CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 6

Fossils are the preserved remains or traces (such as bones, shells or impressions) of plants and animals that lived long ago, found buried in the rocks of the earth's crust. Two methods of determining their age: (i) Relative dating: when we dig into the earth, the fossils found in layers closer to the surface are more recent than those found in deeper layers, because the deeper layers were laid down earlier. (ii) Carbon dating (isotope dating): the ratio of different isotopes of the same element, such as carbon-14 to carbon-12, is measured in the fossil material. Carbon-14 decays at a known rate after death, so this ratio tells how old the fossil is.