Why are radium and radon continually produced in Earth’s crust?

Radium and radon are continually produced in Earth’s crust because they are intermediary products within natural radioactive decay chains.1

Because all isotopes of radium and radon are radioactive and have relatively short half-lives on a geological time scale, any primeval amounts of these elements would have vanished long ago. Instead, they persist because they are constantly replenished by the slow decay of long-lived parent isotopes, such as uranium-238, which have been present since the formation of the solar system.2341

Specifically, radium-226 is formed as a decay product of thorium-230, which is part of the uranium-decay series. Radon, in turn, is generated by the radioactive decay of radium. As these decay chains operate throughout nature, these elements are continuously created within soil and rocks containing uranium mineral deposits.321