An element with an atomic number of 90. The ²³²Th isotope is radioactive and decays with a half-life of 1. 4 * 10¹⁰ years through a series of intermediate isotopes to a stable isotope of lead. The intermediate isotopes emit a wide range of gamma rays, the most prominent being that of thallium, ²⁰⁸Tl. It is assumed that formations are in secular equilibrium; that is, the relative proportions of parent and daughter isotopes remain constant, and the measured spectrum is directly related to the amount of ²³²Th. The concentration in the Earth's crust is about 12 parts per million, ppm, by weight.

Thorium-bearing minerals are rare. Thorium is a trace element associated with clays and heavy minerals. It is very immobile so that quantity measured today probably was present at the time of deposition. A log of thorium is presented in parts per million. It is often a good measure of clay content.