Radiometric Dating

How do we know the age of a Fossil?

It all starts in the upper Atmosphere. Earth’s Atmosphere is constantly bombarded with Cosmic Rays from the sun as solar winds. These Cosmic Rays consist of particles like helium nucleus (α-particle), electrons (β-particle), neutrons, and other elementary particles. These particles travel at very high speeds and when they enter the earth’s atmosphere they collide with other atoms and form new particles like a proton or a neutron. 78% of the earth’s atmosphere contains Nitrogen which increases the probability of hitting the Nitrogen atoms by those particles. In this way when a neutron hits a Nitrogen atom it replaces a proton with itself. As the mass of proton and neutron are similar, the mass number of the atom doesn’t change but the number of protons is decreased by 1.

The stable nitrogen atom N-14 is converted into unstable radioactive carbon atom C-14. It is an isotope of carbon and consists of 6 protons and 8 neutrons. This radioactive carbon then reacts with oxygen to form radioactive CO2. The radioactive Carbon then enters the food chain by trees. Plants take CO2 and convert it into molecules like glucose, starch, and other complex carbon compounds. Animals and other organisms eat plants to get the carbon they need and make up their body with that carbon. In this way, the radioactive carbon reaches every single living organism. Living organisms replace every carbon molecule with a new one throughout their lifetime. When an organism dies the replacing cycle stops and the carbon contents remain the same. C-14 in the dead organisms then decays slowly. To know how the decay process helps us in calculating the age of something we need to know about Half-Life.


Atoms with more neutrons or protons than required for an atom to be stable are radioactive(Unstable). They achieve stability by the Decay process in which they emit α and β particles, positrons, or absorb electrons. It is very hard to predict when a radioactive atom decays. This event has two probable outcomes either an atom decays or not with a chance of happening is 50% for either of those two events. It is like predicting a coin flip, we cannot surely tell whether the coin lands with heads up but we can surely tell that if a million coins are flipped, half of those will always land with heads up. A single mole of a sample contains 6*10^23 atoms which make the perfect conditions for half of the atoms to decay. Half-Life of a radioactive substance is the time it takes to decay half of its mass. It is a constant specified to a radioactive atom. Half-Life of some of the elements is as follows.

  • Carbon-14:-  5730 Years
  • Uranium-235:-  704 million Years
  • Potassium-40:-  1.28 Billion Years
  • Rubidium-87:-   48.8 Billion Years

Carbon-14 decays into Nitrogen-14 when a neutron of Carbon-14 converts into a proton by emitting a β-Particle and an Anti-Neutrino. When Paleontologists discover a fossil, they take a sample of the fossil and test the C-12 to C-14 ratio in it by mass spectrometry. When that organism died, the carbon ratio is equal to that of the earth’s atmosphere. As C-14 atoms are constantly formed the ratio in the atmosphere remains constant. By comparing both ratios they can determine the age of that fossil by using carbon-14’s half-life. We can know the age of a fossil by using the Carbon Dating method up to 50,000 years. Beyond that, we use other radioactive nuclei to find the age of an object or a fossil.

Earth’s Age

We were told that earth is 4.2 Billion years old but how did they know? We use Radiometric Dating to even date the Earth. The oldest rock found on earth is a Zircon crystal which is 4.5 Billion years old. We use Uranium-Lead dating to find the age of rocks. Considering a rock crystal consisting of trace amounts of U-235 and no Lead in it initially, by knowing the amount of Lead present in the rock we and can calculate its age using U-235’s Half-Life. To make sure the age is accurate scientists dated meteorites and found them to be the same.

Future of Radiometric Dating

Since the industrial revolution, we have pushed huge levels of CO2 into the atmosphere. It disturbed the balance of C-12 and C-14. The usage and testing of atomic bombs in the past century ejected radioactive nucleoids all over the world which makes it difficult for future geologists to study the earth.

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