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?

Question

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

Answer 1

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.