CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 11(a)

Fossils: the preserved remains or traces of organisms that lived long ago, such as bones, teeth and shells, or impressions of bodies and leaves, found in rocks. Formation: usually a dead organism decomposes completely. Sometimes the body, or a part of it, is buried quickly in mud, silt or sand (often at the bottom of a lake or sea), where it cannot decompose completely. Over a long time the mud hardens into rock that keeps the impression of the body, and hard parts such as bones and shells are replaced by minerals and turn to stone. This preserved form is a fossil. Two methods of determining the age of fossils: (i) Depth of the layer (relative method): fossils found in layers closer to the surface of the earth are more recent than those found in deeper layers. (ii) Carbon dating (isotope dating): the ratio of different isotopes of the same element, such as carbon-14 to carbon-12, is detected in the fossil material; the less carbon-14 is left, the older the fossil.