The earth is the place of human birth and activities. How old is it?
Someone used the salt content of seawater to estimate the age of the earth, and the result was about 100 million years. This is obviously not true, because the earth was born long before the ocean appeared.
In recent years, isotopic dating method has been used to determine the age of the earth's crust. The most commonly used method is called uranium lead method.
Uranium is a radioactive element. Under natural conditions, uranium degenerates at a certain rate and finally forms lead and helium. This transformation process is very stable and extremely slow. Uranium with an atomic weight of 238 takes about 4.5 billion years to change half its original mass. Therefore, we can accurately calculate the age of the rock according to the proportion of uranium and lead in the rock.
The oldest rock measured by this method is 4 billion years old. So people estimate that the age of the earth is about 4.5 ~ 4.6 billion years. It is generally estimated to be 6 billion years since the earth began to accumulate and breed.