So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil. Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium.

What type of rock is used for absolute dating?

For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years .Radiometric dating.Dating methodMaterial datedAge range datedPotassium-40 to argon-40Volcanic rocks20,000 to 4.5 billion years ago4 more rows•May 20, 2011