CBSE Class 10 Science 2018 · Paper 31(B) · Question 11(b)

Fossils are the preserved remains, impressions or traces of organisms that lived in the remote past, such as a dead insect or the impression of a leaf preserved in rock. Two methods of determining the age of fossils: (i) Relative method (by digging): when we dig into the earth, the fossils found closer to the surface are more recent than those found in the deeper layers, because newer layers settle over older ones. (ii) Carbon dating: the ratio of different isotopes of the same element (carbon-14 to carbon-12) in the fossil material is detected. Carbon-14 decays at a fixed rate after the organism dies, its amount halving every $5730$ years, so this ratio gives the age of the fossil.