Geologic Time Scale: Eons, Eras, Periods and Epochs

The geologic time scale is a complex and fascinating system for understanding Earth's history and the evolution of life. It is based on the physical and biological evidence preserved in the rock record, which has been accumulated over centuries of geological research.

Geologic Time Scale: Major Eons, Eras, Periods and Epochs

The Geologic Time Scale is divided into four eons, ten eras, 22 periods, and several epochs and ages. Each eon, era, period, and epoch is defined by major geological or paleontological events. The eons are the Hadean, Archean, Proterozoic, and Phanerozoic. The Phanerozoic Eon is the eon of visible life, and is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.

Eons are the largest units of geologic time, spanning hundreds of millions to billions of years, and each represents a significant phase in Earth's history. Eon is the equivalent of a chronostratigraphic eonothem. The four eons are:

The Hadean Eon

Hadean Eon (4.54 to 4.00 billion years ago): The Hadean Eon is the earliest eon in Earth's history, and is named after Hades, the Greek god of the underworld. It was a time  of intense volcanic activity and bombardment by asteroids and comets. The Earth's crust was forming during this time, and the atmosphere was composed of mostly carbon dioxide and water vapor.

The Archean Eon

Archean Eon (4.00 to 2.50 billion years ago): is named after the Greek word "arkhaios," meaning "ancient." It was a time of great change on Earth, as the planet cooled and life began to emerge. The first single-celled organisms appeared during this eon, as well as the first cyanobacteria, which began to produce oxygen.

The Proterozoic Eon

Proterozoic Eon (2.50 to 0.541 billion years ago): is named after the Greek words "protos," meaning "first," and "zoe," meaning "life."  The Proterozoic Eon was a time of great diversification of life, with the first animals appearing during this time. The Proterozoic Eon also saw the development of the first multicellular organisms, as well as the first complex ecosystems.

The Phanerozoic Eon

Phanerozoic Eon (541 million years ago to present): is the eon of visible life, and is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. The Phanerozoic Eon is named after the Greek words "phaneros," meaning "visible," and "zoe," meaning "life." It is the eon in which we live, and is characterized by the emergence of all major groups of plants and animals.

Eras

Eras are subdivisions of eons, and they span tens to hundreds of millions of years. Eras are defined by major changes in the Earth's climate, geography, and the diversity of life. Era is the equivalent of a chronostratigraphic erathem. The three eras of the Phanerozoic Eon are: 

The Paleozoic Era

The Paleozoic Era (541 to 252 Ma): The Paleozoic Era spans from 541 million years ago to 252 million years ago. It is also known as the "age of ancient life." During this time, the Earth's continents were combined into a single supercontinent called Pangaea. The Paleozoic Era is known for the rise of invertebrates, fish, amphibians, reptiles, and the first land plants. The Paleozoic Era is named after the Greek words "palaios," meaning "ancient," and "zoe," meaning "life." It was a time of great change on Earth, as life moved from the oceans onto the land.

The Mesozoic Era

The Mesozoic Era (252 to 66 Ma): The Mesozoic Era spans from 252 million years ago to 66 million years ago.  It is also known as the "age of dinosaurs." During this time, the dinosaurs were the dominant land animals on Earth. The Mesozoic Era is named after the Greek words "mesos," meaning "middle," and "zoe," meaning "life." It was a time when the dinosaurs dominated the planet. The Mesozoic Era also saw the rise of mammals and birds

The Cenozoic Era 

The Cenozoic Era (66 Ma to present). The Cenozoic Era began 66 million years ago and continues to the present day. It is also known as the "age of mammals." During this time, the continents have drifted to their present positions, and the mammals have diversified to fill many different niches on Earth. The Cenozoic Era is named after the Greek words "kainos," meaning "recent," and "zoe," meaning "life." It is a time of great diversity among mammals, and is also marked by the rise of human civilization.

Periods

Periods are major subdivisions of geologic time. Geologic periods are subdivisions of eras. They are defined based on major changes in the fossil record, which reflect changes in the Earth's environment and the evolution of life. Geologic periods are typically hundreds of millions of years long.  Geologic periods are defined based on changes in the fossil record, as well as changes in the Earth's physical features, such as its climate and geography. The Phanerozoic era is divided into 12 periods:

Paleozoic Periods

• Cambrian Period (541-485.4 million years ago). The Cambrian Period is often called the "Age of Trilobites," because these marine arthropods were the most common animals of the time. Other important lifeforms that evolved during the Cambrian Period include the first vertebrates and the first land plants.
  
• Ordovician Period (485.4-443.8 million years ago). The Ordovician Period was a time of continued diversification of life on Earth. The first coral reefs appeared during this time, and fish became more common.
  
• Silurian Period (443.8-419.2 million years ago). The Silurian Period is often called the "Age of Fishes," because fish became the dominant vertebrates of the time. The first jawed fish appeared during this time, as well as the first land animals, such as insects and scorpions.
  
• Devonian Period (419.2-358.9 million years ago). The Devonian Period was a time of great change on Earth. The first amphibians appeared during this time, and land plants began to spread widely. The Devonian Period is also known for its giant fish, such as Dunkleosteus.
  
• Carboniferous Period (358.9-298.9 million years ago). The Carboniferous Period is often called the "Age of Coal Forests," because the vast amounts of coal that we use today were formed from the plants that lived during this time. The Carboniferous Period was also a time of diversification for reptiles and amphibians.
  
• Permian Period (298.9-252.2 million years ago). The Permian Period was the last period of the Paleozoic Era. It was a time of great change on Earth, as the climate became drier and the continents began to drift together. The Permian Period ended with the largest mass extinction in Earth's history, which killed off about 90% of all marine life.
  

Mesozoic Periods

• Triassic Period (252.2-201.3 million years ago). The Triassic Period was a time of recovery from the Permian mass extinction. The first dinosaurs appeared during this time, as well as the first mammals and birds.
  
• Jurassic Period (201.3-145 million years ago). The Jurassic Period is often called the "Age of Dinosaurs," because dinosaurs were the dominant land animals of the time. The Jurassic Period was also a time of great change in the Earth's environment, as the continents continued to drift and the climate became warmer.
  
• Cretaceous Period (145-66 million years ago). The Cretaceous Period was the last period of the Mesozoic Era. It was a time of continued diversification for dinosaurs and other animals. The Cretaceous Period ended with the second-largest mass extinction in Earth's history, which killed off all dinosaurs except for birds.
  

Cenozoic Periods

• Paleogene Period (66-23.03 million years ago). The Paleogene Period is the first period of the Cenozoic Era. It was a time of recovery from the Cretaceous mass extinction. Mammals began to diversify during this time, and the first primates appeared.
  
• Neogene Period (23.03-2.58 million years ago). The Neogene Period is the second period of the Cenozoic Era. It was a time of continued diversification for mammals and other animals. The first hominins appeared during this time, as well as the first grasses.
  
• Quaternary Period (2.58 million years ago to present). The Quaternary Period is the last period of the Cenozoic Era. It is the period in which humans evolved and became the dominant species on Earth. The Quaternary Period is also known for its ice ages.
  

Epochs

Epochs are subdivisions of geologic periods. They are also defined by changes in the Earth's rock record, but these changes are generally less significant than those that define periods. Geologic epochs typically last for hundreds of thousands to millions of years.  Epoch is the equivalent of a chronostratigraphic series. There are also 11 subepochs/subseries which are all within the Neogene and Quaternary.

Examples of geologic epochs

• Paleocene epoch: (66.0-56.0 million years ago) The Paleocene epoch was the first epoch of the Cenozoic Era. It began with the extinction of the dinosaurs and ended with the rise of the first mammals and birds.
• Eocene epoch: (56.0-33.9 million years ago) The Eocene epoch was a time of warm, wet climates and lush forests. Mammals diversified rapidly during this time, and the first primates appeared.
• Oligocene epoch: (33.9-23.0 million years ago) The Oligocene epoch was a time of cooling and drying climates. The first hominins appeared during this time.
• Miocene epoch: (23.0-5.3 million years ago) The Miocene epoch was a time of continued cooling and drying climates. The first grasses appeared during this time, and hominins began to walk upright.
• Pliocene epoch: (5.3-2.6 million years ago) The Pliocene epoch was a time of relatively warm and stable climates. Hominins became more diverse during this time, and the first Homo species appeared.
• Pleistocene epoch: (2.6 million years ago to present) The Pleistocene epoch is the current epoch. It is characterized by a series of ice ages and interglacial periods. Humans evolved during this time, and the first Homo sapiens appeared.
• Holocene epoch: (11,700 years ago to present) The Holocene epoch is the current interglacial period. It has been a time of relatively stable climates and human expansion.

Ages

Ages are the smallest subdivisions of geologic time. They typically last only a few million years and are defined by specific changes in the fossil record or climate.

Examples of geologic ages:

• Meghalayan age: (4,200 years ago to present) The Meghalayan age is the current geologic age.  It is characterized by a relatively stable climate and a period of human-driven environmental change.
• Northgrippian age:  (8,270-4,200 years ago) The Northgrippian age was a time of relatively stable climate. Humans began to develop agriculture and build cities during this time.