Geologic time of earth is the vast period that has passed since the beginning of the Earth. The history of the planet Earth goes back as far as 4.5 billion years. It took many more years for life to form on Earth. About 3.5 billion years ago, first, living organisms were originated in the oceans. According to scientists, marine life evolved for another million years, and some later spread to land. The first to spread on land were plants, not animals. Man, compared to plants and animals, was born much later.
In typical day-to-day life, we use time frames such as hours, days, weeks, and years. Something that happened a hundred years ago is called old while something that occurred a thousand years ago is known as ancient. However, in terms of geologic time, the duration is different. There, a period of 1000 million years ago, is probably called old. Hence, this long time is difficult to interpret using ordinary time frames. Because of this, geologists use a different time scale called the geologic time scale. Here, Geologists divide the entire geologic time into different magnitude units. Simply, it is the calendar used by geologists. However, the geologic time scale is a dynamic tool, and it can always update with new findings.
Table of Contents
Structure of the Geologic time of earth scale
In descending order, the geologic time scale divides into several subdivisions, namely Eons, eras, periods, and epochs. On this scale, the earliest events are recorded at the bottom.
The Geologic Time Scale v.5.0.
| Eon | Era | Period | Millions of years ago | Key geological and biological events |
|---|---|---|---|---|
| Phanerozoic | Cenozoic | Quaternary | 2.58 | Humans develop |
| Neogene | 23.03 | Ice age begins | ||
| Paleogene | 66 | The rise of the mammals | ||
| Mesozoic | Cretaceous | 145 | Extinction of dinosaurs | |
| Jurassic | 201.3 | First birds | ||
| Triassic | 251.9 | First mammals First dinosaurs |
||
| Paleozoic | Permian | 298.9 | Formation of Pangaea | |
| Pennsylvanian | 323.2 | Abundant coal-forming swamps | ||
| Mississippian | 358.9 | First reptiles | ||
| Devonian | 419.2 | First amphibians | ||
| Silurian | 443.8 | First upright-growing land plants | ||
| Ordovician | 485.4 | First jawless fish | ||
| Cambrian | 541 | Golden age of invertebrates | ||
| Precambrian | 4000 | Earliest shelled animals Earliest fossil records of life |
||
Essentials of Geology, Frederick K. Lutgens, Edward J. Tarbuck, 11th edition
Eons
First, geologic time divides into Eons. An Eon is a substantial period. The geologic time of the Earth can divide into three Eons. Namely, Archean, Proterozoic, and Phanerozoic.
The first two Eons are commonly known as the Precambrian (4000 – 541 million years ago). It comprises the first 4 billion years of Earth’s history. This period accounted for 88 percent of Earth’s history. During this period, the Earth’s continental crust was created. Moreover, first, living organisms appeared in the Precambrian oceans. Phanerozoic is the Eon in which we live today. And it began about 541 million years ago.
Eras
Next, Eons again split into eras. The Phanerozoic Eon that we live in today divide into three main eras; Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). As the names suggest, these eras experienced a profound worldwide change in life forms.
The Paleozoic era (541 -251.9 million years ago) saw a variety of new life forms. During the early Paleozoic era, all major invertebrate groups formed in the oceans. Invertebrates are creatures without backbones such as jellyfish, worms, and mollusks. These life forms diversified dramatically. Later in this era, insects and plants migrated into lands, and the first amphibians emerged on the Earth.
The Mesozoic era (251.9 – 66 million years ago) is often termed the “Age of Reptiles.” During this period, reptiles, especially Dinosaurs, ruled the Earth. However, at the end of this era, Dinosaurs and many other giant reptiles were extinct from Earth.
During the Cenozoic era (66 million years ago – present), mammals became the dominant life form. Therefore, this era is commonly known as the “Age of Mammals.” Also, this era saw the emergence of the first flowering plants.
Periods
Each era again subdivides into periods. Compared to eras, each period is distinguished by a slightly less profound change in life forms.
Epochs
Finally, each period subdivides into smaller units called epochs. There are seven epochs in the Cenozoic era. Among the seven, the last three epochs are the most notable periods in the realm of human history. During the Tertiary period, humans were genetically separated from their apelike predecessors. In the Pleistocene epoch, human evolution saw a significant change. Subsequently, the genus homo emerged on the African continent. Also, the Pleistocene epoch experienced the last ice age. During this time, 30 percent of the Earth’s surface was covered with thick glaciers. However, the beginning of the Holocene epoch marked the end of the last ice age. And this new warmer climate induces agriculture and led to the Agricultural Revolution. At present, we live in the Holocene epoch.
Time scale of Cenozoic Era
| Era | Period | Epoch |
|---|---|---|
| Cenozoic | Quaternary | Holocene (0.012 million years ago - present) |
| Pleistocene (2.58 million – 0.012 million years ago) | ||
| Tertiary | Pliocene (5.3 – 2.58 million) | |
| Miocene (23.0 -5.3 million years ago) | ||
| Oligocene (33.9 – 23.03 million years ago) | ||
| Eocene (56 – 33.9 million years ago) | ||
| Paleocene (66 – 55 million years ago) |
Essentials of Geology, Frederick K. Lutgens, Edward J. Tarbuck, 11th edition
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