One day, back when I was living in a trailer park in Te Puke, New Zealand, I boiled an egg.
Now, if I am eager to eat a recently boiled egg, it is my custom, once the egg is done, to remove it from the heat and add cold water and let it overflow in the sink until the egg has cooled down enough for me to take off the shell.
And so it was on this occassion, that the surface had cooled and I’d begun to peel the egg.
But something extraordinary* happened — the egg, which had beeen cold to the tough, quickly became unbearablly hot to the touch. I had to drop it, and cool it down again. Several times I cooled the surface, and it quickly heated up again. This sat me down, to stare at the egg (and wait for it to cool more properly, because I was burning my fingers).
The heat stored in the center of the egg was working its way out to the cooled surface, and heating it up again. Fast. And frequently! How much heat was in there? A lot, it seemed, and it could only come out at a certain rate.
This of course got me thinking about the Earth, and the heat in its core. As far as I know, a lot of the heat in the core of the earth — what we call geothermal heat — originates from the Earth’s formation, all those billions of years ago. How much of it is the result of pressure and gravity, and how much is slowly being released from storage? I imagine that any heat we get from the sun never penetrates the crust, and simply radiates away, to the surface and out into space. I expect that most of the heat in there is from the process of the Earth’s formation, and that no significant amount of heat is being added to this depleting original store.
And the Earth is slowly — ever so slowly — cooling. just as its rotation is slowling (and our days are very slowly getting longer. I once did the calculation to find that the dinosaurs had a significantly shorter day than us — I think it was something like a 23 (or was it a 21?) hour day. I should really redo that calculation.
But the point is — long after the egg has been removed from the boiling water and been placed in the cold — even when the boiling water is poured away and forgotten about — the heat is still stored there in the egg. Just like all that heat billions of years ago when the Earth formed. A lot of it is still there, working its way out. And yes, eventually the egg cooled down, and I ate it, but… I forget what my point was.
*In this case, when I use the word ‘extraordinary’ I actually mean “very, very ordinary, but unexpected”