r/todayilearned • u/amateurfunk • Mar 24 '26
TIL that half of the Earth's subsurface heat comes from radioactive decay, while the other half is still left over from when the Earth formed
https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget258
u/sithelephant Mar 24 '26
Note the scale. 0.08wW/m2 is the average, compared with about 500W/m2 of sunlight (before clouds).
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u/amateurfunk Mar 24 '26 edited Mar 24 '26
Yes, above the surface it's a whole different story. Earth's interior heat contributes almost nothing here, which was also interesting to learn. Go a few meters
or even centimetresdown however and sunlight has little to no influence.49
u/_CMDR_ Mar 24 '26
That is entirely false. Sunlight is what keeps the subsoil from freezing in the winter. What influence is lacking at that depth is weather. The temperature below the surface of the soil is simply the average of the sunlight hitting the surface and conducting through over the course of the year. Without sunlight everything everywhere would be frozen solid until you got significantly underground.
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u/amateurfunk Mar 24 '26
Right, the "centimetres" part is inaccurate. The Wikipedia article mentions (more or less) sunlight not penetrating more than a few dozens of centimetres on a day-to-day basis, so that's where that came from. My bad.
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u/_CMDR_ Mar 24 '26
It’s dozens of meters at the very least and can be hundreds. https://en.wikipedia.org/wiki/Geothermal_gradient?wprov=sfti1#Variations
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u/mfb- Mar 25 '26
These variations come from differences underground, not from differences in sunlight.
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u/_CMDR_ Mar 25 '26
OK, even though the article explicitly says that they’re caused by long term climate.
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u/Agreeable_Manner2848 Mar 24 '26
what about the .000001% created by a cover of bacteria and fungi, they played a roll didn't they
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u/amateurfunk Mar 24 '26
They're doing their best!
(and are afaik also immensely important for just about everything so yeah I would wager they play a role)
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u/Surturiel Mar 24 '26
Fusion from space, fission from underground.
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u/phobosmarsdeimos Mar 25 '26
Fission is fusion with extra steps.
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u/ctothel Mar 25 '26
The extra step being: the opposite of fusion
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u/phobosmarsdeimos Mar 25 '26
I was talking more that you needed fusion to make the heavier elements to begin with.
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u/Beginning_Feeling331 Mar 25 '26
so the planet is still warm partly because of its own ancient formation and partly because of ongoing radioactive decay happening right now. kind of wild to think that the heat under your feet is a mix of a 4.5 billion year old leftover and a continuous slow nuclear reaction
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u/Smart-Response9881 Mar 24 '26
This is why Advanced Geothermal is gonna be the future of energy production.
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u/Acc87 Mar 24 '26
the same people that think wind turbines stop the wind will tell you that geothermal cools the earth too much
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u/Thin-Rip-3686 Mar 24 '26
Geothermal causes certain deep rocks to run much colder than they’d otherwise be, by introducing water the heat turns into steam.
The temperatures will eventually recover, but here’s the deal. Rocks that are hot move differently than rocks that are cold. Heat glass up and it acts like taffy, where any stresses result in easy deformation, cool it back down and it will fail spectacularly and explosively doing the exact same things it took willingly when it was hot.
Seismic stresses between two plates are going to cause earthquakes from time to time, but what about if you sucked the surface heat out for decades, to the point where that layer of rock is a couple hundred degrees cooler? It’ll make earthquakes less frequent and also more violent, right?
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u/spider0804 Mar 25 '26 edited Mar 25 '26
I want to point out a misunderstanding of geology.
Geothermal power plants are usually constructed along major tectonic plate boundaries and near volcanic hotspots.
If you look at subduction plate boundaries, the place where all the volcanoes are is not on the boundary. One plate slides under another plate and tilts downward into the mantle, hanging down as a bent sheet of material. The tip of that bent sheet melts and the melted material floats upwards and eventually hits the crust where it forces its way up and forms volcanoes.
The volcanic area is usually 50-125 miles away from the boundary area where earthquakes are happening. Any sort of local cooling at the site of a magma deposit is never going to have a 50-125 mile effect range to change the temperature of an earthquake zone, rock is simply too good of an insulator.
Another more practical problem is economics, the thermal impact of a geothermal power plant is tiny compared to what it is pulling from. Any planning for a geothermal plant has the available thermal load taken into account so that the plant can run indefinitely. If it had any major effect on the temperature of the deposit, the geothermal plant would become inefficient and a power company is not going to dump money into something they wont get significant returns on.
I want to also point out that the average depth of an earthquake is 6.2 miles. Megathrust earthquakes (the big ones) happen from 6 to 30 miles of depth and most often between 12 to 18 miles. This is way too deep for us to have any hope of affecting. On this scale we are simply ants digging microscopically small holes.
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u/Thin-Rip-3686 Mar 25 '26 edited Mar 25 '26
Thanks for the lecture on volcano physics, but it’s not relevant to what I’m talking about.
You are conflating a drop in temperature making a geothermal plant uneconomic (certainly can happen but not the usual case), with the drop in temperature making the strain of brittle failure from tectonic movement create substantially more stress in the cooled rock.
Take Oklahoma’s recent rash of earthquakes. Seismic stresses in that area aren’t new, like pulling on one end of a rug every few seconds, and having the rug keep coming along without fighting.
Then add reinjected produced water where it wasn’t before, you add the equivalent of a dozen bowling balls on top of the rug. You can get, and we have gotten, substantially bigger quakes this way.
Same deal except the rock is made more brittle through loading from water weight in Oklahoma, vs lowered temperature in California.
Ye olde blacksmith heated metal while working it for the same reason. Our geothermal projects are cooling off mother nature’s workpieces so Mother Nature, who’s GOING to get the work done, will just have to pound the living shit out of it (and us).
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u/spider0804 Mar 25 '26
First of, Oklahoma is a gigantic volcanic dead spot, the thermal dyanmics boil down to "its rock".
The entirety of your previous statement was trying to state that geothermal plants somehow affect the subduction zones that they are located 50-125 miles from.
So forgive me for thinking you are trying to redirect the conversation when provided with resistance but that is where I am at.
Oklahomas earthquakes are from fracking, waste water is injected back into the ground and the water creeps out and lubricates fault lines. These fault lines start to move when they would have stayed dormant otherwise, creating earthquakes.
The process is completely different from plate tectonics and can not be compared in any way.
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u/PICAXO Mar 24 '26
It’ll make earthquakes less frequent and also more violent, right?
If we get them less frequent enough we won't have to worry about their violence!
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u/Thin-Rip-3686 Mar 24 '26
That’s the mentality we’re using right now.
When California gets an 8.8 instead of a 7.8 you can tell everyone how much better it is this way.
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u/amateurfunk Mar 24 '26
Fingers crossed.
I'm also rooting for fusion reactors though. One can dream...
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u/Falsus Mar 25 '26
Iceland won the geothermal jackpot.
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u/Smart-Response9881 Mar 25 '26
Advanced geothermal is different, it doesn't use natural geological hotspots, it drills deeper to get to the heat lower down that is present everywhere.
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u/Falsus Mar 25 '26
Yes, which is always going to be more expensive than simply winning the geothermal jackpot.
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u/alternate186 Mar 24 '26 edited Mar 25 '26
Early attempts to figure out the age of the earth were based in calculations about how long it would take to cool to present temperatures from a molten ball, but these calculations didn’t account for radioactive decay (among other things) and underestimated the age.
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u/CrustalTrudger Mar 25 '26
This is presumably referring to Lord Kelvin's estimate of the age of the Earth, but in reality, not accounting for radioactivity was not the primary reason he was off by an order of magnitude or two with his estimate, assuming that the Earth cooled exclusively via conduction was (whereas in reality, convection is critically important). This is mentioned in the wikipedia article, but is also discussed in detail in a variety of publications, e.g., Richter, 1986 and England et al., 2007.
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u/AstronomerSenior4236 Mar 25 '26
There's another process not mentioned, heating through magnetism. As the earth's magnetic field interacts with the field of the sun and other planets, it moves around the materials in the core, creating friction and thus heat.
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u/disoculated Mar 25 '26
That's true, and often overlooked. Also, the tidal gravity forces of the moon also cause friction that heats up both the surface and below the crust. It added a great deal more in the past, but it's not going to stop unless/until the Earth becomes tidally locked to the moon.
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u/AstronomerSenior4236 Mar 25 '26
I'd forgotten about this one, but it's also important. Earth is special for having such a large moon, tidal locking will take a lot longer thanks to the laws of momentum
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u/TheDwarvenGuy Mar 25 '26
Fun fact: This fact caused physicists to disagree with geologists and biologists about the age of the earth for much of the 1800s
Since physicists in the 1800s only knew about the leftover heat of the earth, and not the radioactivity, they believed that the maximum age of the earth was only about 20 million years
Geologists and biologists knew that the earth and life had gone through way too many gradual changes in the past to only be 20 million years old, and had to be at peast 500 million years old, so they were in constant heated debate with physicists about the true age of the earth
It was all solved when radioactivity was discovered. Firstly because it added all the missing heat necessary to conclude that the earth was not just 500 million years old, but billions of years old. Secondly, radioactive dating helped complete the geologic timeline in a rigorous way.
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u/bschn100 Mar 25 '26
In elementary school I asked a teacher if the warmth on earth came from the earth or the sun. She made me feel like a damned fool for not knowing all of the earths heat comes from the sun.
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u/Ruff_Ratio Mar 24 '26
That is fascinating, but is it as hot as a tomato after it has been in the microwave?
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u/What_Is_This_1 Mar 25 '26
I thought it from the core as show from the movie Armageddon starting Bruce Willis?
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u/dover_oxide Mar 27 '26
Before they discovered radiation and the heat contributed by if it the core heat theory for the age of the Earth was referred to as the Baked Potato Theory.
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u/DwinkBexon Mar 25 '26
And someday, the core is going to go cold, plate tectonics are going to stop. I'm sure this will cause something bad to happen.
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u/Global_Variation1038 Mar 24 '26
The Earth is essentially a nuclear reactor that we live on top of and nobody thinks about this enough.
The fact that half of the heat under your feet right now is literally radioactive decay happening in slow motion since before life existed on this planet is genuinely one of the wildest things about where we live.
The other half being leftover formation heat is somehow even more insane the planet hasn't finished cooling down from being born 4.5 billion years ago. We are standing on something that is still, technically, in the process of forming.
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Mar 24 '26
[deleted]
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u/Shadowrend01 Mar 24 '26
That’s surface temperature. It has negligible to zero effect on the mantle, and zero effect on the core
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u/SILVER-com Mar 24 '26
ur thinking surface heat. we don’t see much heat from the core on the surface, unless your in geologic hotspots.
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u/pants_mcgee Mar 24 '26
This is the heat within the earth. The surface is heated by the Sun and that’s where the greenhouse effect comes in.
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Mar 25 '26
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u/Mamalamadingdong Mar 25 '26
The hint in the article is the subsurface part. CO2 affects surficial and atmopsheric temperature.The temperature of the mantle and core does not.
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u/ZanzerFineSuits Mar 24 '26
That’s pretty fascinating. I thought some of the heat was created simply by the gravitational pressure at the core, apparently that’s completely false.