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-238 be used to determine the age of rocks even though they are extremely old?

Uranium 238 is only found in igneous or volcanic rocks. So no fossils can be dated directly using U 238. Because of the huge differences in the half lives of Carbon 14 and Uranium238 they cannot be used together. Carbon 14 can only be used to date fossils of a very recent age.

Why is uranium-238 used to measure the age of objects with an age of 4500 million years?

Dating rocks The half-life of uranium-238 is 4500 million years. The age of the rock can be calculated if the ratio of uranium to lead is known. As the rock gets older the proportion of lead increases. If half of the uranium-238 has turned into lead-206 the rock will be 4500 million years old.

Why is uranium better for radioactive dating of older things than 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.