How and when did the universe and the Earth begin? Scientific answers to such questions involve the existence of immense timespans, often referred to as ‘deep time’. Deep time is necessary for many of the processes that have created the Earth’s crust. But how deep is deep? How old is the Earth? And what methods do scientists use to find out?
Elements are defined by atomic number, or the number of protons in the atom’s nucleus. Elements may differ in the number of neutrons in the nucleus, determining the various forms or isotopes the element can occur as. The nuclei of some atoms are stable, others are unstable, meaning they can lose or capture other atomic particles to form different isotopes or elements, which may in turn be stable or unstable. Unstable atoms are ‘radioactive’, and this process is called ‘radioactive decay’. Argon-40, carbon-14 and uranium-238 and many other isotopes are unstable. Lead-206 and carbon-12 are stable.
The timing of the decay of a single radioactive atom is completely unpredictable. A ‘parent’ uranium-238 atom might decay to its ‘daughter’ isotope, thorium-234, in the next split-second or it may happily exist in a mineral for the next five billion years before decaying. However, if you have a large enough number of uranium-238 atoms, the number of decays per second in the group is quite predictable. The rate of decay is normally expressed as the ‘half-life’—the time taken for half the amount of unstable parent isotope to decay to daughter isotopes. Half-lives vary dramatically. It is just 0.09 milliseconds for lithium-11 but 4.47 billion years for uranium-238. There is no evidence that such decay rates have varied over time. They cannot be altered to any significant extent by the environment, including mineral hosts.
A critical principle was established in the early 20th century: the number of decays per second is purely a function of the amount of the radioactive isotope present. Experiments conducted on a range of isotopes over the next century allowed reliable tables of half-lives to be published and refined. For geologists this principle would prove very timely: if you can measure the parent to daughter isotopes ratio in a mineral or rock and know the half-life, then such materials have an inbuilt ‘geological clock’. This allows dating of rocks and minerals and ultimately a means of determining the age of the Earth.
But what assumptions underpin radiometric age dating of minerals or rocks apart from constant half-lives? Simple clocks require that no daughter isotope was included when a mineral formed and that there has been no subsequent loss of parents or daughters. In more complex clocks the partial or total loss of daughter isotopes can be calculated and even used to date geological events. This is the case for the work-horse of radiometric dating, which involves the dating of zircons and other minerals.
Such radiometric methods are validated through studies that have involved application of different and independent isotopic age dating systems. An example is dating of the Cretaceous-Palaeogene boundary, a geological marker dividing the very last material deposited in the Cretaceous period from the very first material deposited in the Palaeogene, and best known for the disappearance of dinosaurs. Independent radiometric age dating methods applied to this boundary have delivered very similar ages of around 66 million years ago (66 Ma or mega-annum). Independent corroborating evidence using different methods always gives force to arguments in the sciences.
Armed now with a number of radiometric dating methods, geologists have scoured the crust in search of the oldest preserved rocks and minerals. Zircon has been the typical target mineral. The U-Pb or Pb-Pb series are the preferred dating method (though other methods have been used). The search has focused on ancient crust exposed in Greenland, Canada, Siberia, South Africa and Australia. For some time, the record for the oldest minerals (zircons) has stood at 4.37 billion years (Ga or giga-annum) extracted from rocks at Jack Hills in Western Australia’s Yilgarn. The oldest preserved rock stands at 4.03 Ga for the Acasta Gneiss Complex in Canada’s Slave Province.
These are the ages of the oldest known Earth materials, but not of the earth itself. There was a short period after formation (around 4.5 Ga) before crustal temperatures dropped to the point that crustal rocks could be preserved. Hence, another approach to finding the age the Earth has been to date materials that originated during the formation of our Solar System. These are meteorites—silicate or metallic remnants from the early formation of the Solar System that occasionally land on Earth. The oldest age dates on meteorites are around 4.57 Ga, setting an outside boundary for the Earth’s age.
Seeing deep time in other geological processes
Radioactive decay is not the only method for developing geological clocks.
The plate tectonics model elegantly underpins much of modern geology. It explains many features on the Earth’s surface including the lack of old ocean crust compared with the maximum ages of adjacent continental rocks. The growth of continents and the cycles of formation and breakup of supercontinents such as Gondwana, indicate long timeframes in the evolution of the crust. The adjacent edges of the Australian and Antarctic continents share similar geological characteristics and complementary shape (as does the facing sides of the African and South American continents). As you head south from Australia or north from the Antarctic, the ocean crust becomes progressively younger until you reach a spreading ridge where new ocean crust is being created at this very moment. At an average rate of around 6 cm a year it would have taken Australia 85 Ma or so to move this far north. The ocean crust that first formed as these continents parted is radiometrically dated at ~85 Ma. These independent methods thus corroborate each other.
Other methods for age dating include optically-stimulated light emission dating of quartz, the orientation of the magnetic field in iron-rich rocks reflecting the occasional reversals of the north and south magnetic poles, and the ‘apparent wandering’ of the Earth’s magnetic poles as continents drifted. All these point to deep geological time.
The earth has come a long way during its 4.5 billion year history—the geological clocks have been ticking.
 The concept of ‘deep time’ was developed by 18th century geologist James Hutton. See J. Repcheck, The Man Who Found Time: James Hutton and the Discovery of the Earth's Antiquity (Perseus Books, 1983).
 For uranium-238 (conventionally written 238U) the 238 defines the isotope and represents the sum of protons (92) and neutrons (146) in the 238U nucleus. 235U has 92 protons but only 143 neutrons.
 C. Lewis, The Dating Game: One Man's Search for the Age of the Earth (Cambridge University Press, 2000).
 See D.H. Bailey, How reliable is geologic dating? 2020, https://www.sciencemeetsreligion.org/evolution/reliability.php and R.C. Wiens, Radiometric Dating: A Christian Perspective, 2002, https://www.asa3.org/ASA/resources/Wiens.html (URLs in this article accessed March 2020).
 An example is the potassium-40 (parent) to argon-40 (daughter) system. The inert gas, argon, does not easily fit into mineral structures, but there are methods to assess the amount of inherited argon or subsequent loss of potassium or argon.
 S.L. Harley and N.M. Kelly, ‘Zircon: Tiny but timely’. Elements Vol.3, 2007, pp13–18. (available at https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxnZW9xdWltaWNhaW5nZW5pZXJpYXVuYW18Z3g6MjA2NTJlMmJhY2U5NGVlMA).
 H. Baadsgaard et al., ‘A radiometric age for the Cretaceous–Tertiary boundary based upon K–Ar, Rb–Sr, and U–Pb ages of bentonites from Alberta, Saskatchewan and Montana’. Canadian J. Earth Sci. Vol.25, 1988, pp1088–1097.
 J.R. Reimink et al., ‘A comparison between zircons from the Acasta Gneiss Complex and the Jack Hills region’. Earth and Planetary Science Lett., 2020, p531 (on-line).
 J.N. Connelly et al., ‘Pb–Pb chronometry and the early Solar System’. Geochim. Cosmochim. Acta Vol.201, 2017, pp345–363.
 S.E. Williams et al., ‘Australian-Antarctic breakup and seafloor spreading: Balancing geological and geophysical constraints’. Earth Sci. Reviews Vol.188, 2019, pp41–58; M.H. Monroe, Australia Separates from Antarctica, 2008, https://austhrutime.com/australia_antarctica_separation.htm. Adapted in part from Shaping a Nation: A Geology of Australia by Geoscience Australia. https://www.ga.gov.au/data-pubs/data-and-publications-search/publications/shaping-a-nation.
 A. Cox and R.B. Hart, Plate Tectonics: How it Works (John Wiley and Sons, 2009).
 P. Lyle, The Abyss of Time: A Study in Geological Time and Earth History (Dunedin Academic Press, 2016).
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