The Allahabad High Court ordered a “scientific survey”, including carbon dating, of a “Shivling” said to have been found at the Gyanvapi mosque complex in Varanasi.

What is carbon dating?

  • Carbon dating is a widely-used method to establish the age of organic materials, things that were once living.
  • Living things have carbon in them in various forms.
  • The dating method is based on the fact that Carbon-14 (C-14), an isotope of carbon with an atomic mass of 14, is radioactive, and decays at a well known rate. This is how it works:
  • The most abundant isotope of carbon in the atmosphere is C-12.
  • A very small amount of C-14 is also present.
  • The ratio of C-12 to C-14 in the atmosphere is almost static, and is known.
  • Plants get their carbon through photosynthesis; animals get it mainly through food.
  • Because plants and animals get their carbon from the atmosphere, they too acquire C-12 and C-14 in roughly the same proportion as is available in the atmosphere.
  • When they die, their interactions with the atmosphere stops.
  • While C-12 is stable, the radioactive C-14 reduces to one half of itself in about 5,730 years — known as its ‘half-life’.
  • The changing ratio of C-12 to C-14 in the remains of a plant or animal after it dies can be measured, and can be used to deduce the approximate time when the organism died.

Limitations of carbon dating:

  • It cannot be used to determine the age of non-living things like rocks.
  • The age of things that are more than 40,000-50,000 years old cannot be arrived at through carbon dating.
  • This is because after 8-10 cycles of half-lives, the amount of C-14 becomes almost very small and is almost undetectable.

Radiometric dating:

  • Instead of carbon, decays of other radioactive elements that might be present in the non-living material become the basis for the dating method.
  • These are known as radiometric dating methods.
  • Many of these involve elements with half-lives of billions of years, which enable scientists to reliably estimate the age of very old objects.
  • Two commonly employed methods for dating rocks are potassium-argon dating and uranium-thorium-lead dating.
  • The radioactive isotope of potassium decays into argon, and their ratios can give a clue about the age of rocks.
  • Uranium and thorium have several radioactive isotopes, and all of them decay into the stable lead atom.
  • The ratios of these elements present in the material can be measured and used to make estimates about age. 

Cosmogenic nuclide dating:

  • There are also methods to determine how long an object has remained exposed to sunlight.
  • These are again based on radioactive decays and are particularly useful in studying buried objects or changes in topology.
  • The most common of these is called cosmogenic nuclide dating, or CRN, and is regularly applied to study the age of ice cores in polar regions.


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