# Average temperature of the earth - volume, not surface **URL:** \ **Category:** Factual Questions\ **Created:** [August 2, 2012, 6:47pm UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209 "2012-08-02T18:47:34Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![ArmenE](https://avatars.discourse-cdn.com/v4/letter/a/e9bcb4/32.png) [@ArmenE](https://boards.straightdope.com/u/ArmenE)\ **Post date:** [August 2, 2012, 6:47pm UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/1 "2012-08-02T18:47:34Z")
Do we have a decent enough handle on what happens below the mantle that someone might be able to provide me with an average temperature of the Earth? I don’t mean the surface temperature, but the temperature through the whole volume of the thing. I guess this is something like a measure of the thermal energy of the Earth? ---
**Author:** ![s0meguy](https://avatars.discourse-cdn.com/v4/letter/s/a3d4f5/32.png) [@s0meguy](https://boards.straightdope.com/u/s0meguy)\ **Post date:** [August 2, 2012, 6:58pm UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/2 "2012-08-02T18:58:20Z")
They’re making estimates on planets that we can only see because they caused a tiny anomaly in the light from that star. Thus I’m sure there are ones of Earth too - but I think inaccurate ones. What is beyond a few km deep, we only know theoretically. That is to say it is a matter of speculation. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [August 2, 2012, 7:59pm UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/3 "2012-08-02T19:59:46Z")
The deeper you go, the hotter it gets. While some of the heat is due to all of that mass being compressed together, a lot of it actually comes from radioactive decay. > [@](#): > > Temperature within the Earth increases with depth. Highly viscous or partially molten rock at temperatures between 650 to 1,200 °C (1,202 to 2,192 °F) is postulated to exist everywhere beneath the Earth’s surface at depths of 50 to 60 miles (80 to 100 kilometers)[citation needed], and the temperature at the Earth’s inner core/outer core boundary, around 3,500 kilometres (2,200 mi) deep, is estimated to be 5650 ± 600 kelvins.[6][7] The heat content of the earth is 1031 joules.[1] From wikipedia, here: > **[Geothermal gradient](https://en.wikipedia.org/wiki/Geothermal_gradient)** > > Geothermal gradient is the rate of change in temperature with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plate boundaries, temperature rises in about 25–30 °C/km (72–87 °F/mi) of depth near the surface in the continental crust. However, in some cases the temperature may drop with increasing depth, especially near the surface, a phenomenon known as inverse or nega St... As was already mentioned, a lot of this is theoretical. It’s not like we’ve actually stuck a thermometer down into the earth’s core or anything. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 3, 2012, 3:39am UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/4 "2012-08-03T03:39:44Z")
> [@](#): > > While some of the heat is due to all of that mass being compressed together,… Almost none of it. Just _being_ under pressure doesn’t give you any heat at all. The process of _becoming_ compressed produces heat, but that all happened billions of years ago. There’s been more than enough time since then that, if there were no other heat source, we’d have shed almost all of that original heat by now. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [August 3, 2012, 4:18am UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/5 "2012-08-03T04:18:03Z")
> [@Chronos](#): > > Almost none of it. Just _being_ under pressure doesn’t give you any heat at all. The process of _becoming_ compressed produces heat, but that all happened billions of years ago. There’s been more than enough time since then that, if there were no other heat source, we’d have shed almost all of that original heat by now. In fairness couldn’t “all of that mass being compressed together” also be considered “residual heat from planetary accretion” per below? > **[Geothermal gradient](https://en.wikipedia.org/wiki/Geothermal_gradient)** > > Geothermal gradient is the rate of change in temperature with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plate boundaries, temperature rises in about 25–30 °C/km (72–87 °F/mi) of depth near the surface in the continental crust. However, in some cases the temperature may drop with increasing depth, especially near the surface, a phenomenon known as inverse or nega St... > [@](#): > > Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth’s interior. Away from tectonic plate boundaries, it is 22.1°C per km of depth (1°F per 70 feet of depth) in most of the world.[1] Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets. \*\*The Earth’s internal heat comes from a combination of residual heat from planetary accretion (about 20%) \*\*and heat produced through radioactive decay (80%).[2] The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[3] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[4] Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth’s heat production would have been much higher. Heat production was twice that of present-day at approximately 3 billion years ago,[2] resulting in larger temperature gradients within the Earth, larger rates of mantle convection and plate tectonics, allowing the production of igneous rocks such as komatiites that are not formed anymore today.[5] ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [August 3, 2012, 4:33am UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/6 "2012-08-03T04:33:53Z")
Also note that the normal “rule of thumb” geothermal gradient figures of 10-30ish degrees C per KM only apply to the crust, since the radioactive elements that create the bulk of the earth’s heat are concentrated in the crust. It’s probably more like a degree per kilometer once you get past the Moho. Realistically, for the purposes of this exercise you could probably just ignore the crust since it’s less than 1% of the earth’s total volume. Just call the top of the mantle 700 degrees C or so and then add 1 degree/km all the way down. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 3, 2012, 1:53pm UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/7 "2012-08-03T13:53:25Z")
> [@](#): > > In fairness couldn’t “all of that mass being compressed together” also be considered “residual heat from planetary accretion” per below? I think that’s what I _was_ considering it to mean. Though I misremembered the figure as being much smaller than 20%, so my statements of “almost all” were a bit stronger than they should have been. And **GreasyJack** , if anything, wouldn’t the radioactive elements be more concentrated in the core, since they tend to be quite dense? ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [August 3, 2012, 2:49pm UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/8 "2012-08-03T14:49:17Z")
> [@GreasyJack](#): > > Also note … the radioactive elements that create the bulk of the earth’s heat are concentrated in the crust. Cite? I’d expect the bulk of the radioactive elements - uranium, thorium etc - to be at the core because of their density. ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [August 3, 2012, 6:01pm UTC](https://boards.straightdope.com/t/average-temperature-of-the-earth-volume-not-surface/630209/9 "2012-08-03T18:01:26Z")
> [@Chronos](#): > > And **GreasyJack** , if anything, wouldn’t the radioactive elements be more concentrated in the core, since they tend to be quite dense? Radioactive elements are what are called “lithophiles” which means they prefer to bond with oxygen, which means that they are very easily integrated into silicate minerals and rocks. In contrast, the elements that are relatively depleted at the earth’s crust like iridium and platinum are “siderophiles” which means they preferentially bond or occur with elemental iron. Even though elements like uranium are quite dense, the rocks and minerals in which uranium commonly occurs are far lighter than the tightly-packed elemental iron in the core. See here: [Goldschmidt classification - Wikipedia](http://en.wikipedia.org/wiki/Goldschmidt_classification) Now, granted, there has been a bit of trouble reconciling the geochemically-derived theories of the composition of the mantle and core with geophysical observations. One of the specific problems is that it seems difficult to account for the amount of heat deeper in the earth with most of the radioactive elements tied up in the crust and upper mantle. The idea that there are large amounts of radioactive elements trapped in the lower Earth somewhere, or even a natural nuclear reactor, is intuitively appealing and has been proposed many times, but thus far these proposals have more problems than merely tweaking the current model which has most of the radioactive elements nearer the surface and the heat in the core being mostly residual and frictional rather than directly derived from radioactive decay. The (heavily caveated) analogy my geochem professor used is that the radioactive elements in the crust are sort of like an electric blanket in that they’re not actually heating the core, but they’re greatly slowing the rate at which the center of the earth loses heat to space. It’s been a while since I took the relevant classes, but a bit of googling doesn’t seem to indicate there’s been any major upheavals in this regard. In fact, one of the problems with trying to find a cite that concisely sums up the issue is that there are so many articles in popular publications about the various radioactive material in the core theories that have come and gone, but not much that actually explains the current conventional wisdom. There’s a lot of higher level class outlines and such that deal with it obliquely and in very technical terms, but here’s a geo-101 [online text](http://www.indiana.edu/~geol105/images/gaia_chapter_3/earth_differentiation.htm) that says the bit about (some) radioactive elements in the crust explicitly: > [@](#): > > This differentiation caused the heavy metals (iron, nickel and related elements) to be concentrated in the core of the earth, whereas the light elements (oxygen, silicon, aluminum, potassium, sodium, calcium etc.) were enriched in an outer layer of the earth that is now termed the mantle and the crust. Not everything, however, goes simply by density. Uranium and Thorium are very heavy elements, and we should therefore expect them to be enriched in the core. Yet, contrary to expectation they are concentrated in crust and mantle. The reason for this aberration is the circumstance that ion size and chemical affinities of U and Th prevent them from being incorporated in the dense, tight crystal structures that iron assumes at the high pressures encountered in the earth’s core. Because they can fit much more easily into the more open crystalline structures of silicate and oxide minerals, they are concentrated in crust and mantle.