- The geological time scale is a vital tool for scientists studying the history of the Earth, including the evolution of life and the changes in Earth’s climate and geology over time. By dividing time into eons, eras, periods, and epochs, scientists can organize and study the events that have occurred throughout Earth’s history.
- Fossils provide a record of past life on Earth, and the distribution of fossils through time can help scientists identify when certain organisms evolved or went extinct.
- The subdivisions of the geological time scale are arranged in a hierarchical manner with eons being the largest units of time and epochs being the smallest.
- The geological time scale provides a framework for the study of Earth’s history and the evolution of life on the planet.
- By dividing geological time into smaller, more manageable units, scientists can better understand the sequence of events and the relationships between them.
Division of Geological Time Scale
- It’s important to note that the Geological Time Scale (GTS) is a way to divide Earth’s history into different time intervals based on significant geological events and changes. This allows scientists to study and understand the Earth’s history and how it has evolved over time.
Here’s a summary of the different subunits of the GTS:
Eon:
The largest time period of the GTS, represents billions of years. There are only four eons in Earth’s history: the Hadean, Archean, Proterozoic, and Phanerozoic.
Era:
A division of an eon, representing tens to hundreds of millions of years. The Phanerozoic eon, which began about 541 million years ago, is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.
Period:
A division of an era, representing millions of years to tens of millions of years. For example, the Mesozoic era is divided into three periods: the Triassic, Jurassic, and Cretaceous.
Epoch:
A division of a period, representing hundreds of thousands of years to tens of millions of years. The Cenozoic era is divided into three epochs: the Paleogene, Neogene, and Quaternary.
It’s worth noting that the boundaries between these subunits of the GTS are not always well-defined, and may vary depending on the region being studied. The GTS is constantly being updated and revised as new data and discoveries are made.
Hadean Eon
- The Hadean eon (4,540 – 4,000 mya) represents the time before a reliable (fossil) record of life.
- Temperatures are extremely hot, and much of the Earth was molten because of frequent collisions with other bodies, extreme volcanism, and the abundance of short-lived radioactive elements.
- A giant impact collision with a planet-sized body named Theia (approximately 4.5 billion years ago) is thought to have formed the Moon.
- The moon was subjected to Late Heavy Bombardment (LHB – lunar cataclysm – 4 billion years ago).
- During the LHB phase, a disproportionately large number of asteroids are theorized to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars.
- Volcanic outgassing probably created the primordial atmosphere and then the ocean.
- The early atmosphere contained almost no oxygen.
- Over time, the Earth cooled, causing the formation of a solid crust, leaving behind hot volatiles which probably resulted in a heavy CO2 atmosphere with hydrogen and water vapor.
- Liquid water oceans exist despite the surface temperature of 230° C because, at an atmospheric pressure of above 27 atmospheres, caused by the heavy CO2 atmosphere, water is still liquid.
- As the cooling continued, dissolving in ocean water removed most CO2 from the atmosphere.
- Hydrogen and helium are expected to continually escape (even to the present day) due to atmospheric escape.
Archean Eon
- The beginning of life on Earth and evidence of cyanobacteria date to 3500 mya.
- Life was limited to simple single-celled organisms lacking nuclei, called Prokaryota.
- The atmosphere was without oxygen, and the atmospheric pressure was around 10 to 100 atmospheres.
- The Earth’s crust had cooled enough to allow the formation of continents.
- The oldest rock formations exposed on the surface of the Earth are Archean.
- Volcanic activity was considerably higher than today, with numerous lava eruptions.
- The oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.
- By the end of the Archaean, plate tectonics may have been similar to that of the modern Earth.
- Liquid water was prevalent, and deep oceanic basins are known to have existed
- The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia.
- The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria.
Proterozoic Eon
- It is the last eon of the Precambrian “supereon”.
- It spans from the time of the appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life (such as corals) on Earth.
- Bacteria begin producing oxygen, leading to the sudden rise of life forms.
- Eukaryotes (have a nucleus), emerge, including some forms of soft-bodied multicellular organisms.
- Earlier forms of fungi formed around this time.
- The early and late phases of this eon may have undergone Snowball Earth periods (the planet suffered below-zero temperatures, extensive glaciation, and as a result drop in sea levels).
- Snowball Earth: The Snowball Earth hypothesis proposes that Earth’s surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago).
- It was a very tectonically active period in the Earth’s history.
- It featured the first definitive supercontinent cycles and modern orogeny (mountain building).
- It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic.
- In the late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma).
Phanerozoic Eon
- The boundary between the Proterozoic and the Phanerozoic eons was set when the first fossils of animals such as trilobites appeared.
- Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 541 million years ago.
- Plant life on land appeared in the early Phanerozoic eon.
- Complex life, including vertebrates, begin to dominate the Earth’s ocean.
- Pangaea forms and later dissolves into Laurasia and Gondwana.
- Gradually, life expands to land and all familiar forms of plants, insects, animals and fungi begin appearing.
- Birds, the descendants of dinosaurs, and more recently mammals emerge.
- Modern animals—including humans—evolve at the most recent phases of this eon (2 million years ago).
The Phanerozoic eon is divided into three eras:
- The Palaeozoic, an era of arthropods, amphibians, fishes, and the first life on land;
- The Mesozoic, which spanned the rise, reign of reptiles, the climactic extinction of the non-avian dinosaurs, the evolution of mammals and birds; and
- The Cenozoic, which saw the rise of mammals.
- The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic, which are further subdivided into 12 periods.
Frequently Asked Questions (FAQs)
1. What is the Geological Time Scale, and why is it important?
Answer: The Geological Time Scale is a framework that divides Earth’s history into distinct intervals based on significant geological and biological events. It helps scientists organize and understand the vast expanse of Earth’s history, providing a chronological sequence of major events such as mass extinctions, evolutionary developments, and geological processes. This scale is crucial for studying the history of life on Earth and for interpreting geological processes that have shaped the planet over billions of years.
2. How is the Geological Time Scale divided, and what are the major units?
Answer: The Geological Time Scale is divided into hierarchical units, with the largest units being eons, followed by eras, periods, epochs, and ages. The current division of time includes the Phanerozoic Eon (the most recent eon), which is further divided into the Cenozoic, Mesozoic, and Paleozoic eras. Each era is then subdivided into periods, and periods into epochs. For example, the Cenozoic Era includes the Paleogene and Neogene periods, and the Quaternary epoch, where we find the present Holocene epoch. This hierarchical structure allows scientists to categorize and discuss specific intervals in Earth’s history.
3. How do scientists determine the ages of rocks and events in the Geological Time Scale?
Answer: Scientists use various methods to determine the ages of rocks and events in the Geological Time Scale. One common method is radiometric dating, which involves measuring the decay of radioactive isotopes in rocks. For example, the decay of uranium to lead is often used for dating rocks. Fossils are another crucial tool for dating, as certain organisms existed only during specific time periods. Additionally, stratigraphy, the study of rock layers and their sequence, helps establish the relative ages of rocks and events. By combining these methods, scientists create a comprehensive timeline of Earth’s history in the Geological Time Scale.
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