GEOLOGY

Detailed explanation-1: -Complete answer: The relative age of a rock can be determined if there is an index fossil present in the rock. A fossil must be widely distributed and it must also represent a type of organism that existed for a brief time period, if all these conditions are present then the fossil can be useful as an index fossil.

Detailed explanation-2: -Index fossils are of organisms that lived over a wide area. They lived for a fairly short period of time. An index fossil allows a scientist to determine the age of the rock it is in.

Detailed explanation-3: -The numerical ages of rocks in the Geologic Time Scale are determined by radiometric dating, which makes use of a process called radioactive decay – the same process that goes on inside a nuclear reactor to produce heat to make electricity. Radiometric dating works because radioactive elements decay at a known rate.