Geologic Time

Our understanding of the age of the Earth has radically changed over time. Geologists have developed a geologic time scale to reflect our best understanding of Earth’s history. This helps people around the world communicate and is revised regularly to reflect current knowledge.

Determining the age of Earth
People have taken many approaches to determining the age of Earth. Aristotle thought Earth had always existed, whilst Christian philosophers came up with an age of approximately 6,000 years based on the Bible.
James Hutton, the founder of modern geology, carefully observed processes of weathering and deposition. He realised that the deposition and folding of layers of rock would require much more than 6,000 years. Some scientists used modelling to determine Earth’s age. Comte du Buffon estimated Earth’s age at 75,000 years based on cooling a globe with a similar composition to Earth. Lord Kelvin used data from increasing temperature with depth and came up with 20 million years. John Perry modelled Earth with convection and estimated 2 – 3 billion years.
Arthur Holmes perfected the technique of radiometric dating in the early 1900s, giving scientists a powerful tool to measure the age of Earth materials and meteorites. The oldest minerals on Earth are 4.375 billion-year-old zircon crystals from the Jack Hills of Western Australia. Meteorites have been dated to 4.55 billion years old.

Zircons from the Jack hills in WA are the oldest minerals found on Earth so far. (A Cavosie, J Valley. Detrital zircons BSE micrographs NAI 2014 Annual Science Report.jpg, Public Domain)



Using rock layers to tell time
Steno’s rock strata became the key to matching rocks in different locations. William Smith, an English surveyor, noticed that fossils were always in the same order in rock layers, even in different locations. Smith formalised the Law of Superposition (in an undisturbed sedimentary sequence, old material is on the bottom, young on top) and the Law of Strata (each layer has distinctive fossils). These laws are the basis for relative dating and allow geologists to match rocks in different locations.
Relative dating allowed geologists to develop useful geologic maps and to define the major subdivisions of geologic time used today. They recognised mass extinctions that marked the end of one era and beginning of another. However, they needed radiometric dating to determine exactly when this occurred. 
William Smith’s geologic map of Britain and Wales is recognised as a major milestone in geology. (William Smith 1815, public domain)

Isotopes help refine the time
Relative dating provided an order of events, but radiometric dating revealed the length of geologic time divisions. Arthur Holmes published The Age of the Earth in 1913 which contained the first geologic time scale with dates.
Since Holmes’ publication, geologists have refined the dates of geologic periods. This requires finding igneous rocks near the boundaries of each time and carefully dating them. The more data collected from different localities, the more accurate the scale.
This geologic time scale shows the occurrence of some major animal lineages in the Phanerozoic. The bar at right presents all of geologic time. (F Lerouge 2015, Wikimedia Creative Commons)


Explore the vastness of geologic time
  • Make your own geologic time scale.
  • Learn how scientists pieced together events in Earth’s history with UCMP Berkeley.
  • See what the Earth looked like in the deep past and what it may look like in the future with Smithsonian Magazine’s Interactive Earth.