Science

Why is Earth Hot?


Arenal Volcano in Costa Rica.

A recent article on ice volcanism on Enceladus, one of Saturn’s moons, made me think about why Earth is hot and has tectonic activity. As a geologist, I think about plate tectonics often. Most people don’t think about plate tectonics much in their daily lives. After all, unless you live on a fault that happens to move or are affected by a volcanic eruption or a tsunami, plate tectonics doesn’t have much affect on your daily life.

Tectonic plates move extremely slowly on human timescales, so they’re difficult to wrap one’s mind around. Yet, the plates are moving. Moving on the order of centimeters per year, places such as California and East Africa will eventually rift away while South America and Asia will come crashing together as the Pacific Ocean closes over millions of years. Considering that the Earth is approximately 4.5 billion years old, that kind of motion is significant. Over the course of Earth’s history, tectonic plates have moved a fair amount. Geologists believe there have been several supercontinents, such as the most recent one Pangea, and that oceans have opened and closed many times.

The theory that Earth’s lithosphere is broken up into several moving plates was only fully accepted by the scientific community in the 1960s. Quickly, though, plate tectonics became recognized as a paradigm of geology. Today, plate tectonic theory is essential in understanding Earth’s geology, from volcanic deep sea vents at the very bottom of the ocean to weathering processes at the top of Mt. Everest. Scientists also realize that plate tectonics has played an important role in shaping our planet’s atmosphere and hydrosphere and may have played an important role in the origin of life on Earth. For instance, some scientists have proposed that volcanoes at the bottom of the ocean may be the places where life first originated.

In the Earth, there are three main sources of heat that drive plate tectonic activity:

1. Heat of initial planetary accretion
2. Heat of planetary differentiation (the sinking of heavier material to form Earth layers, such as the mantle and core)
3. Heat of radioactive decay

Rock is actually a fairly poor conductor of heat. Lavas flows, for instance, can remain molten for hundreds of years below a thin crust of solidified rock. Remarkably, much of the heat of initial accretion and subsequent differentiation still remain trapped in the Earth. Radioactive decay also contributes to keeping Earth hot. Elements such as potassium (K), uranium (U), and thorium (Th) decay and release energy as a result. Many of the most radioactive elements, such as uranium, are fairly heavy and are thus concentrated in Earth’s core. So, radioactive decay is somewhat concentrated in Earth’s center and the associated heat takes awhile to be released into space as a result.

Plate tectonics and volcanic activity are two ways which Earth releases heat. Heat is conducted to Earth’s surface very slowly, and– more significantly– heat energy is transformed into kinetic energy through physical convection of the mantle, a solid yet slowly-moving (like tar or honey) layer in the Earth on top of which tectonic plates float and are moved around.


The layers of the Earth. These layers formed as denser materials sank to the middle of the Earth while lighter materials floated to the top. This differentiation released energy which still helps heat the Earth.

Eventually, Earth will cool down. Indeed, she has already cooled down significantly. Komatiites, for instance, may represent gigantic volcanic eruptions which only occurred early on in Earth’s history in a time when Earth was much hotter. There is some controversy about the nature of komatiites, but most geologists agree that Earth is cooler now than she was back in the Archean. In millions of years– billions, perhaps– Earth will cool and plate tectonic activity will cease.

On other planetary bodies, such as the moon and Mars, plate tectonic activity has already ceased. The moon has been tectonically dead for billions of years while Mars has no current volcanism, though there may have been volcanic eruptions on Mars as recently as several thousand years ago.

Why are the moon and Mars dead tectonically while Earth is still very active tectonically? The key lies in surface area to volume ratio. Earth has a sort of golden ratio in this regard. The moon and Mars have just too much surface area (relative to their volume) through which heat can be released into cold space. Thus, Earth is hot while they are cold.

Evelyn

Evelyn is a geologist, writer, traveler, and skeptic residing in Cape Town, South Africa with frequent trips back to the US for work. She has two adorable cats; enjoys hiking, rock climbing, and kayaking; and has a very large rock collection. You can follow her on twitter @GeoEvelyn. She also writes a geology blog called Georneys.

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17 Comments

  1. Very nice post, Evelyn. Tectonics is a fascinating subject to me, despite not living too near any faults or volcanoes. I just love thinking about the ramifications for the future of the planet. One of the few times I regret there not being some kind of afterlife is that I'll never be able to, say, pop down to see what the continents look like in a couple million years.

    Also, I like how you were personifying the Earth as a female. Not for any 'Mother Earth'-related reasons, though. It amused me largely because I was imagining you talking about the Earth the way a sea captain talks about his ship. 'She's a good planet, aye, and sails true.'

  2. "Earth has a sort of golden ratio in this regard." There we go, evidence for an intelligent designer, that's it, I'm now a creationist. Well done! :)

    (On the other hand: great post!)

  3. I didn't realize how important differentiation was until recently, when I was asked why the Earth was hot. I had to look it up. I think it's fascinating that the Earth has retained so much energy since its formation… but when you think about how much gravitational potential energy there is in the Earth, it's incredible it's not as hot as the surface of the Sun. :)

    The other thing that amazes me is how slow convection is. I can't remember the number, but isn't it something like cm/year?

  4. Cool! I mean hot! I mean… Oh, bother.

    So here's a thinker: what about Venus? Is there evidence of tectonic activity or volcanism on Venus?

    It's quite close to Earth's size, and we know that it's quite hot down there, to put it mildly. Is it possible that there's still some energy trapped under the surface, enough to drive tectonic activity and volcanism? Or, for a more out-there possibility, could it be that all the heat inside the planet has been dumped into the atmosphere? (Although that's probably not necessary to explain the violent winds and high surface temperature, given its proximity to the Sun.)

  5. Evelyn, I hope you're stashing your posts for your resume collection; as someone who took Oceanography for my Natural Sciences requirement, these are good primer posts. Perhaps when you're done with your worldly travels you can teach? Unlike my Physics requirement, which was a huge hall full of disinterested non-physics majors, Oceanography had labs and field trips and was more intimate. Seems like you would be good at that, for sure.

    Anyway, a new book I have on rocks and fossils says that the speed at which tectonic plates move varies: the fastest is over 15 cm (6 inches) a year around the East Pacific Rise near Easter Island, the average 6 cm (2.5 inches) and the slowest around the Arctic Ridge 2.5 cm (1 inch). It goes on to say that on average this is about the rate at which fingernails grow, and is very noticeable in natural or manmade structures on the plate boundaries. Would you agree with these figures? I get that the speed of convection currents affects the plate moving speed and the cooler material is sinking, so yeah, what's going to happen when the convection currents stop?

    Like Expatria, I wish I could live to see it all. Ah well, we can go backwards millions of years…

    Do you want a typo alert?:

    concnetrated in Earth’s core

    more significanlty– heat energy

  6. thad: I, for one, support your theory wholeheartedly and plan to vote on it at your next meeting.

    Joshua, that is PRECISELY what Evelyn‘s volcano picture reminds me of. Those commercials were somewhat scary to me when I was a young child, for reasons I cannot adequately explain. They are even scarier now, for reasons I doubt NEED to be explained to anyone visiting this site.

  7. Great post on the basics, Evelyn. I agree with Melusine that you should be keeping a record of these posts. The ability to communicate clearly, and engage those listening / reading is a plus in any profession, especially the sciences. You should cultivate this gift as much as possible.

    Joshua, I remember a theory about Venus’ apparent lack of tectonic activity from over a decade ago: that adequate amounts of water (liquid or locked in the crust rocks) greatly facilitate plate dynamics, and that Venus had lost most of that water to its atmosphere.

    There’s currently a probe orbiting Venus; hopefully some more answers (with no doubt many more questions) will be forthcoming.

  8. "Why is the Earth Hot?" should be a song. You know, like "Why Does the Sun Shine?" Because it's a mass of incandescent gas &mash; a gigantic nuclear furnace — where hydrogen is built into helium at a temperature of millions of degrees, that's why!

    Joshua,

    I'm all for anything which makes life more like a science-fiction movie. (And of all the Star Wars flicks, TESB is the only one really worth emulating.) This is why I read my news online and pretend revolving doors are airlocks.

  9. In the Earth, there are three main sources of heat that drive plate tectonic activity:

    1. Heat of initial planetary accretion

    2. Heat of planetary differentiation (the sinking of heavier material to form Earth layers, such as the mantle and core)

    3. Heat of radioactive decay

    Isn't tidal heating also a source of heat?

  10. Venus Explorer! Yeah, I can’t wait to see what cool stuff it turns up, though it probably won’t get as much press as Water! On! Mars! or even the hydrocarbon lakes on Titan. It’s a shame, because Venus is, if anything, a more mysterious and cooler planet, what with being nearly the same size as earth and having that super-thick atmosphere.

    I guess nobody’s as interested because it would be harder, if not impossible, to colonise the surface than Mars, and the likelihood of familiar life existing there now or in the past is quite low. But, come on, Venus can support aerobots! And maybe some day we can build flying cities there, like Bespin! And Billy Dee Williams will be there! How awesome is that?

  11. In my Introductory Astronomy course one of the basic points I hit over and over again is that the Earth and Venus are geologically alive while Mars, Mercury and the Moon are dead for the simple reason of mass-to-surface area ratio being large for Earth and Venus and small for the other three. A handy analogy in class: peas and potatoes. Heat them both up to the same temperature and wait a few minutes. The pea of course cools quickly while the potato remains hot. The same principle applies to babies getting cold easily.

    Tidal heating is a small contributor to internal heat on the Earth. Most of the tidal action induced by the Moon (and the Sun) takes place in the ocean which easily sloshes around and dissipates the energy, and the overall magnitude is quite small due to the large distance of the Moon from the Earth. On the Galilean satellites of Jupiter, especially Io, and on Enceladus at Saturn, tidal heating is the dominant source.

  12. From the Wikipedia entry on Venus…

    “Earth’s crust is in continuous motion, but it is thought that Venus cannot sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100 million years, subduction occurs on an enormous scale, completely recycling the crust.”

    Now that would be something to see… Even spread out over 100,000,000 years, there would surely be some fine fireworks in that period.

  13. So maybe the earth cooling down could offset the atmosphere heating up …

    Seriously though, my guess is that when the earth's cooled down sufficiently, and plate tectonics grinds to a halt, people like Evelyn will be out of a job :P

    But long before that happens, the earth's magnetic field will shut down, as there's no more liquid outer core to work up heatcurrents and create magnetic flows. This will in turn result in the sun's various deadly radiations reaching the surface unimpeded and killing everything that's not sufficiently protected. The atmosphere will still be there protecting us from the stuff it already does, but that alone is not enough.

  14. A very nice article writeup, but I have one nit to pick. Since I took geochemistry a long time ago, I may just be out of date on this, but your comments about radioactive materials being concentrated in the interior doesn't fit with what I remember about differentiation.

    While it is true that most of these elements are low on the periodic table, the most common compounds they form (like uranium oxides) have the property that they don't mix well with the iron in the core. It's also only slightly enriched in mantle rocks. In fact, with all of that churning of material in plate tectonics the place uranium is most heavily enriched is in the crust, especially the continental crust (something like 130 times chondritic values).

    With all of that, the reason it is so much hotter in the interior is the point you made about rock being a lousy heat conductor: it just takes a long time to get that heat out of the interior, while the crust cools off much faster.

    Okay, so am I really out of date on this one?

    DK

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