The more lead the rock contains, the older it is. The long half-life of uranium-238 makes it possible to date only the oldest rocks. Also, the half-life of potassium-40 is only 1.3 billion years, so it can be used to date rocks as young as 50,000 years old.

Why can uranium be used to date rocks?

Because the radioactive decay occurs at a known rate, the density of fission tracks for the amount of uranium within a mineral grain can be used to determine its age.

Why is uranium-238 used?

Depleted uranium (uranium containing mostly U-238) can be used for radiation shielding or as projectiles in armor-piercing weapons. Where does it come from? U-235 and U-238 occur naturally in nearly all rock, soil, and water. U-238 is the most abundant form in the environment.

Why would we use uranium-238 over carbon-14?

In other instances, why might Uranium–238 be more useful than Carbon-14? Answer: Because the range of Carbon-14 is less spread out than Uranium-238, Carbon-14 might be more useful for younger fossils. Uranium-14 might be more useful for older fossils.