The term ‘geological time’ refers to the interval of time represented by the history of the Earth, from its formation to the present day. Events in the Earth’s history, such as the deposition of sediments, eruptions of volcanoes, or the evolution and extinction of animals and plants, are interpreted by geologists from the records they have left in sequences of rocks. Geologists may refer to the age of rocks either by their absolute or numerical age (ranging from thousands to billions of years), or by a relative age (named episodes in Earth history, similar to the way in which historians refer to periods such as the Ming dynasty).
Numerical age: how old is the Earth?
The question of the age of the Earth has interested humans from at least the time of the ancient Egyptians. Some cultures, such as the Hindus, considered the Earth to be thousands of millions of years old. Early in the 17th century, Irish Anglican Archbishop James Ussher made a calculation, based on a literal interpretation of the Bible, that the Earth was created in the year 4004 BC. Until the mid 19th century, this led most Christians to believe that the Earth was only few thousand years old. In the late 18th and early 19th centuries the science of geology became established, and geologists studying the extremely slow rates at which sediments were deposited and rocks eroded realised that the Earth must be millions or even hundreds of millions of years old.
In 1897 the English physicist Lord Kelvin suggested that the Earth was 20 to 40 million years old, based on a calculation of the time taken for it to cool from a molten sphere. His estimate was shown to be wrong when it was discovered that radioactive decay generated heat within the Earth, so that the Earth must have taken much longer to cool and therefore had to be much older. In the 20th century it was discovered that the rate of decay of each radioactive element is constant, so that it can be used as a geological ‘clock’. Using this technique, called radiometric dating, geologists have determined the age of meteorites, which are believed to have been formed at the same time as the Earth. The age of the Earth is now estimated to be about 4700 million (4.7 billion) years.
Relative age and the geological time scale
The relative geological time scale was established during the 19th century in Europe, where geologists observed that rock sequences in certain areas were characterised by distinctive assemblages of fossils. These sequences were given names based either on the areas where they were first recognised (for example, Devonian after the English county of Devonshire), or on the distinctive nature of the rocks (for example, Carboniferous after the extensive coal deposits)
It soon became clear that similar rock sequences containing the same fossils could be recognised in widely separated areas, and the same name was applied to those sequences wherever they occurred in the world. Because younger rocks are laid down on top of existing, older deposits (geologists call this the principle of superposition), it was possible to determine the relative ages of the sequences by observing which ones lay above or below others in the same area. In this way, rock deposits from around the world representing intervals of time could be arranged in a single sequence, from oldest to youngest. This sequence is called the geological time scale, which is shown in the chart on the following page.