The lake of a horse or dog can be determined by the teeth, the tree by the thickness of the trunk or by the annual growth rings, and how do you know the age of the Earth and the rocks that make it up? The Bible - the oldest collection of religious works - claims that the Earth is very young - it is just over 7 thousand years old. This ridiculous figure is refuted by the science and history of human society. However, to give the correct answer to a question on the age of the Earth and time of various geological events is not so simple. It took many years of hard work of a large army of scientists - geologists, biologists, paleontologists, physicists - to determine the age of the Earth.
- Geologists and paleontologists have studied the sedimentary rocks (sand, clay, limestone, marl, etc.) of the Earth's crust from the top layers - the youngest to the bottom - the oldest. By the remnants of organisms, they have restored the true history of life on Earth. The development of plants and animals went from simple forms to complex ones, from the lowest to the highest due to changes in living conditions. Scientists have singled out five eras in the history of the Earth and life, in each era several periods, and in turn epochs and centuries.
- Each subsequent period differs from the previous one in the appearance of new, more highly organized plants and animals, climate changes, land, and sea location.
- In each era, period or epoch of geological history there were certain animals and plants.
- According to the remains of organisms, they are called the leading fossils, determine the relative geological age of deposits of the Earth's crust, ie, trace what was before and what later. Paleontologists establish the same age (simultaneity) of layers located at a great distance from each other and restore ancient geography (paleography) for different periods, eras. For example, if trilobites are found in the Earth's layers, these layers are attributed to the marine sediments of the Palaeozoic era, and if dinosaur residues are found, the layers containing them are attributed to the continental sediments of the Mesozoic era.
- However, it is impossible to find out in this way how many years geological eras, periods, epochs and epochs lasted, i.e. to determine their absolute geological age in thousands or millions of years.
- Attempts to calculate the absolute age of the Earth's layers have been made for a long time, but only in recent years, when physics and chemistry have achieved great success, it was possible to develop methods to accurately measure the time of the distant past.
- Physicists and chemists have discovered that the atoms of some elements - uranium, thorium, radium, etc. - are the most important elements of the past. - change all the time, "disintegrate", forming other elements. The transformation of atoms, or decay, is accompanied by radiation, i.e. the radiation of small charged particles. Therefore, such elements are called radioactive and the process of their transformation is called radioactive decay. It turned out that radioactive decay always occurs at the same rate. It is not affected by high temperature and pressure in the Earth's interior. In science, it is customary to determine the rate of radioactive decay by the time it takes to decay half the amount of element available at the beginning. This time is called the half-life period. It varies from element to element. Half-life of rubidium-87 occurs in 50 billion years, potassium-40 in 1.25 billion years, uranium-238 in 4.52 billion years, radium in 1590 years. Constant decay rates for each radioactive element make it possible to use them as an accurate clock to measure the age of rocks.
- The decay of a type of radioactive uranium (so-called isotopes) produces helium and lead elements. It is estimated that about 90 million years are needed to accumulate 1 G of lead in 100 G of uranium (i.e. 1%). Thus, by determining the percentage of lead in uranium, it is possible to determine how much time has passed since the beginning of the decay process or the beginning of rock formation.
- With the help of radioactive elements, scientists have calculated the age of the Earth, which is defined as at least six billion years! This method is used to determine the age of the oldest rocks on the Earth's surface.
- A very interesting and precise radiocarbon method is used to determine shorter time intervals. They are defined by the age of up to 50 thousand years, and the error does not exceed 400 years. In the tissues of living organisms, along with ordinary carbon (its atomic weight 12) contains a small and constant amount of its isotope or radioactive carbon with atomic weight 14. The half-life of radioactive carbon is short, 5760 years. In the remaining organic residues (bones, trunks of trees sunk in the swamp, tissues, etc.), carbon-14 gradually loses radioactivity. Its amount is very small and can be detected by very precise instruments. It was possible to check this method by studying archeological monuments, the age of which was known from historical documents.
In short, these are the methods that science has at its disposal. There is no doubt that new, even more, precise ways of measuring the time of the distant past will soon be discovered.