The first stage is the formation of the earth's sphere, and its time limit is about 4600~4200 Ma ago. About 4.6 billion years ago, the primitive earth began to differentiate from the solar nebula, with low temperature, light and heavy elements fused and no layered structure. Once the primitive earth is formed, it is conducive to the continuous accumulation of materials in the solar nebula (falling of meteorites), the volume and mass will increase, and the temperature will also increase due to gravity differentiation and radioactive element transformation (Figure 1-4). When the material in the primitive earth heats up to a molten state, elements such as iron and nickel with high density and low melting point first precipitate and settle to the center of the earth to become iron-nickel nuclei; Low-density Fe-Mg-Al silicate is enriched upward to form mantle and crust. Extensive volcanic eruption increased the thickness of the primitive crust, changed its material composition and structural characteristics, and the liquid and gaseous components with lower density formed the primitive hydrosphere and atmosphere. Since then, the earth has started the interaction between different circles and the frequent exchange of material and energy.
Figure 1-4 Nuclear fission and meteorite impact inside the Earth
The second stage is Archean and Proterozoic, and the time limit is about 4200 ~ 543 Ma. The geological bodies of this period are still preserved in some parts of the earth, but most of them have undergone the transformation of geological processes in the later period, and the situation is complicated. Because of the primitive crust, atmosphere and hydrosphere, the emergence of land and life has created a new era in geological history. Therefore, the geological process in this period is unique, which is of great significance to understand the evolution and development of the earth. The earliest life record on earth is the spherical single-celled bacterial fossils preserved in sedimentary rocks in South Africa 3.8 billion years ago. In the following 2 billion years, bacteria and algae proliferated rapidly, which had an important impact on the evolution of the original atmospheric composition. The photosynthesis of algae produces oxygen, which oxidizes the original atmosphere to form gases such as CO2 and N2, and the original atmospheric composition begins to transform into modern atmospheric composition. The early evolution of organisms was extremely slow, and it was not until eukaryotes (65.438+0.8 billion years ago) and even mollusks (600 million years ago) appeared in the middle and late Proterozoic that the marine biosphere was initially formed. From about 4.2 billion years ago, islands made of volcanic eruption materials appeared as the earliest land, and external geological processes such as weathering, denudation, transportation, sedimentation and diagenesis began to occur on the earth, forming sedimentary rocks (sedimentary rocks exposed in southwest Australia, containing zircon for 4.2 billion years). Since Proterozoic (2.5 billion years ago), the difference between land and sea has been remarkable, and the ancient continent has been evolving and growing under the interaction and influence of continental crust and ocean crust.
The third stage is Phanerozoic, with a time limit of 543Ma. Phanerozoic lasted for a relatively short time, but during this period, life was extremely prosperous, geological evolution was very rapid, and geological processes were rich and colorful. In addition, geological bodies are all over the world and widely preserved, making it extremely possible to observe and study them, becoming the main research object of geological science and laying the basic theory and knowledge of geology.