I read an interesting article about Lord Kelvin's 1862 calculation of the age of the earth. Assuming that the Earth is just a cooling ball of rock, he could figure out how long it took to cool from the melting temperature of rocks (about 3000 degrees C) to the current heat flux at the surface. He calculated that the Earth was about 100 million years old (and no more than 400 million years old). Of course, all the geologists at that time completely disagreed with that number, although they had no way of figuring out absolute age on their own. Then about 40 years later, once radioactive elements were discovered (and specifically that radioactivity can generate heat), the geologists figured out where Lord Kelvin went wrong. Kelvin assumed there were no internal heat sources in the Earth, and if there were, the Earth would be much older. It only takes about a 10 km layer of granite with a normal concentration of radioactive elements to account for the current heat flux of the Earth.

I never understood this explanation. I didn't believe that there is enough radioactivity to compensate for the Earth's heat flux. If so, I should be flooded with radiation all the time. Well, in this article I read, the author showed that radioactivity doesn't actually account for the Earth's heat flux. Obviously, there isn't a 10 km layer of granite covering the surface of the Earth. And if you include radioactivity into Lord Kelvin's calculations, it only increases the age of the Earth by about 50 million years! Yes, radioactivity is a source of internal heat, but it is not that large. But geologist at the time were so excited to prove the Earth was so much older than Lord Kelvin said, they didn't worry about actually calculating it. And this "myth" has been passed down the years.

What really explains the age of the Earth is convection. Convection of the mantle transports heat from the core to the crust without diffusion. Back in the day, one of Lord Kelvin's students, John Perry, modified his mentor's calculations to include convection, and got an age of around 2.2 billion years (much closer to the actual age of 4.6 billion years). But this solution was never picked up by geologists because (A) radioactivity was a much higher-profile topic, (B) it was considered just a squabble between physicists, and (C) it makes a much less entertaining story.