Rocks contain information about our planet, because they capture the various geological epochs that the Earth survived during its existence. Humanity in the XIX century understood the geological processes that lead to the formation of mountains, but did not know how to determine the time during which these processes took place, and the age of the fossils was unknown. Only in the 20th century, thanks to advances in physics, did this problem become possible to solve. Consider the question of how to determine the age of rocks.
Classification of methods for determining the age of rocks
In the general case, when answering the question of how to determine the age of rocks, all methods can be divided into two main types: some determined the relative, while others determined the absolute indicator. Initially, there were only those methods that allowed to reveal the relative value. They made it possible to interpret the geological history as a sequence of processes following one after another. Then came the methods for determining the absolute indicator, which are based on knowledge in the field of radiometry.
At first glance, it may seem that it is rather complicated. However, this is not so, students study this topic in detail by geography in the 7th grade.
How to determine the age of rocks? The technique for finding the studied quantities is based on the following fundamental geological laws:
- superposition law;
- laws of intersection and horizontal layering;
- study of inclusions and inclusions;
- correlation of mountain layers.
Absolute age
If we talk about how scientists determine the age of rocks, it must be said that they use the most accurate methods and methods. With their help, it is possible to obtain information in exact or approximate figures that indicate how many thousands and millions of years ago specific geological processes took place.
At the beginning of the 20th century, physicists made a revolutionary discovery: they showed that in nature there are so-called radioactive isotopes of chemical elements. What does this mean? The fact is that most of the elements of the periodic table (92 elements) are stable, that is, their atoms exist unchanged for quite a long time. However, some of them can spontaneously decay with the formation of other more stable chemical elements with the emission of elementary particles and the release of a certain amount of energy. Such atoms are called radioactive isotopes, and they contain the answer to the question of how to determine the age of rocks in geography.
Currently, there are several methods for determining the desired values, which are based on the use of the radioactivity of chemical elements, for example, the dendrochronological method, the study of the density of craters, thermoluminescence. The most accurate methods for determining the age of rocks are radiometric methods.
Radioisotope dating
Radioisotope dating as a method of radiometry is a technique for assessing the absolute age of geological materials, such as rocks, minerals and organic matter. This technique is based on the use of the physical laws of radioactive decay of unstable isotopes of chemical elements.
In 1907, the phenomenon of radioactivity was discovered, the essence of which is the decay of the atomic nucleus. At the beginning of the 20th century, the famous New Zealand physicist Ernest Rutherford expressed the opinion that it is possible to use the phenomenon of radioactivity to determine the age of minerals by determining the amount of initial radioactive elements in this substance and determining the amount of their decay products. The method of radioisotope dating began to develop actively since the seventies of the twentieth century.
Thus, this high-precision method allows you to understand how to determine the age of minerals and rocks.
Radioactive elements suitable for determining the age of rocks
In short, how to determine the age of rocks, it should be noted that not every radioactive chemical element is suitable for this purpose. In order to be able to use it in the radioisotope dating method, the following prerequisites must be met:
- This chemical element should be fairly common.
- Its half-life should be neither too long nor too short, and it must also be consistent with the time intervals that need to be measured.
- The number of daughter elements formed after radioactive decay should be distinguishable from the random number of these elements that has been present in the mineral since its formation.
Finding the absolute age of the mountains is not an easy task. As a rule, scientists obtain accurate numbers by averaging numerical indicators obtained in a series of experimental measurements. Moreover, to improve the accuracy of the results of radioisotope dating, they try to use information about the isotopes of various radioactive elements.
Characterization of radioactive elements
The main characteristic of any radioactive chemical element is its average lifetime, that is, the period of time through which its atom undergoes radioactive decay. The table below provides information on the average life time of some radioactive elements that can be used to answer the question of how to determine the absolute age of rocks. In parentheses are the mass of the nucleus in atomic units of mass.
| Radioactive element | Child element | Average life time in years | Ability to use |
| samaria (147) | neodymium (143) | 105 * 10 9 | old metamorphic rocks |
| rubidium (87) | strontium (87) | 47 * 10 9 | any type of rock |
| uranium (238) | lead (206) | 4.51 * 10 9 | most accurate method |
| potassium (40) | argon (40) | 1.3 * 10 9 | commonly used method |
| uranium (235) | lead (207) | 713 * 10 6 | similar to a pair of uranium (238) -lead (206) |
| beryllium (10) | boron (10) | 1,5 * 10 6 | sedimentary rocks |
| thorium (230) | radium (226) | 75 * 10 3 | marine deposits younger than 1 million years old |
| carbon (14) | nitrogen (14) | 5730 | materials of biological origin |
Methods of Samarium-Neodymium and Rubidium-Strontium
The long average lifetime of samarium is opposed to the fact that neodymium accumulates very slowly in rocks, therefore this method of radioisotope analysis gives the most accurate results only for the age of those mountains, whose life cycle is much longer than billions of years.
The rubidium-strontium method is often used to find the age of ancient mountains and a wide variety of all kinds of minerals. It gives accurate results for endogenous rocks.
Uranium Lead Methods
Considering the question of how to determine the age of rocks using the radioactive decay of a uranium atom, it should be noted that there are two independent reactions that manifest in a certain way. Both methods are often used together, which allows you to obtain independent information about the absolute age of the breed. These high-precision methods are quite complex, but they provide the most reliable information. Initially, the methods were used to determine the age of only minerals containing uranium. Currently, they are also used to study sedimentary mountain layers, such as limestone. The accuracy of the method is quite high, for example, it determines the age of rocks of the Archean period with an accuracy of 1-2 million years.
Beryllium method 10
A relatively young method of radioisotope dating, which began to be used in the seventies of the last century, and in recent decades it has become widespread. Using this method, the age of mountains is measured, which lies in the range from several hundred thousand years to 15 million. Along with the potassium-argon method, it is the only reliable one in this time period. It is usually used to estimate the age of volcanic and other sedimentary mountain layers.