Geologic Time Scale

What is Geologic Time Scale?

The geological time scale is a system of chronological dating that relates geological events and processes to the Earth’s history. It is divided into several time intervals or epochs, each with distinct sets of rock layers, fossil evidence, and other geological markers. These epochs are based on major changes in the Earth’s environment, such as mass extinctions, geological events, or evolutionary milestones. The main divisions of the geological time scale are:

1. Eon: The largest division of time, it is divided into Phanerozoic, Proterozoic, and Archean aeons.
2. Era: Divided into Paleozoic, Mesozoic, and Cenozoic eras, it covers major events such as the emergence of life, the rise and fall of dinosaurs, and the evolution of mammals and humans.
3. Period: Divided into smaller intervals such as the Permian, Jurassic, and Miocene periods, it covers specific geological and biological events.
4. Epoch: The smallest division of time, it is defined by changes in the Earth’s climate, fauna, and flora.

The geological time scale is an important tool for understanding the history of the Earth and the evolution of life on our planet. It allows geologists to correlate rock formations and fossils from different parts of the world and provides a framework for studying the complex interactions between the Earth’s physical and biological systems.

Aeons of Geological Time Scale

The geological time scale is a way of dividing the history of the Earth into different periods and epochs based on changes in the rock record, changes in life forms, and changes in climate. The aeons of the geological time scale are the largest divisions of time, each spanning billions of years.

There are four aeons in the geological time scale:

1. Hadean Eon: This aeon spans from the formation of the Earth around 4.6 billion years ago to the end of the Late Heavy Bombardment around 3.8 billion years ago. The Hadean Eon is characterized by the formation of the early Earth, including the differentiation of the core, mantle, and crust and the emergence of the first oceans and atmosphere.
2. Archean Eon: This aeon spans from the end of the Hadean Eon around 3.8 billion years ago to the beginning of the Proterozoic Eon around 2.5 billion years ago. The Archean Eon is characterized by the emergence of life on Earth, including the first prokaryotic cells and the development of photosynthesis.
3. Proterozoic Eon: This aeon spans from the beginning of the Proterozoic Eon around 2.5 billion years ago to the beginning of the Phanerozoic Eon around 541 million years ago. The Proterozoic Eon is characterized by the evolution of complex life forms, including eukaryotic cells, multicellular organisms, and the first animals.
4. Phanerozoic Eon: This aeon spans from the beginning of the Phanerozoic Eon around 541 million years ago to the present day. The Phanerozoic Eon is characterized by the diversification of life on Earth, including the emergence of plants, insects, reptiles, mammals, and birds, as well as major geological events such as the formation of the supercontinent Pangaea, the evolution of dinosaurs, and the development of the human species.

Eras of Geological Time Scale

Each Aeon is subdivided into smaller time periods known as Eras. The time period of Eras is smaller than that of aeons. There are several eras in each aeon. Precambrian Era is included in Proterozoic Eron, while the rest of the three eras are included only in Phanerozoic Aeon.

1. Precambrian Era: This era spans from the formation of Earth about 4.6 billion years ago until the beginning of the Paleozoic Era, around 541 million years ago. During this time, the Earth’s atmosphere and oceans formed, and life evolved from single-celled organisms to complex multi-cellular life forms.
2. Palaeozoic Era: This era lasted from 541 to 252 million years ago and is characterized by the appearance of invertebrate and vertebrate life forms, including fish, amphibians, reptiles, and early land plants. The era ended with a mass extinction event that wiped out many of the dominant species at the time.
3. Mesozoic Era: This era lasted from 252 to 66 million years ago and is known as the “Age of Reptiles.” It was dominated by the evolution and diversification of dinosaurs, birds, and mammals, and ended with a catastrophic event that wiped out the dinosaurs.
4. Cenozoic Era: This era began 66 million years ago and continues to the present day. It is known as the “Age of Mammals” and is marked by the rise of modern mammalian species, including primates, whales, and humans.

Periods of Geological Time Scale

Each era is further divided into periods and epochs, providing a more detailed timeline of Earth’s history. There are 12 distinct periods in GTS. Each era contains multiple periods;

• Palaeozoic Era includes Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.
• The Mesozoic Era includes Triassic, Jurassic, and Cretaceous periods in it.
• Cenozoic Era includes Paleogene, Neogene, and Quarternary Periods in it.

Detail of the Periods of GTS

1. Cambrian Period: The Cambrian period was a geological period that occurred approximately 541 to 485 million years ago. It is widely known as the “Cambrian explosion” due to the sudden appearance of a diverse array of complex organisms with hard exoskeletons. This period marks the beginning of the Paleozoic era and was characterized by the emergence of new animal phyla, the evolution of complex eyes, and the development of early forms of ecosystems. The Cambrian period also saw the formation of the supercontinent Gondwana and the explosion of a wide range of marine organisms, including trilobites, brachiopods, and echinoderms. The end of the Cambrian period was marked by a mass extinction event that wiped out many of the earlier Cambrian life forms.
2. Ordovician period: The Ordovician period is a geologic period that occurred from about 485 to 443 million years ago. It followed the Cambrian period and preceded the Silurian period. The name of the period comes from the ancient Celtic tribe of the Ordovices, who lived in what is now Wales.During the Ordovician period, the Earth was much different than it is today. Most of the land was located near the equator, and there were two large supercontinents, Gondwana and Laurentia. The climate was relatively warm, and there were extensive shallow seas that covered much of the Earth’s surface.Life during the Ordovician period was diverse and complex. Marine life, including trilobites, brachiopods, and graptolites, flourished during this time. The first jawless fish appeared, as well as the first land plants. The Ordovician period also saw the first major diversification of vertebrates.The end of the Ordovician period was marked by a major extinction event, which wiped out up to 85% of marine species. The cause of this extinction is still debated, but it may have been due to a combination of factors, including a drop in sea level and a glaciation event. Despite this mass extinction, life continued to evolve and diversify, leading to the emergence of new species during the following Silurian period.
3. Silurian Period: The Silurian period is a geologic time period that lasted from 443.8 to 419.2 million years ago. It is named after the Silures, a Celtic tribe that lived in Wales where many of the rocks of this period were first studied.During the Silurian period, the supercontinent of Gondwana continued to break apart, and North America and Europe began to move closer together. This led to the formation of the Caledonian Mountains, which stretched from Scandinavia to Scotland.The oceans were dominated by jawless fish and primitive jawed fish, and the first land plants and animals appeared, including millipedes, scorpions, and spiders. Coral reefs also became more widespread, and some of the first modern-day fish, such as sharks and bony fish, evolved during this period.The end of the Silurian period was marked by a mass extinction event, which wiped out many of the early marine animals, including the trilobites. This extinction paved the way for new life forms to evolve during the following Devonian period.
4. Devonian Period: The Devonian period was a geological period that occurred approximately 419.2 million to 358.9 million years ago. It is often called the “Age of Fishes” as it was a time of significant diversification and evolution of fish, including the rise of jawed fish and the development of bony fish. The Devonian period also saw the emergence of the first land-dwelling animals, such as amphibians and insects. The period ended with a mass extinction event that wiped out many marine species, including trilobites, and paved the way for the emergence of new species during the following Carboniferous period.
5. Carboniferous Period: The Carboniferous period was a geological period that lasted from approximately 359 to 299 million years ago. It is named for the abundant coal deposits that were formed during this time, due to the massive growth and subsequent burial of vast forests of plants.During the Carboniferous period, the continents were clustered together in a supercontinent called Pangaea, which was located near the equator. The climate was warm and humid, with frequent rainfalls that supported the growth of the large plants that dominated the landscape.The Carboniferous period was also a time of significant evolutionary diversification, with the emergence of many new plant and animal species. Early reptiles and amphibians evolved during this time, as did the first insects with wings.The Carboniferous period came to an end with a series of global environmental changes, including widespread glaciation, rising sea levels, and a decrease in atmospheric oxygen levels. These changes had a profound impact on life on Earth and set the stage for the subsequent evolution of new species during the Permian period.
6. Permian Period: The Permian period is a geologic period that lasted from about 299 million years ago to 252 million years ago. It is the last period of the Paleozoic era and precedes the Mesozoic era. During this time, the supercontinent of Pangaea began to form and diverse groups of animals, such as amphibians, reptiles, and insects, emerged. The end of the Permian period was marked by the largest mass extinction in Earth’s history, where an estimated 90% of all marine and 70% of all terrestrial species died out.
7. Triassic Period: The Triassic period lasted from approximately 251 to 201 million years ago. It followed the Permian period and was succeeded by the Jurassic period. During the Triassic, the Earth’s landmasses were grouped together in a single supercontinent called Pangaea. The climate was generally hot and dry, and many new species of plants and animals evolved. Some of the notable organisms that lived during this period include the first dinosaurs, early mammals, and marine reptiles such as ichthyosaurs and plesiosaurs. The end of the Triassic was marked by a mass extinction event that wiped out many species, paving the way for the rise of the dinosaurs in the Jurassic period.
8. Jurassic Period: The Jurassic Period lasted from approximately 201.3 million years ago to 145 million years ago. It is the middle period of the Mesozoic Era, which is also known as the Age of Dinosaurs. During the Jurassic Period, the supercontinent Pangaea began to break apart, leading to the formation of new oceans and the development of diverse marine and terrestrial ecosystems.The Jurassic Period is best known for the proliferation of dinosaurs, which dominated the land during this time. Some of the most well-known dinosaurs from the Jurassic Period include the long-necked Brachiosaurus and Apatosaurus, the armoured Stegosaurus, and the carnivorous Allosaurus and Velociraptor. Other notable animals from this time include marine reptiles such as ichthyosaurs and plesiosaurs, as well as the first birds.In addition to the abundant and diverse fauna, the Jurassic Period also saw the evolution of many new plant species, including the first true flowers. The warm and humid climate of the period allowed for the growth of lush forests and widespread vegetation.The end of the Jurassic Period was marked by a series of mass extinctions, possibly caused by volcanic activity and climate change. These extinctions paved the way for the rise of new groups of animals and plants in the subsequent Cretaceous Period.
9. Cretaceous Period: The Cretaceous Period was the last and longest period of the Mesozoic Era. It lasted from 145 million to 66 million years ago and was characterized by the dominance of dinosaurs, particularly the herbivorous ornithischians and the carnivorous theropods. The period saw the emergence of flowering plants, and the evolution of new types of marine animals, including molluscs and plankton. The end of the Cretaceous was marked by a mass extinction event that wiped out the dinosaurs and many other groups of animals. It is thought to have been caused by a combination of factors, including an asteroid impact and volcanic activity.
10. Palaeogene Period: The Palaeogene Period, also known as the Oligocene period, is a geological period that occurred from 33.9 to 23 million years ago. It followed the Eocene Epoch and preceded the Miocene Epoch. During this period, the Earth’s climate was relatively warm, but there were some fluctuations that led to the formation of ice caps in Antarctica. The continents continued to move, and the collision of India with Asia caused the rise of the Himalayan Mountains. Mammals continued to evolve, and some of the earliest primates, including the ancestors of modern apes and humans, appeared during this period. The Palaeocene Period ended with a mass extinction event that affected many marine species.
11. Tertiary Period: The Tertiary Period, also known as the Neogene Period, is a geological time period that started approximately 23 million years ago and ended about 2.6 million years ago. It is the third and last period of the Cenozoic Era, following the Paleogene Period and preceding the Quaternary Period.During the Neogene Period, the Earth underwent significant changes, including the formation of the Himalayas, the opening of the Drake Passage, and the global cooling and drying that led to the spread of grasslands and the extinction of many large mammals.The Neogene Period is divided into two epochs: the Miocene and the Pliocene. During the Miocene epoch, the climate was warm and the Earth was dominated by tropical and subtropical forests. In the Pliocene epoch, the climate began to cool, and grasslands became more prevalent, leading to the evolution of many modern plant and animal species.The Neogene Period is important for its rich fossil record, which includes many important evolutionary transitions, such as the emergence of modern apes, the diversification of early hominids, and the emergence of many modern mammal families.
12. Quarternary Period: The Quaternary Period is the most recent geological period, spanning from 2.6 million years ago to the present. It is characterized by the presence of humans, the widespread glaciation of the northern hemisphere, and the emergence of modern animal and plant species. The Quaternary is divided into two epochs, the Pleistocene and the Holocene. The Pleistocene is marked by multiple glacial and interglacial periods, while the Holocene is the current interglacial period that began around 11,700 years ago. The Quaternary Period is significant as it encompasses the evolution and spread of humans across the globe, and the impact they have had on the environment.

Epochs of Geological Time Scale

Some latest periods are subdivided into epochs.

• The Palaeogene period is divided into Palaeocene, Aeocene, and Oligocene epochs.
• The Neogene period is divided into Miocene and Pliocene epochs.

Detail of the Epochs of GTS

1. Palaeocene Epoch: The Palaeocene epoch is a geologic time period that lasted from about 66 to 56 million years ago. It is the first epoch of the Cenozoic era, following the mass extinction event that marked the end of the Mesozoic era. During the Palaeocene, the Earth’s climate was warm and tropical, and many new plant and animal species emerged, including the first mammals and modern primates. The Palaeocene is also characterized by the diversification of marine life, with the emergence of new species of fish, sharks, and marine reptiles. It was a time of significant geological and biological change, setting the stage for the evolution of the modern Earth.
2. Aeocene Epoch: The Aeocene Epoch is a geological period that occurred between 56 and 33.9 million years ago. During this time, the Earth experienced a period of global warming, resulting in high sea levels and the development of tropical and subtropical forests in many areas. The Eocene was also marked by the diversification and expansion of mammals, including the emergence of the first primates. Some of the key events that took place during the Eocene include the separation of Australia from Antarctica, the collision of India with Asia, and the formation of the Alps and Himalayas mountain ranges.
3. Oligocene Epoch: The Oligocene epoch is a geological time period that occurred from approximately 33.9 to 23 million years ago. It is a part of the larger Paleogene period, which followed the end of the Cretaceous period and the extinction of the dinosaurs. During the Oligocene, the Earth’s climate was relatively warm and tropical but began to cool towards the end of the epoch. This cooling was accompanied by the formation of large ice sheets in Antarctica and the expansion of grasslands and savannas in other parts of the world. Many mammal groups, including primates, evolved and diversified during the Oligocene. Fossils from this time period are found in many parts of the world, including North America, Europe, Asia, and Africa.
4. Miocene Epoch: The Miocene epoch occurred approximately 23.03 to 5.33 million years ago, following the Oligocene epoch and preceding the Pliocene epoch. It is often referred to as the “middle epoch” of the Tertiary period and is characterized by the diversification and evolution of many modern plants and animals. During the Miocene epoch, the Earth was generally warm and humid and saw the emergence of grasslands, savannas, and forests, as well as the spread of many mammal groups, including apes, cats, dogs, horses, and elephants. It is also notable for a series of global climate changes and fluctuations in sea level, as well as major geological events such as the formation of the Alps and the Andes mountain ranges.
5. Pliocene Epoch: The Pliocene epoch occurred from about 5.3 million to 2.6 million years ago. It is the most recent epoch of the Tertiary period and the last epoch of the Neogene period. The Pliocene is characterized by the expansion of grasslands and the emergence of modern human ancestors, including the australopithecines and early members of the Homo genus. The climate during this epoch was relatively warm, and sea levels were high, resulting in the formation of shallow seas and extensive coral reefs. Many of the large mammals that existed during the epoch, such as mammoths, sabre-toothed cats, and giant sloths, went extinct at the end of the epoch, likely due to a combination of climate change and hunting by humans.
6. Pleistocene Epoch: The Pleistocene epoch is a geological time period that began around 2.6 million years ago and ended about 11,700 years ago. It is characterized by a series of ice ages and glacial periods, during which large portions of the Earth’s surface were covered in ice. The Pleistocene epoch is also known for the evolution of many modern animal species, including Homo sapiens. Many of the world’s iconic large mammals, such as mammoths, woolly rhinoceroses, and sabre-toothed cats, roamed the Earth during this time period. The Pleistocene epoch is often considered the last major geological period before the current Holocene epoch.
7. Holocene Epoch: The Holocene epoch is a geological period that began approximately 11,700 years ago and continues to the present day. It is characterized by a relatively stable and warm climate, which has allowed human civilization to flourish. During this epoch, the Earth’s climate has gone through several natural cycles of warming and cooling, but the overall trend has been towards a warmer climate. The Holocene epoch is also marked by significant changes in sea level, the development of agriculture, and the rise of human societies around the world. Today, the Earth is facing a new period of rapid climate change, which many scientists attribute to human activity.