Because uranium is soluble to some extent in all natural waters, any material that precipitates or is grown from such waters also contains trace uranium, typically at levels of between a few parts per billion and few parts per million by weight. In contrast, thorium is not soluble in natural waters under conditions found at or near the surface of the earth and so materials grown in or from these waters do not usually contain thorium. As time passes after the formation of such a material, the uranium-234 in the sample decays to thorium-230, with a half-life of 245,000 years. The thorium-230 is itself radioactive with a half-life of 75,000 years and so instead of accumulating indefinitely (as for instance is the case for the uranium-lead system) it instead approaches secular equilibrium with its parent isotope. At equilibrium, the number of thorium-230 decays per year within a sample is equal to the number of uranium-234 decays per year in the same sample.
Uranium-thorium dating has an upper age limit of somewhat over 500,000 years, defined by the half-life of thorium-230, the accuracy with which we can measure the thorium-230/uranium-234 ratio in a sample, and the precision to which we know the half-lives of thorium-230 and uranium-234. Note that to calculate an age using this technique the ratio of uranium-234 to its parent isotope uranium-238 must also be measured.