I think I got it that the earth is uncomfortably warm inside, even for firewalkers.
I do not understand why the crust is cool.
You got 8,000 miles of really hot stuff with an eggshell-thin crust of rock for insulation–and rock is not my first choice for an insulator.
Sure; that heat radiates off into space. But hey; when I pull a big ol’ hot rock out of a fire, at no point is the outside cool to the touch when it’s still hot right below the surface.
In simple terms, rock actually is a fairly good insulator. At the core of the Earth, the temperature is hot enough to melt iron (though the inner core has pressures high enough to make it solid), and the temperatures decrease as you go toward the surface. The mantle is a viscous semisolid that allows convective heat transfer from the core to reach near the surface (and resultant dramatic processes such as volcanoes), but thin as it is, the silicate crust is a good enough insulator to ensure the surface stays cool.
In addition to that, the earth radiates heat away at a rate that would make it quite a bit cooler than it is if it weren’t for the atmospheric greenhouse effect.
But even a good insulator will still heat up given enough time; it just transfers the heat more slowly, doesn’t it?
What keeps the crust from heating up over time?
Or is the loss of heat to surrounding space rapid enough to dissipate heat from the core?
I have no idea if this would have anything to do with it, but could the surface area have something to do with it as well? I know the earth’s surface is basically smooth, but couldn’t it act like a giant heatsink? By the time the heat made it to the surface, it would be so spread out that it wouldn’t seem nearly as warm.
The earth’s crust is pretty thin. Picture the skin of an apple. The divergence of the heat intensity because of increasing area wouldn’t amount to much.
The surface of the earth is in thermal equilibrium with it’s surroundings. All of the heat that arrives at the surface for whatever reason must be radiated away in order to maintain the surface temperature. Radiation for a body with any given set of thermal characteristics is proportional to the 4th power of the temperature. The surface temperature must rise and fall so as to keep input heat equal to output heat.
The heat flux at the surface is about 90mW/square meter–not very hot if you are standing on it. Since there is a lot of surface, that’s a big total but the total heat trapped inside by the insulating crust is enormous. I am sure the right answer–indeed the only possible answer–is that the crust is a good enough insulator to retard the heat flow from inside the earth so we don’t burn our tootsies. For me it’s just one of those things that is very counterintuitive when you consider that you have this giant 8000 mile-diameter burning ball and a very thin crust made of rock. It just seems like over time a rocky crust would equilibrate to the temperature inside so that there is not such a sharp gradient in just the outer 15 miles. Obviously not, for which I am grateful.
The crust is already heated up as far as it goes by the core of the earth. The crust is continually heated by conduction from the core, but it’s also continually losing heat to space. The equilibrium point is where the temperature is right now.
The insulating property depends on thickness, which is why two blankets are warmer than one. And the crust is tens of miles thick; at that thickness it becomes an excellent insulator. The small amount of heat conducted through it doesn’t raise the surface temperature very much.
It needs to also equilibrate to the temperature outside.
If you were to take two 1’X1’X1’ copper cubes and sandwich a ceramic insulator of 1’X1’X1mm between them, and then apply 0[sup]o[/sup]C to the end of one cube and 100[sup]o[/sup]C to the end of the other cube, it would be a pretty good bet that after an hour, each cube would be it’s relative temperature and the ceramic would, over the space of 1mm run the gammit from 0[sup]o[/sup]C to 100[sup]o[/sup]C.
The earths crust plus the atmosphere are that 1mm.
Your rock from the fire is tiny, but have you actually equiped one with sub surface temperature sensors and measured the change in temperature from surface to core?
When you took the french fry out of the boiling oil, it was too hot to touch. Now, a few minutes later, you can touvh it up without burning yourseld, even though the inside is still warm when you bite into it.
Substitute “earth” for “french fry” and “billions of years” for “a few minutes.”
The earth was entirely molten. It’s had a few billion years to cool down. (Granted, internal heating from radioactivity has extended the process.) Now, what you have is a planet that has cooled down enough that its outer crust is solid, even though the interior is still hot and juicy. Add several more billion years of cooling time, and it will be mostly solidified, with only a tiny hot center. Finally, it will be totally solid.
Tempreture = Pressure : Pressure = Tempreture. The more pressure any substance is under the more heat it will hold. The rock and metal in the core are under tremendous pressure and thus retain most of its heat. Wile the crust radiates its heat very easly. This is the same principle that airconditioners work under.
Thanks. I pretty much have the concept, I think; what is counter intuitive to me is that the gradient would only be a degree or so for every 60 feet. I’d expect rock to conduct heat much better than that. I realize my intuition is wrong, but if I put a big old plate of rock 20 miles thick on a blistering hot ball of goo with a mass many times the plate of rock, I’d be thinking the rock would heat up to where I couldn’t walk on it. I would be wrong, of course.
Think of a brick fireplace. It’s insulated from the surrounding wooden wall by maybe 1 ft of brick. You can leave burning coal in there indefinitely without burning down the wall. Of course brick isn’t exactly “rock” but it’s not that different.
I can remember reading about the Russians drilling deep into the crust and encountering very high temperatures. Very interesting project I thought, although it ultimately failed in its primary aim.
Ah no. Kelvin showed that the heat of a hot earth would be lost quickly due to simple radiation. What heats the inner earth up is radioactivity. The pressure = heat (capacity) thing is not relevant here