Faster cooling, but smoother gradients. It means that if you go down a hundred miles, it will be warmer, but the entire center of the earth will be closer to th

Faster cooling, but smoother gradients. It means that if you go down a hundred miles, it will be warmer, but the entire center of the earth will be closer to the same temperature. If you assume a solid, you assume a certain constant thermal conductivity, as Kelvin did, even though with a liquid, that's not true. Basically, he assumed that the heat was diffusing slowly, and that it couldn't go above a certain temperature at the core (i.e. not a gas or liquid), and so you had hard limits on the total reservoir of heat in the planet and could calculate a linear fit on it. It turns out the effective thermal conductivity of the center of the earth is much higher than the thermal conductivity of e.g. continental crust.