The Earth has been formed for 4.6 billion years. How do geologists understand the Earth's past history? One important task is to analyze the strata. Strata generally refer to layered rocks formed during geological history, including sedimentary rocks, igneous rocks, and metamorphic rocks formed from the transformation of the two. For example, the oldest sedimentary rock layer currently found on Earth dates back to 3.9 billion years ago, which proves that water and atmosphere existed on Earth at least 3.9 billion years ago. Because water existed, weathered rock debris could be deposited in water.
Strata formed between 3.9 billion and 2.54 billion years ago have undergone significant metamorphism due to their long age and the impact of several major crustal movements and high-temperature magmatic activities. This resulted in deformed rock structures, harder rock textures, and more complex rock compositions. This period often produced large-scale sedimentary iron deposits, such as the Anshan Iron Mine in Liaoning. The surface temperature during this period was high, around 70°C (compared to the current average of 15°C), and there was a lack of oxygen, making it unsuitable for the survival of most organisms but favorable for the growth of cyanobacteria, which could form fossils in the strata. However, due to the limited variety of cyanobacteria at that time, they played a minor role in stratigraphic classification. Additionally, the strata contained numerous volcanic eruption materials, along with frequent and large-scale crustal movements, resulting in inconsistent strata. Therefore, geologists had to use radioactive elements in the strata to determine their ages. This period is called the Archean Eon.
Between 2.54 billion and 541 million years ago, cyanobacteria reached their peak development on Earth. They often consolidated marine debris around themselves, forming spectacular stromatolites and even reefs. Additionally, eukaryotic algae and multicellular algae appeared, and by about 600 million years ago, primitive animals also emerged. However, at that time, the number and types of animals were very limited, consisting only of sponges from the Porifera phylum, jellyfish from the Cnidaria phylum, and worms from the Annelida phylum. Due to the relatively weak crustal movements and magmatic activities during this period, the strata underwent less metamorphism, allowing geologists to obtain more information about ancient climate, geography, environment, crustal movements, and magmatic activities from the strata. This period differed significantly from the Archean Eon in all aspects, so paleontologists called it the Proterozoic Eon.
Since 541 million years ago, the Phanerozoic Eon has begun. A large number of organisms appeared after the Cambrian Explosion, leaving numerous fossils in the strata. The newer the strata, the more complex and diverse fossils can be found. Scientists can infer the ancient biota, climate, geography, and environment, as well as the crustal movements, magmatic activities, sea-land changes, and the formation of sedimentary mineral deposits from the strata and fossils, thereby exploring the history of the Earth. The study of strata is closely related to fossil research and is the foundation of geological science, making it a separate discipline - stratigraphy.