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Fossils are generally found in sedimentary rock not igneous rock.
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.
Using changes in Earth's temperature or the Sun's use of energy failed because energy from nuclear reactions was unknown.Radiometric dating (often called radioactive dating) is a way to find out how old something is.The method compares the amount of a naturally occurring radioactive isotope and its decay products, in samples. It is the main way to learn the age of rocks and other geological features, including the age of the Earth itself.By measuring the amount of carbon-14 left in dead organic material the approximate time since it died can be worked out.For example, in 1991, two hikers discovered a mummified man, preserved for centuries in the ice on an alpine mountain.
So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.