eli5/ Why is there magma at the center of the earth? What does it do? How does it stay so hot for millions of years without “losing” its temperature?
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Well… its a lot of very very hot molten rock and metals; a good chunk of the center of the planet is iron, nickle and other metals and metals like to hold lots of heat.
Normally these metals would love to throw their heat out into space, but a few problems: space is a vacuum. Energy can only radiate into space either as light or as infrared energy (if you’ve ever sat a distance from a fire but still felt warm on the fire side of your face, that’s infrared). And for a while, just after the planet was a giant spinning ball of molten metal, that’s what it did; the outer layers shot off some reddish visible light and gobs of IR energy. But as the outer layers shed their energy, they became cooler, solidifying and forming our rigid rocky outer crust, which then became a nice insulating blanket, trapping all of the molten core’s energy inside. Furthermore, we have this pesky sun which keeps showering us in additional energy as we revolve around it, so our outer shell never gets super cold to be a great heatsink for the core heat.
Heat still flows from the core out to the crust and slowly trickles out, but now we have a thick outer crust and even better, we have a thick atmosphere that even further insulates our planet’s heat. Eventually, the core of our planet will cool sufficiently that it will start to cause us problems (I mean we won’t have to worry about volcanic activity or plate tectonics anymore, so that’s nice) but that’s bajillions of years in the future and if we’re still stuck on Earth by then we’re kindof in bigger trouble anyway.
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