Until now, scientists have not found a way to determine the exact age of the Earth directly from rocks, because the oldest of them are destroyed and processed by tectonic activities.
Even if the primary rocks still exist, they have not been found yet.
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Assuming that the Earth and other celestial bodies of the solar system formed simultaneously, scientists were able to determine their probable age.
The age of our planet, as well as rocks of the moon & meteorites, are measured by the decay of long-lived isotopes of radioactive elements. Isotopes occur naturally in minerals and rocks. Their half-lives range from 700,000,000 to over 100,000,000,000 years.
Similar dating methods are based on physics and are commonly known as radiometric.
There are no tectonic processes on the Moon, therefore, there are more ancient rocks on it than on Earth. However, as a result of six Apollo missions and three Luna programme, very few stones were delivered to Earth. The studied rocks vary greatly in age, which reflects different epochs of their formation.
The oldest of the samples date back to 4.4 – 4.5 Ga. It is the figure that provides the minimum framework for the origin of our closest planetary neighbor.
Ancient rocks with an age exceeding 3.5 billion years are found on all 6 continents of our planet. The oldest among those found are:
- Rocks in the Minnesota River Valley and Northern Michigan (USA),
- 3.5 – 3.7 billion years old;
- Supracrystal rocks in the west of Greenland, from 3.7 to 3.8 billion years;
- Gneisses in Northwest Canada Near Big Slave Lake, 4.03 billion years old.
An interesting feature of these ancient rocks is that they originated not from the primary crust but from lava flows and sediments, which indicates the beginning of Earth history long before their formation.
One of the foundations of the current age estimated for our planet is zircon mineral grains from the western and central parts of Australia. Grain age is estimated at 4.4 billion years old. In this case, the initial crystal rocks could not be found.
More than 70 different types were identified and studied among them; their estimated age ranges between 4.53 and 4.58 billion years old.
According to scientists, the best estimation should be based not on the dating of individual species but on consideration of all the elements as part of a single evolving system.
Particularly, the ratio of lead isotopes 207 and 206, which changes due to the decays of uranium-235 and uranium-238 respectively, is considered to be the key.
Studying the ratio of Pb-207 and Pb-206, scientists calculated the age of the Earth, meteorites, and therefore the entire solar system. That has given the figure 4,540,000,000 years with an error of less than a percent.