Comment by kragen

As I understand it, most of the heat that was deposited by radioactive decay in the crust three billion years is still in the crust, not having had time to conduct its way to the surface, and most of heat deposited in the crust even today is from radioactive decay in rocks in the crust itself. But https://en.wikipedia.org/wiki/Geothermal_gradient#Heat_sourc... says that's wrong: "An estimated 45 to 90 percent of the heat escaping from Earth originates from radioactive decay of elements, mainly located in the mantle."

https://en.wikipedia.org/wiki/Geothermal_energy#Resources says 20% of the Earth's internal heat content is residual heat from planetary accretion 4.5 billion years ago, but of course that's mostly not in the crust. It also says, "the conductive heat flux averages 0.1MW/km²."

At the given thermal gradient of about 28°/km, a nominal thermal conductivity of 3.3W/m/K for granite (https://www.sciencedirect.com/science/article/abs/pii/S00137...), we can derive a heat flow rate, which works out to 0.09 MW/km², in good agreement with the Wikipedia number. By dividing by a nominal specific heat of 0.7J/g/K and some density estimate like 2.4g/cc, can we get a speed in meters per second? The units don't quite work out; the specific heat times the density gives us 1.7MJ/m³/K.

So I guess I don't know how I came to my conclusion, so maybe it's wrong. How did you come to yours?