Why is it hot underground?

Way back in the Middle Ages, miners began to notice that the deeper they dug into the earth, the toastier it got. Who knows what they made of the heat, but the physicist Lord Kelvin - of temperature fame, naturally - had a theory: Earth started off hot, and has been cooling down ever since, like a baked potato pulled from the oven. What's more, Kelvin was confident this idea would allow him to calculate the age of our potat-- planet... Imagine pulling two recently-baked potatoes out of a freezer - one that's been in there for just one minute, the other for half an hour. The minute-old-one would still feel like a hot potato, while the half-hour frozen spud would have cooled well below the skin — you'd have to poke all the way to the center to feel its residual warmth. And so in principle, you can tell how long ago a potato was cooked just by feeling how warm it is right beneath its surface. Which is exactly what Kelvin did - except with the earth. And scientific rigor. He took temperature measurements from the mines, put them into his calculations and, got... 20 million years. Which is, of course, very, very wrong — somehow, the hot temperatures just under Earth's skin made it seem, to Kelvin, that our planet was pretty much fresh out of its cosmic oven, when we now know that it's four and a half THOUSAND million years old. Kelvin's error is usually attributed to the fact that he didn't know about radioactivity, which creates a ton of heat in Earth's core and helps keep the planet warm. But heat from radioactive decay moves so slowly through solid rock that taking radioactivity into account only improves Kelvin's estimate by... pretty much nothing. Kelvin's real oversight was in thinking of the Earth like a baked potato — a solid lump through which heat slowly diffuses. Earth's mantle — the thick layer between the crust and the core — is mostly solid, but it isn't rigid. In fact, the rock closest to the molten outer regions of the core gets so hot that it becomes slightly more pliable, like warmed candle wax. And like the hot air above a candle, the warm rock rises in convection currents - over millions of years - spreading heat more evenly throughout the planet. This stirring carries tremendous amounts of heat from the core to the crust, fueling volcanoes, maybe helping to drive plate tectonics, and heating mine shafts to temperatures that make Earth seem like it's fresh out of the cosmic oven. Even though it's not.

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