Heredity, Natural Selection, and Evolution

Highlights of Evolution

How are fossils dated?

Fossils are dated using two methods: Relative dating determines the age of the surrounding rock, giving an approximate age to the fossils therein based on their distance from the surface (older rocks are generally deeper from the surface). Other fossils within the rock can also be used to give an approximate date. The second method is absolute dating, based on the known rates of radioactive decay within rocks. By measuring the ratio between the radioactive forms of an element like uranium-238 to its nonradioactive, “decayed” form, scientists can determine when the rock formed.


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