The exact age of the Earth is unknown and is difficult to determine directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed by the process of
plate tectonics. Nevertheless, scientists have been able to determine the probable age of the
Solar System and have been able to calculate an age for the Earth by assuming that the Earth and the rest of the solid bodies in the Solar System formed at the same time.
The ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and that decay. These dating techniques are known as radiometric dating.
Scientists have settled on the age of the earth of about 4.6 billion years.