Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years. So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.

What type of rock do scientists use for radiometric absolute dating?

Sedimentary rock Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the sediments they have ended up in. However, there are radiometric dating methods that can be used on sedimentary rock, including luminescence dating.

Why is uranium 238 used for dating rocks?

Uranium-lead dating can be used to find the age of a uranium-containing mineral. Uranium-238 decays to lead-206, and uranium-235 decays to lead-207. The two uranium isotopes decay at different rates, and this helps make uranium-lead dating one of the most reliable methods because it provides a built-in cross-check.