaning a stable balance (rather than a "detailed-balance" in the thermodynamic sense), the same as how it is used by physicists in the concept of "equilibrium temperature of a planet" (which technically also refers to a steady state that is not a thermodynamic thermal-equilibrium). $\endgroup$
    – benjimin
    Commented May 14, 2020 at 0:14
  • $\begingroup$ I suggest you to add this information in your answer. $\endgroup$
    – untreated_paramediensis_karnik
    Commented May 14, 2020 at 6:19
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The pressure at the core is higher, so higher temperatures are thermodynamically more favourable there.

More importantly, the Earth is not in thermal equilibrium. Heat can't move outward from the core nearly so efficiently as from the surface off the planet, for example, so the surface cools a lot more quickly.

There are also mechanisms which continue to generate new heat deep underground, but not at the surface: friction from the motion of material under the surface, and decay of radioactive elements there.

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  • $\begingroup$ What do you mean by high pressure favouring high temperature? Doesn't that require greater compressibility etc to be applicable? Also, if friction is generating the heat, what is generating the motion? $\endgroup$
    – benjimin
    Commented Oct 21, 2016 at 20:40
  • $\begingroup$ Gravity can generate the motion. That's what title forces are. $\endgroup$
    – Bill Alsept
    Commented Oct 21, 2016 at 21:16
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Earth's core is losing energy to the universe. Just very very slowly. Also radioactive elements in the mantle and the core heat up Earth's interior. Also the friction of infalling material to the core adds heat to Earth's core. Also all those layers of rock prevent Earth from losing heat.

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Gravity continues to generate energy. At the surface that heat can dissipate easier than at the core.

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  • $\begingroup$ So all the downloaders seem to think that tidal forces can exist without gravity?? Also I noticed that the approved answer mentions pressure. Gravity helped caused that pressure $\endgroup$
    – Bill Alsept
    Commented Oct 21, 2016 at 20:24
  • $\begingroup$ I would still like the down voters to explain why my answer is wrong. The approved answer says both pressure and friction. Gravity is what causes that. If there were no renewed internal heat then eventually the planet would reach equilibrium but that will never happen because of gravity. $\endgroup$
    – Bill Alsept
    Commented Oct 21, 2016 at 20:36
  • $\begingroup$ Tidal forces are produced by the moon. These do contribute, but are not the main effect. Gravity is never said to create energy, only to change its form. (If the heat were coming from gravity, a corresponding amount of net mass would need to be e.g. decreasing in height.. this is applicable in the formation of a star or planet, but not applicable to an existing planet.) Answers like gravity, pressure, and friction are simply incorrect in this case. $\endgroup$
    – benjimin
    Commented Oct 21, 2016 at 21:24
  • $\begingroup$ Tidal forces are caused by gravity and not the moon. Just like stars gravity creates a condition that radiates energy. And yes I do believe gravity is causing energy to change its form into heat. $\endgroup$
    – Bill Alsept
    Commented Oct 21, 2016 at 21:30
  • $\begingroup$ Stars produce energy from nuclear fusion. $\endgroup$
    – benjimin
    Commented Oct 21, 2016 at 21:34

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