How Old are the Oldest Rocks on Earth?

Earth has accumulated from the original solar nebula to form a planet until now. The current best estimate of the age of the earth is 4.55 billion years. The age of the earth is generally referred to as its astronomical age. The astronomical age of the earth refers to the time from the beginning of the earth to the present. This time is closely related to the hypothesis of the origin of the earth. The geological age of the earth refers to the time from the beginning of the geological process on the earth to the present. It is estimated that it will take hundreds of millions of years from the formation of the primitive earth through the early evolution to a layered structure, so the geological age of the earth is less than its astronomical age.

Age of the earth

There are different concepts about the age of the earth. The astronomical age of the earth refers to the time when the earth began to form. This time is closely related to the hypothesis of the origin of the earth. The geological age of the earth refers to the time since the beginning of geological processes on the earth. It is estimated that it will take hundreds of millions of years from the formation of the primitive earth through the early evolution to a layered structure, so the geological age of the earth is less than its astronomical age. The oldest known rock on Earth is 4.374 billion years old ^[1]

To measure the time elapsed by the earth, we must find a scale with a constant rate and a large range. The rate of change of some scales found earlier is not constant in Earth history. After the discovery of radioactive elements in 1896, people have found a physical process that changes at a constant rate as a scale to determine the age of rocks and the earth.

The ancient Chinese speculated that since the development,

A long time ago, people tried to estimate the age of the earth using the general physical and chemical processes that occur on the earth, such as the accumulated thickness of sedimentary rocks on the surface of the earth, the increase in the salinity of seawater over time, and the cooling rate inside the earth. But the rate of change of these processes is not constant in the history of the earth, so it is impossible to get a correct age estimate. It wasn't until the discovery of radioactive elements in 1896 that people found a physical process that changes at a constant rate to determine the age of rocks and the earth. As far as the test level is concerned, it can be considered that the decay rate of radioactive elements is constant under any physical and chemical conditions. According to the principle of radioactive decay, if radioactivity is known

There are some ancient stable plots on every continent of the earth, such as the west

Using theories about the origin of elements, the upper limit of the age of the earth can be given. After the formation of elements, the solar nebula was formed, and then planets such as Earth were formed from the solar nebula. According to the theory of nuclear synthesis, the ratio of uranium isotopes U and U at the time of element formation is about 1.64: 1. After they form, they decay at their own rate, and U decays faster than U. Therefore, the abundance ratio of these two isotopes of uranium on the earth is 1: 137.88. From these two ratios, we can estimate the age of the element to be 6.6 billion years. Although different theories estimate the abundance ratio at the time of uranium isotope formation, this age will not be less than 5 billion years.

Uranium two

In earth history, layered deposits containing galena have been formed at various times from 3.7 billion years ago to modern times. The lead of these galena is from the crust and at different times

A large number of igneous rocks were formed in different periods of earth history, and many of them were hardly affected by late metamorphism. If different rock samples are taken from the same rock mass of these igneous rocks for lead isotope analysis, it can be found that there is often an excellent linear relationship between their lead isotope compositions j and j .

The most reliable estimate of the age of the earth is by means of the age of the meteorite. Planetary bodies in the solar system are formed at about the same time. Meteorites are broken pieces of asteroids. Due to the small size of the asteroid, its internal radioactive energy is generally insufficient to cause remelting, so the products of the radioactive decay series in the meteorites record the time of asteroid body condensation. If Pb / Pb is plotted against Pb / Pb for all meteorites, they all lie on a straight line (Pb-Pb isochron). Earth's modern lead also falls near this line. This further proves the hypothesis that all meteorites were formed at about the same time as the earth. Calculations based on the Pb-Pb isochrones of various meteorites and their different minerals show that the age of the earth is 45.3 to 4.57 billion years. With the Rb-Sr isochron method, the age values of various meteorites also mainly fall between 45.4 and 4.57 billion years. Two achondrites have been age-determined using the Sm-Nd isochron method, ranging from 45.5 to 4.56 billion years. The moon of the earth, the moon, is the member of the solar system closest to the earth, and its internal energy is not enough to cause strong melting, so the moon's surface still retains many of the original materials when it was formed. According to the Rb-Sr isochron method, the age of the oldest rocks on the moon's surface is 45.2 to 4.600 billion years, and the age of dust is 4.6 billion years. Therefore, the formation time of the planets in the solar system is most likely around 45.5 to 4.57 billion years.

The above methods all depend to varying degrees on the assumption that all planetary bodies of the solar system are formed simultaneously. More precise research results prove that there is still a small time difference in the origin of the planets of the solar system. Therefore, to obtain a more accurate age value of the earth needs further research.