Earth's geologic age is organized into a hierarchical system that reflects significant changes in the planet's geology, climate, and life forms over time. This organization includes several primary divisions:
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Eons: The longest time intervals, which are divided into eras. The current eon is the Phanerozoic, which began about 541 million years ago.
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Eras: Each eon is further divided into eras, characterized by major events in Earth's history, such as mass extinctions or significant geological changes. For example, the Phanerozoic Eon contains three eras: Paleozoic, Mesozoic, and Cenozoic.
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Periods: Eras are divided into periods, which are defined by specific events in geological history, such as the appearance of particular life forms or notable geological developments.
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Epochs: Periods can be further divided into epochs, focusing on finer details in the geologic record, such as climatic changes or evolutionary milestones.
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Age: Each epoch can be divided into ages, which represent smaller time frames characterized by specific changes in the Earth’s geology or biota.
The organization also considers significant geological and biological events:
- Ice Ages: Major glacial and interglacial periods that have dramatically affected Earth's climate, sea levels, and ecosystems.
- Comet Impacts: Events such as mass extinctions caused by asteroid or comet impacts, which play a crucial role in shaping biological evolution and geological change.
- Solar Activity: Fluctuations in solar radiation can influence Earth's climate and geological processes over millions of years.
- Continental Drift: The movement of Earth's continents over geological time, impacting climate, ocean currents, and the evolution of life.
The system of geological time is flexible to accommodate new discoveries, but generally, significant changes (like those mentioned above) at intervals of roughly 10 million years may provide a basis for defining new units within this hierarchical structure. Each division reflects a combination of major tectonic events and predominant life forms that characterize each time unit, painting a picture of Earth's dynamic history.