Geologic Time

Before scientific methods were used to investigate geologic time, ideas about time and Earth history came from religious theories. The Hindu and Mayan religions taught about endlessly repeating cycles of time, each lasting for billions of years. Ideas in western culture about the age of Earth were just as imprecise, and just as incorrect. In the 1650s, the Irish clergyman and scholar James Ussher (1581–1656) used apparent genealogy within the Bible's Book of Genesis to determine that the Earth was created in 4004 B.C. Ussher based his results on the only information he had available to study the question of Earth's age. Ussher's estimates have been shown to be flawed, but unfortunately have been repeated by more modern writers who oppose modern views of Earth's age. Isaac Newton also speculated on the age of Earth, using the investigative techniques of the time that could be considered archaic today. As early as the eighteenth century, scientists knew that Earth's lifetime must have been immense. But geologists were not able to measure the dimensions of Earth's history until mass spectrometers became available in the 1950s. The mass spectrometer is an instrument used to separate different varieties of atoms from each other. Before that time, educated guesses had been made by comparing the rock record from different parts of the world and estimating how long it would take natural processes to form all the rocks on Earth.

Georges Louis Leclerc de Buffon (1707–1788), for example, calculated Earth to be 74,832 years old by figuring how long it would take the planet to cool down to the present temperature. Writing around 1770, he was among the first to suggest that Earth's history can be known about by observing the planet's current state.

James Hutton (1726–1797) did not propose a date for the formation of Earth, but is famous for the statement that Earth contains "no vestige of a beginning—no prospect of an end." The German geologist Abraham Werner (1750–1817), the first scientist to make use of a stratigraphic column, a diagram of order of sedimentary layers. An original approach to geological history was suggested by the French zoologist and paleontologist Georges Cuvier (1769–1832), who observed that specific fossil animals occurred in specific rock layers, forming recognizable groups, or assemblages. William Smith (1769–1768) combined Werner's and Cuvier's approaches, using fossil assemblages to identify identical sequences of layers distant from each other, linking or correlating rocks which were once part of the same rock layer but had been separated by faulting or erosion.

In 1897, the physicist Lord Kelvin (1824–1907) developed a model for Earth history, which assumed that Earth has been cooling steadily since its formation. Because he did not know that heat moves around in currents in the earth (convection), or that Earth generates its own heat from the decay of radioactive minerals buried inside it, Kelvin proposed that the earth was formed from 20 to 40 million years ago.

In the late eighteenth century, geologists began to name periods of geologic time. In the nineteenth century, geologists such as William Buckland (1784–1856), Adam Sedgwick (1785–1873), Henry de la Beche (1796–1855), and Roderick Murchison (1792–1871) identified widespread rock layers beneath continental Europe, the British isles, Russia, and America. They named periods of time after the places in which these rocks were first described. For instance, Cambrian Period was named for Cambria (the Roman name for Wales), and Permian, for the Perm province in Czarist Russia. Mississippian and Pennsylvanian Periods widely used by American geologists were named for a U.S. state and a region around the upper reaches of a large river, respectively. By the mid-nineteenth century, most of the modern names of the periods of geologic time had been proposed; all of them are still in use.