The important task of geological science is to reveal the movement law of the materials that make up the crust in the process of geological development.A large number of data show that the distribution of these materials in the crust is uneven in time and space, and their existence state is also inconsistent.
These characteristics mark the stage of geological development.Accurate determination of the time of geological development stage and determination of absolute age are important steps to understand the laws in the process of geological development.It is of great significance to determine the absolute geological age.[1]
measuring method
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【1】 Denudation method
The age of seawater can be obtained by measuring the total amount of a certain component in seawater and its annual input.It is measured that the total amount of Na+in the ocean is 1.22 × 1022g. Because most of the Na+input into the ocean comes from the weathering and erosion of continental rocks, it can be calculated that the amount of Na+input into the ocean from rivers does not exceed 6.9 × 1013g per year by calculating the annual erosion amount of rocks. Thus, the age of the ocean is 1.22 × 1022g/6.9 × 1013g=1.8 × 108 years (180 million years).The ocean floor age of the fossil record obtained by ocean drilling is also 180 million years.Since the actual erosion rate depends on the slope of the surface, precipitation, temperature and CO2 content, and is also related to the type of rock and soil, there are many influencing factors. Therefore, this calculation method has generally been abandoned.However, from the perspective of fuzzy concept and statistical significance, it is still of certain reference value to use this method to infer the age of a certain period.
This method has too many influencing factors and large calculation error, and can only roughly estimate the time experienced in a certain geological period, such as calculating the age of sea and lake.It cannot be used to determine the age of rocks.
【2】 Radioisotope method
This method was first proposed by British physicist Lufu in 1904. The basic principle of this method is that it is assumed that a certain amount of radioactive parent isotope is contained in the rock when it is formed. With the passage of time, the content of the parent isotope gradually decreases, and the daughter isotope formed after the transformation gradually increases. As long as the ratio of the parent isotope to the daughter isotope is measured,This ratio can be used as the time scale since the formation of rocks.
The formula used to calculate the age of rocks is: t=ln (1+D/N)/λ λ is the decay constant;D is the daughter isotope content;N is the parent isotope content
The age of the oldest rocks on the earth measured by radioisotope method is 4-4.3 billion years;The maximum age of meteorites and lunar rocks from other planets is 4.5-4.7 billion years.According to this, the age of the earth is determined to be at least 4.5 billion years.
This is the only method that can measure the time since the formation of rocks. The age of rocks measured by this method is called absolute geological age.
