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  • 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 wrongsomehow, 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 mantlethe thick layer between the crust

  • and the coreis 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.

Way back in the Middle Ages, miners began to notice that the deeper they dug into the

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B2 中高級

為什麼說是地下熱? (Why is it Hot Underground?)

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    簡宇謙 發佈於 2021 年 01 月 14 日
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