CBSE Class 10 Science 2020 · Paper 31/C/1 · Question 27(b)

Fossils are the preserved traces of living organisms that lived long ago, such as bones, teeth, shells, or impressions of their bodies in rock. Formation: normally a dead organism decomposes, but if its body or a part of it is buried quickly in mud, sand or silt, it cannot decompose completely. The sediment hardens around it and keeps the remains or the impression of its body parts (an insect caught in hot mud, for example); as more layers pile up over millions of years, their pressure turns the sediment into rock with the fossil inside. Age: (1) from the depth of the rock layer: fossils found closer to the surface are more recent than those in deeper layers (relative age); (2) by fossil dating: detecting the ratios of different isotopes of the same element in the fossil material, since a radioactive isotope such as carbon-14 decays at a known rate (carbon-14 halves about every $5730$ years).