Nuclear Physics is the study of atomic nuclei and their interactions with each other. Geologists often use Nuclear physics for calculating the exact age of given rock using radiometric dating. By the end of this article, you may have a clear understanding of how the age of rocks is determined.

Basics of Nuclear Physics are covered in this article for better understanding the method involved in determining the age of rocks.

the protons are red and the neutrons are blue in a stable atomic nuclei | Image Credits : Marekich

Atomic Nuclei occupies the central portion of an atom. The nucleus is composed of several protons and neutrons which constitute for 99.9% of total mass of an atom. Generally, atoms are stable but there are some Atomic nuclei which are unstable. These unstable atomic nuclei make an atom, Radioactive.




Radioactivity is a phenomenon which occurs when atomic nuclei are unstable. The unstable nucleus emits energy in form of radiation and also emits subatomic particles to form different atoms known as Isotopes.

The unstable nucleus which emits energy and particles is known as Parent Nucleus or Parent Isotope. The resultant nucleus formed by this process of emission is known as Daughter Nucleus or Daughter Isotope.


Image Credits: Kjerish


This entire process involving the conversion of parent nucleus to daughter nucleus is known as decay. This process of decay may occur naturally or artificially. The process of decay can be initiated by emission or capturing of several particles. Based on these factors decay can be categorized into four types.

They are alpha decay, beta decay, gamma decay and electron capture. These are covered in detail in a different article.

Half-life is the probable time required for half of the Parent Isotope to decay into daughter Isotope. Generally, radioactive atoms which are also known as Radioisotopes take billions or millions of years to decay completely into daughter Nuclei. Therefore, geologists consider half life of the Radioisotope.

As the time passes, the Parent isotope decreases by half during its half-life to form daughter isotope or nucleus.

Half-lives of different Radioisotopes are mentioned below

Uranium (Parent Isotope) decays to Lead (Daughter Isotope) in 700 million years

Thorium (Parent Isotope) decays to Radium (Daughter Isotope) in 75400 years

Using all these basics, we can determine the age of rocks using Radioactivity.

Geologists determine the age of rocks by using radioactivity of the isotopes which are present in the rocks. Within many sedimentary, igneous and metamorphic rocks are radioactive atoms which decay over time as we studied earlier.

Many rocks have sufficient radioactive atoms known as Radioisotopes which are useful for determining the age of rocks. Uranium is most abundantly found in rocks which are used for estimating the age of rock. This process is known as radiometric dating.

Radiometric dating involves the calculation of the percentage of parent nucleus present at present and half life of the radioactive atom.


Problem: Determine the age of Limestone using the Uranium Isotope with half life of 700 million years.

Data Given: The number of parent atoms present in limestone at present is 2,100,000 and number of daughter atoms present in limestone at present are 900,000.


Number of Parent atoms present in limestone (P) = 2,100,000.

Number of Daughter atoms present in limestone (D) = 900,000.

Half-life of Uranium = 700 million years.

Step 1: Calculate the percentage of Parent atoms or Nucleus present now (because they may have been decreased due to decay and form daughter atoms)

Percentage of Parent nucleus or atoms  P%=P/(P+D)

Where P and D are Parent and Daughter Atoms respectively.

From given Information

After the percentage of remaining parent nucleus or atoms is determined, it is to be multiplied with a half life of Uranium Isotope and subtracted from half life again to calculate the age of rocks.

Therefore the limestone (rock) is determined by radiometric dating and is found to be 210 million years old.

Isotopes with less half-life are used to determine the age of young rocks.
Image Credits: Robert_Lavinsky

Determining the age of rocks can useful for predicting the age of our planet or any extra terrestrial landforms. The oldest rocks on the surface of a planet can stand as an evidence for determining the age of a planet or comet.

The age of the planet Earth is estimated to be 4.543 billion years based on the radiometric dating of oldest rocks. Zircon, a mineral found in many igneous rocks is highly resistant to heat and corrosion. Zircon crystals are constitutes of silicon and zinc which don’t undergo metamorphism, unlike other rocks.


Since these Zircon crystals don’t change over time, they can be considered as the oldest rocks on the Earth. Zircon crystals found in sandstone regions of Australia have formed over 4 billion years ago.

Therefore scientists estimated that the age of zircon crystal may the same as Earth since there are no changes in the rocks. The radiometric dating along with relative dating has predicted the exact age of planet Earth.


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