Fossils and geological time

Our planet formed 4.6 billion years ago which, compared with a human lifespan, is almost inconceivably old. Geologists use a number of techniques to determine the age of rocks, for instance:

• basic field observations: these help work out the relative sequence of geological events, such as the order in which sediments were deposited
• fossils: because species evolve and become extinct, fossils of some plants and animals are confined to known, specific periods of geological time
• radioactive elements: because these elements decay at a known rate, in some circumstances they can be used to calculate how many years have passed since a mineral crystallised or a rock was deposited

By careful accumulation and comparison of this type of information over many years, geologists have constructed detailed timelines and timecharts describing the Earth’s history and, every day, new discoveries help to fill in the gaps in our knowledge.