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?


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?

In: 13

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.

Earth has formed billions of years ago when a lot of dust particles lumped together due to gravity. All these collisions released a lot of heat, so Earth was quite hot in the beginning and didn’t have a solid surface. Eventually it cooled down enough for the solid crust to form, but down there it’s still hot. It cools very slowly because it’s so large, and also because there is radioactive decay at work, adding significant amount of heat.

The core of the Earth is actually solid, but still quite hot. The surrounding material is more goopy and molten.

When Earth formed, a lot of heat was produced, and because the planet is *so goddamn big*, it takes a very long time to shed that heat and cool off.

That alone does not explain it, though. Even accounting for that, Earth should have cooled down quite some time ago. There is another thing that keeps adding heat; radioactive decay. Radioactive material in the ground sits there decaying and generating heat. Relatively small amounts of heat, but because Earth is so big (and thus insulated), this heat is enough to keep the cure very hot.

First off: There isn’t exactly magma in the core, or really anywhere beneath the crust – not as you think of it anyway. The core itself consists of solid lump of mostly iron (the inner core) surrounded by a mass of liquid mostly iron (outer core). Outside that the mantle isn’t molten like lava or magma, it’s really more of a plastic material (think something like playdoh or modeling clay).

As for why it’s still hot, it’s primarily because Earth is *really damn big* and most everything present inside it is pretty good at insulating. Ultimately the only way for the planet to lose heat is to radiate it into space from the surface, which is already slow to begin with (Radiation is related to surface area, which increases by the square of the radius. The total heat capacity is related to volume, which increases by the *cube* of the radius). Heat is moved really easily through convection, where hot stuff rises, loses heat to the surroundings, and sinks. This is what happens in the mantle and outer core.

When Earth was completely molten, these convection currents could bring hot material to the surface where it would radiate into space and cool off. Eventually the topmost areas cooled enough to form a crust, but with a crust heat can’t be moved quickly anymore because the convection currents can’t reach the surface anymore. Now the only way for heat to escape is through conduction to the surface. I won’t go too detailed here, but rocks and metals are pretty damn hard to push heat through quickly because of their high density (and thermal mass with it) and high heat capacities, so now there’s an insulating blanket of sorts surrounding the core, several thousands of kilometers thick. Think of how hot something stays when you cover it with a nice thick blanket, or how embers from a fire can stay red hot for days when covered with a thin layer of ash, then make that layer over 6000km thick. Then add some heat source such as radioactive decay, and it can remain hot for a *very* long time.