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In my understanding, the center of the Earth is hot because of the weight of the its own matter being crushed in on itself because of gravity. We can use water to collect this heat from the Earth and produce electricity with turbines. However, I'd imagine that doing this at an enormous, impossibly large scale would not cool the center of the Earth to the same temperature as the surface, since gravity is still compressing the rock together.

However, since energy cannot be created or destroyed, it seems like this energy is just coming from nowhere. I doubt the Earth's matter is being slowly consumed to generate this energy, or that the sun is somehow causing the heating.

I think that I have misunderstood or overlooked some important step in this process. If so, why (or why not) does the Earth's center heat up, and, if not, does geothermal energy production cool it down irreversibly?

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  • $\begingroup$ Similar: physics.stackexchange.com/questions/80159/… $\endgroup$ – BowlOfRed May 20 at 3:14
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    $\begingroup$ Note that geothermal plants have very little to do with the heating of the Earth's center. They depend mostly on the radiogenic heating of the crust (a few thousand kilometers away from the core :)), which indeed is Earth's matter being slowly consumed to generate this energy. $\endgroup$ – Luaan May 20 at 7:57
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    $\begingroup$ This could lead to a nice question on HSM.SE . Prior to the discovery of radioactivity, estimates of the Earth's age conflicted with the thermodynamic calculations based on all (at that time) known heat mechanisms. Anyone want to comment on the sequence of discoveries/analyses related to pure heat-shedding? $\endgroup$ – Carl Witthoft May 20 at 19:00
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    $\begingroup$ @CarlWitthoft Actually, the problem with the calculations wasn't not knowing about nuclear reactions (that was the problem with the Sun). It was about assuming the Earth is homogeneous, and that the heat is distributed only through thermal conductivity. Even today, we don't know how big of a difference the radioactivity makes - some estimates see the radiogenic heat as dominant, others the primordial heat. Radioactivity isn't negligible, but even without it, you get Earth that's billions of years old. $\endgroup$ – Luaan May 22 at 7:24
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Heating because of high pressure is mostly an issue in gases, where gravitational adiabatic compression can bring up the temperature a lot (e.g. in stellar cores). It is not really the source of geothermal heat.

Earth's interior is hot because of three main contributions:

  1. "Primordial heat": energy left over from when the planet coalesced. The total binding energy of Earth is huge ($2\cdot 10^{32}$ J) and when the planetesimals that formed Earth collided and merged they had to convert their kinetic energy into heat. This contributes 5-30 TW of energy flow today.

  2. "Differentiation heat": the original mix of Earth was likely relatively even, but heavy elements would tend to sink towards the core while lighter would float up towards the upper mantle. This releases potential energy.

  3. "Radiogenic heat": The Earth contains a certain amount of radioactive elements that decay, heating up the interior. The ones that matter now are the ones that have half-lives comparable with the age of Earth and high enough concentrations; these are $^{40}$K, $^{232}$Th, $^{235}$U and $^{238}$U. The heat flow due to this is 15-41 TW.

Note that we know the total heat flow rather well, about 45 TW, but the relative strengths of the primordial and radiogenic heat are not well constrained.

The energy is slowly being depleted, although at a slow rate: the thermal conductivity and size of Earth make the heat flow out rather slowly. Geothermal energy plants may cool down crustal rocks locally at a faster rate, getting less efficient over time if they take too much heat. But it has no major effect on the whole system, which is far larger.

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    $\begingroup$ This answer misses heat generated by the coalescence of the Earth's outer core onto the Earth's inner core. $\endgroup$ – David Hammen May 20 at 0:15
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    $\begingroup$ Gravitational heating is still a huge part of the Earth's core heating - it's the source of the first two of your three main contributors. So your introductory paragraph seems rather contradictory with the rest of your answer. Even David's note of the solidification of core material ultimately comes from the gravitational potential energy that came from the Earth's material condensing from a large cloud of stuff to a (relatively) small ball of stuff. $\endgroup$ – Luaan May 20 at 7:55
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    $\begingroup$ Related: The postulated iron catastrophe. Quite fascinating and an example for the immense gravitational energy. $\endgroup$ – Peter A. Schneider May 20 at 14:19
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    $\begingroup$ I think a distinction should be made between being at high pressure, and going to high pressure. The latter heats up a gas, the former does not. Compressing a gas takes work, and some of that work is converted to heat. But a gas just sitting at high pressure doesn't create heat. $\endgroup$ – Acccumulation May 20 at 16:07
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    $\begingroup$ Can this answer add a part about the Tidal heating of the earth? Like the Jupiter moon IO, I would guess earth is heated a bit by the tidal effect of our moon and the sun? $\endgroup$ – Maxter May 22 at 17:19
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First things first: Human activity is not tapping into the heat of the Earth's core. At best, we're tapping into the heat differential between the surface and tens of meters to perhaps a few kilometers below the surface. Temperature in general increases with increasing depth. We humans don't have the technology to penetrate more than a few kilometers below the surface of the Earth, let alone the technology needed to penetrate the six thousand plus kilometers needed to reach the center of the Earth.

That said, the Earth's core does produce heat. It retains a bleep ton heat (read a crude four letter word instead of "bleep") from its initial formation. This initial heat came in two forms. One was a result of collisions. Even more heat was generated when the Earth separated into a core, mantle, and crust. This is where the bleep ton comes into play. The Earth has only had 4.5 billion years to radiate away that huge amount of heat. That's too short of a period of time for that huge amount of heat.

Regarding heat production, the Earth's core produces heat via the conversion of molten material in the Earth's molten outer core to solid material in the Earth's solid inner core. The Earth's core may also produce heat via radioactive decay of material within the Earth's core, but this is highly debatable. The four main long-lived radioactive isotopes (uranium 238 and 235, thorium 232, and potassium 40) are chemically incompatible with migration to the Earth's core. That heat is generated from the formation of the Earth's inner core is widely accepted. That heat is generated in the Earth's core via radioactive decay of uranium, thorium, or potassium in the Earth's core is anything but widely accepted.

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  • $\begingroup$ My point about taking energy from the Earth's core is that taking it from the crust would cool the crust, which would be heated up again by the mantle, which would be heated by the core. $\endgroup$ – Redwolf Programs May 20 at 0:40
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    $\begingroup$ @RedwolfPrograms That's not how the Earth's thermodynamics work. The core is not the only source of heat - the mantle is heated both from below and above, and even inside. We have some idea of what the individual contributions are, but the uncertainties are very large - radioactive heating might be the dominant factor (mostly the crust and mantle), or primordial heating might be (mostly the core). Or they might be roughly equivalent. In any case, neither can be ignored. Of course, both are peanuts compared to sunlight :) $\endgroup$ – Luaan May 20 at 8:05
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    $\begingroup$ @Luaan The mantle is certainly cooled from above. A dead giveaway is that I don't sit on Lava. $\endgroup$ – Peter A. Schneider May 20 at 14:36
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    $\begingroup$ @PeterA.Schneider Obviously it's cooled on average, otherwise the temperatures would keep increasing. That's not what I was talking about. I thought it was clear enough it wasn't a blanket statement claiming that he mantle keeps heating up with no outlet :) $\endgroup$ – Luaan May 20 at 18:02
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    $\begingroup$ There's been some pretty deep holes smithsonianmag.com/smithsonian-institution/… Deepest seem to be from starting on the seafloor, getting a jump over the folks that started on the ground surface. "The oil and gas industry also claims some deep holes, on land and offshore. BP’s Deepwater Horizon holds the offshore record. The drilling rig—lost in an explosion in 2010— managed to get some 30,000 feet below the sea, or about 5 miles" Temps really go up the deeper the drills get. $\endgroup$ – CrossRoads May 21 at 14:13
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We generally tend to underestimate sizes and masses of celestial bodies. A little giveaway is that for all non-astronomical means and purposes we consider the Earth's mass infinite without any measurable error.3

Let's make an estimation: How does the heat stored in the planet Earth relate to humanity's energy production? I'm only interested in an order of magnitude here. Let's assume that the average specific heat of the earth's matter is that of silica (SO2), ca. 0.7 J/(g*K). This leads to the following results:1

Specific heat of silica (J/(kg*K))              7.00E+2
Earth's mass (kg)                               5.97E+24(2)
Earth's energy/K, assuming it's all silica      4.18E+27

World primary energy supply 2015 (Mtoe)         1.36E+4
J/Mtoe                                          4.19E+16
World primary energy supply 2015 (J)            5.60E+20
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Years of world energy supply from ΔT=1K         7.31E+06
========================================================

That's actually less than I thought, by a factor of 100 or so, but still ... long.

It's noteworthy though that this estimate assumes a constant energy supply for the next couple million years. That is rather unlikely since we'll be on our way to a Kardashev Type III civilization, provided we manage to survive all the bottlenecks ahead. As Ray Kurzweil remarked we tend to underestimate exponential growth because we are hardwired for linear relations. A civilization with exponentially growing resource usage (like our current one) will not be able to rely on geothermal energy for geological time frames. (It will not be able to rely on solar energy either, if we extend the time frame just a bit.) If we assume an increase of 2% per year, Wolfram Alpha plots this nice curve which shows when the supply needed in a single year would amount to the Earth's thermal energy corresponding to a 1K difference. Apparently that point would be reached in 800 years, not 7 million. Note how the curve doesn't make a dent until year 500 or so.4

enter image description here


1 The original primary energy consumption number is from the IAEA. Mtoe stands for mega ton oil equivalent, roughly 4,187e+10 J.

(2) Give or take 10^20

3 Obligatory (but somewhat depressing) xkcd.

4 A similar curve (with a time interval of perhaps 150 years instead of 800) could be drawn for the consumption of mineral oil. In the mid-1800s anybody predicting that one day not too far into the future we'll worry about using up all of the easily accessible mineral oil of the Earth would have been laughed out of town.

It's entirely possible that I made a mistake and the result is off by a few decimal digits (although it's probably not too small); I appreciate corrections.

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  • $\begingroup$ Not a correction, but it would be interesting to see how much energy needs to be removed to reduce the earths magnetic field sufficiently for it to no longer protect us. $\endgroup$ – user400188 May 21 at 23:24
  • $\begingroup$ @user400188 In the long run, I expect you'd need to solidify the core completely. Otherwise no matter how much energy you take out, the convective currents will start again as the core continues to solidify and release the extra energy as heat. Of course, this doesn't mean our civilisation would survive even the "short term" disturbance. $\endgroup$ – Luaan May 22 at 7:55
  • $\begingroup$ @Luaan The core would eventually solidify no matter what if humanity continually removes heat at a level higher than tidal and radioactive influx. (Of course that would mean the end of the Earth's magnetic field.) $\endgroup$ – Peter A. Schneider May 22 at 8:16
  • $\begingroup$ @PeterA.Schneider If you keep extracting the heat, yes. I was considering more the case of synchronising the liquid core perfectly with the rotation of the planet (so removing all the little vortices and flows that generate the magnetic field), or magically cooling everything to 300 °K. So you'd stop the magnetic field for some time (probably thousands or hundreds of thousands of years), but it might restart eventually as heat is released from the solidification etc. Though given that would make the mantle warmer, the dynamics would probably be quite weird (and world destroying). $\endgroup$ – Luaan May 22 at 10:46

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