Think about how the geologic time scale was created and how it is divided. Then answer the following questions.
How did scientists form the geologic time scale?
How is this scale organized?
What processes changed the Earth's environment during Precambrian time?
The geologic time scale was formed by scientists studying rock layers and fossils found in different locations around the world. They noticed that certain fossils were always found in specific layers of rock and that similar rocks and fossils could be found in different locations. This led them to understand that the rocks and fossils could be used to determine the relative ages of different layers and create a timeline of Earth's history.
The geologic time scale is organized into different units of time called eras, periods, and epochs. The largest division is the eon, which is further divided into eras. Each era is then divided into shorter periods, which are further divided into epochs. The scale starts with the Precambrian eon, followed by the Phanerozoic eon, which is divided into the Paleozoic, Mesozoic, and Cenozoic eras.
During the Precambrian time, several processes changed the Earth's environment. One of the most significant was the development of photosynthetic organisms, such as cyanobacteria, which released oxygen as a byproduct of photosynthesis. This led to the oxygenation of Earth's atmosphere and the subsequent diversification of life forms. Other processes during Precambrian time included the formation and breakup of supercontinents, volcanic activity, the evolution of multicellular organisms, and the development of complex ecosystems.
To understand how scientists formed the geologic time scale, it is important to know that the Earth's history is divided into different time periods based on significant geological and biological events. The geologic time scale was created through a combination of methods including relative dating, radiometric dating, and a deep understanding of Earth's rock layers and fossils.
1. Relative dating: Scientists used the principle of superposition, which states that in an undisturbed sequence of sedimentary rock layers, the oldest rocks are at the bottom and the youngest are at the top. By studying the different rock layers, scientists could determine the relative ages of these rocks and the events that occurred during each time period.
2. Radiometric dating: Scientists also utilized radiometric dating, a method that measures the decay of radioactive isotopes in rocks and minerals. By comparing the ratio of parent and daughter isotopes, they could calculate the absolute age of the rock or mineral and assign it to a specific time interval.
3. Fossils: Fossils provide important clues about past life forms and the sequence of their appearance and disappearance on Earth. By studying the types of fossils found in different rocks layers, scientists could determine the relative ages and establish connections between different time periods.
The geologic time scale is organized into divisions called eras, periods, epochs, and ages. It is hierarchical, meaning that larger divisions (eras) contain smaller divisions (periods), and so on. Here is the organization of the geologic time scale, from largest to smallest division:
1. Eon: The largest division of time, representing billions of years. The geologic time scale currently recognizes four eons: Hadean, Archean, Proterozoic, and Phanerozoic.
2. Era: Divisions of eons, representing hundreds of millions of years. The Phanerozoic eon is further divided into three eras: Paleozoic, Mesozoic, and Cenozoic.
3. Period: Divisions of eras, representing tens of millions of years. The Paleozoic era, for example, contains periods such as the Cambrian, Ordovician, Silurian, etc.
4. Epoch: Divisions of periods, representing millions of years. Epochs are not always evenly defined or present in every period.
5. Age: The smallest divisions of the geologic time scale, representing hundreds of thousands to millions of years. Ages are often named after significant rock formations or key events.
During Precambrian time (which spans nearly 4 billion years), the Earth experienced several significant changes in its environment. These changes include the formation and breakup of supercontinents, the appearance of oxygen in the atmosphere through the process of photosynthesis, the emergence of early life forms, the evolution of multicellular organisms, and the development of complex ecosystems. Additionally, drastic changes in climate, sea levels, and the composition of the atmosphere also occurred during the Precambrian era.