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Scientist use techniques such as relative dating and radiometric dating to calculate the age of fossils and rocks.
http://explorebuxton.co.uk/wp-content/uploads/2017/01/fossils-970x546.jpg

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Fossils are found in sedimentary rocks within the layers. The process of reading the order is called stratigraphy. When scientists look at the rock layers, they are able to identify the age of the fossils.
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Another way of calculating the age of fossils and rocks is radiometric dating. Radioactivity was discovered by a French physicist named Henri Becquerel.
http://www.nobelprize.org/nobel_prizes/physics/laureates/1903/becquerel.jpg

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In 1907, they found that the lead ratio in uranium increased along with its geological age. Scientist discovered that the nuclear decay of radioactive elements is decaying at a steady rate. They were able to determine the age of rocks by calculating the decay of radioactive elements.

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Scientist also uses a process called correlation, which is using fossils to match rocks with the same age. Furthermore, index fossils help identify rocks. In order for a fossil to be an index fossil, it needs to live in a specific time period, can be easily identified, and found in many places.
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