Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Ionium–thorium dating
The radioactive element uranium-238 is soluble in water. However, when it decays into thorium, the latter element is insoluble and so precipitates out to become part of the sediment. Thorium-232 has a halflife of 14.5 billion years, but thorium-230 has a halflife of only 75,200 years, so the ratio is useful for dating sediments up to 400,000 years old. Conversely, this technique can be used to determine the rate of ocean sedimentation over time.