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Hank clarifies, corrects, and generally straightens out the origins of the terrific heat inside the Earth. It's not only from the collisions and pressure that date back to Earth's formation, it also involves the transport of heavier elements toward the core and, more recently, the radioactive decay of elements. Sorry for the errors, Internet!
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A few weeks ago in an episode about volcanoes I lied to you. I told you a lie that was told to me by a teacher in high school and I apologize for not checking that fact. That's how these misconceptions get passed around and we try to be careful about that here at SciShow but in this case we messed up. I said that the Earth's heat was caused by the pressure of the Earth pushing down on itself. This misconception is based on the fact that if you compress gas it heats up. Work is done to compress the gas and that energy gets stored in the gas molecules, they speed up and thus more heat. You can see it in action right here. I can compress the gas in this piston really fast and boom. Hot enough to light that cotton on fire. Pretty neat but not neat enough to liquify the mantle of the earth.

(Intro)

You see the gas was only heated up because it was compressed but the Earth is not made of gases, it's made of solids and liquids which compress very little and the compression they experienced happened long ago. Gravity is no longer doing work on the Earth, though that is indeed where a great deal of the heat of the Earth came from. Before the Earth was formed it was basically an asteroid belt but eventually enough mass ended up in one place that it started drawing all of those asteroids together. This was a pretty violent process. Every gigantic asteroid that got sucked up into the Earth added massive amounts of energy to the system. The energy of those collisions and the friction between those bodies liquefied the Earth. The entire thing was a ball of lava, that is why it and every other planet is nearly spherical. In a gravity neutral environment liquids form spheres. Amazingly after 4 and a half billion years some of that energy is still around. The Earth has cooled off of course, forming a thin, delicious crust but much off that heat is still there, yet we haven't really even started yet. There are two other important sources of heat.

One is the transport of heavier elements to the center of the Earth. This also caused by gravity. That resulted in a tremendous amount of frictional heating. This appears to mostly be at a steady state now but early in the Earth's history it was a major source of heat.

And then we have a major contributor that continues to heat the earth to this day: nuclear decay. Certain isotopes of elements are unstable, we call them radioactive because they have a time limit and once their time is up they decay into other isotopes and elements and in the process they release energy.The largest contributors to this heating are uranium, thorium, and potassium. Now we're not sure how much of the Earth's heat comes from each of these sources. Our best guess is that about 50% of the Earth's heat comes from radioactive decay while the rest is left over from the process of planetary accretion.

So the Earth is in fact cooling, radiating about 40 terawatts of heat, while radioactive decay only produces 20-30 terawatts. But the energy content of the planet is so massive and the crust formed such a nice layer of insulation that we remain an extremely hot ball of mostly magma that we can occasionally use to generate power for ourselves. And no, it doesn't have anything to do with pressure.

Thanks for watching this episode of SciShow, especially to all of the people who pointed out that we were wrong in our volcano power video. If you notice us getting anything wrong don't hesitate to bug us about it on Facebook or Twitter or in the comments below. And if you want to keep getting smarter with us here at SciShow you can go to youtube.com/scishow and subscribe.