Geological Time Scale: Life on earth started about \(4\) billion years ago and humans were one of the last organisms to evolve. We wrack our brains just trying to imagine what would have happened a few hundred years ago. To help us comprehend the full expense of time, scientists have turned to the rocks.

Geologists are now able to describe crucial events in life’s history- from its origin to disastrous extinction events. There was no idea what happened in what order. To know when and how life originated, how it looked, Geological Time Scale was made by geologists. In this article, we will provide detailed information about the geological time scale. Scroll down to learn more!

Geological Time Scale: Definition

Geological Time Scale is the tabulated form showing the sequence and duration of the eras and the periods with their dominant form of life since the beginning of life on the earth. (MODERN’S \({\rm{abc + }}\) of Biology)

History of Geological Time Scale

Let’s have a look at the history of geological time scale:

1. Scientist Nicolas Steno published the first law of stratigraphy in \(1669\).He argued that the layers closer to the surface must be younger than the layers below them. So, he thought, the farther we dig, the older the fossils that we get.
2. Based on Steno’s ideas, Italian geologist Giovanni Arduino in the \(1760{\rm{s}}\) went a step further and began naming the layers of rock. But as rock layers do not appear in this same order all over the world, there was no way for geologists to compare rocks from one location to another.
3. Finally, in \(1819\), English geologist William Smith figured out the solution to this problem- Fossils. Smith could match the ages of ancient organisms by comparing their remains present in the rocks regardless of how far apart they were. For example, Smith realised that fossils of many early species of trilobites are found below ammonite fossils, which are in turn below certain species of shellfish. So, anyplace in the world where we find these first trilobites, we know that we are looking at a rock that is older than ammonites lived.
4. Steno, Arduino and Smith, modern scientists have used these and other clues to create what we call the Geological Time Scale or \({\rm{GTS}}\). The First Geological Time Scale was published in \(1913\) by the British geologist Arthur Holmes.

Geological Time Scale: Description

The Geological Time Scale has been reworked many times to reflect the latest knowledge of Earth’s history. It is organised into \(5\) subgroups: – Eons, Eras, Periods, Epochs and Ages.

Eons are the biggest which are divided into eras and eras further divided into periods, epochs and ages.

1. Eons

i. These are the largest slices of time, ranging from a half-billion to nearly \(2\) billion years long.
ii. Earliest Eon is known as Hadean. It began with the very formation of the Earth itself, around \(4.6\) bya and ends \(4\) bya.
iii. This is the only Eon that does not have any fossils.
iv. During this time there was volcanic activity, cosmic bombardment, temperatures were hot enough to melt rocks.
v. Small amounts of organic carbon have been discovered in Hadean rocks which can be the evidence of earliest life.
vi. After the Hadean eon, the Archaeaneon came when cooling of earth’s crust occurred and continents formed, which ran from \(4\) billion to \(2.5\) billion years ago.
vii. First sign of life appeared.
viii. Mostly \({\rm{C}}{{\rm{O}}_2}\) was there in the atmosphere until a prokaryote called photosynthetic cyanobacteria came into existence.
ix. The Proterozoic eon came after the Archean eon, \(2.5\) billion years ago.
x. Meaning of the Proterozoic eon meaning “earlier life”.
xi. Around this time, photosynthetic bacteria along with some multicellular forms of life came which converted reducing atmosphere into oxidizing one.
xii. Ancestral eukaryotes came during this eon whose cells had a nucleus with organelles wrapped in a membrane.
xiii. Around \(541\) mya, the current Phanerozoic Eon came which means ‘visible life’.
xiv. During this time, trees, dinosaurs, newts, aardvarks, and humans came.

2. Eras

i. The Phanerozoic Eon is divided into three Eras, as, Paleozoic era, Mesozoic era and Cenozoic era.
ii. \(541\) million years ago the Paleozoic era came.
iii. Here diversification of visible life occurred.
iv. This rapid growth and diversity in the ocean were so huge that all the eons that came before it- Hadean, Archaean, Proterozoic are collectively known as the Precambrian.
v. Fossil record of \(25\) mya reveals the appearance of complex animals which had shells, exoskeletons, etc.
vi. Of all these complex animals, Trilobites evolved first.
vii. The Mesozoic era is the age of reptiles, where dinosaurs, pterosaurs and a variety of marine species evolved and many of them including dinosaurs became extinct.
viii. Mammals, modern birds, and flowering plants also evolved during this era.
ix. \(66\) mya, this mesozoic era came to an end with another episode of devastation and the cenozoic era came.
x. The Cenozoic era is our era. It is the era that we are in today and marks the rise of mammals.
xi. By \(40\) million years ago, mammal’s groups like whales, bats, rodents and primates came.
xii. But \(34\) mya, the climate began to change again during which ice started to grow at the poles taking up much of the planet’s water.
xiii. These drier conditions created a new habitat, the grassland, where ancestral horses and antelope were hunted by the earliest cats and dogs.
xiv. \(7\) million years ago species of apes evolved.
xv. \(2.6\) mya, ice expanded even more, and earth entered a glacial period called Ice Age.
xvi. During this age, mammoth, ground sloths evolved.
xvii. Then \(15000\) years ago the climate started warming up.
xviii. By \(11,700\) years ago, modern humans inhabited nearly all the corners of the globe.
xix. This is the era during which mammals, birds, and flowering plants are dominant.

3. Periods- Periods in geological time scale refers to the unit which is smaller than eras but bigger than epochs. All the eras are divided into smaller units called periods. The Palaeozoic era is divided into periods- Permian, Carboniferous, Devonian, Silurian, Cambrian, Ordovician. All the other periods are given in the table below.

4. Epochs- Periods are further divided into epochs like the Quaternary period of Cenozoic era is divided into \(3\) epochs, i.e., Holocene, Pleistocene and Pliocene.

5. Ages- Epochs are divided into even smaller subunits called ages. It is the smallest division of the Geological Time Scale.

The Geological Time Scale: Chart

Below we have provided the chart of geological time scale for your reference:

Summary

From the above discussion, we can conclude that Steno, Arduino and Smith, modern scientists, have contributed to the formation of the Geological Time Scale or GTS. The First Geological Time Scale was published in \(1913\) by the British geologist Arthur Holmes. Geological Time Scale is organised into \(5\) subgroups: – Eons, Eras, Periods, Epochs and Ages. Eons is the largest in the GTS.

Eons are divided into Eras which are further subdivided into Periods, Epochs and Ages.  Earliest Eon is known as Hadean followed by the Archaean eon, Proterozoic eon and then Phanerozoic Eon. The Phanerozoic Eon is divided into three Eras, as: – Paleozoic era, Mesozoic era and Cenozoic era. The Cenozoic era is our era. It is the era that we are in today and marks the rise of mammals. Eras are further divided into periods, i.e., Cenozoic era is divided into Quaternary and Tertiary.

FAQs on Geological Time Scale

Q.1. What are the 5 major divisions of geologic time?
Ans: 5 major divisions of geologic time are- eons, eras, periods, epochs, and ages.

Q.2. What are the 11 periods on the geologic time scale?
Ans: 11 periods on the geologic time scale are-
1. Quaternary
2. Tertiary
3. Cretaceous
4. Jurassic
5. Triassic
6. Permian
7. Carboniferous
8. Devonian
9. Silurian
10. Cambrian
11. Ordovician

Q.3. What is the largest time scale?
Ans: Eon is the largest geological time scale.

Q.4. Which era do we live in?
Ans: Cenozoic era is the era we live in.

Q.5. What period makes 90% of the earth’s history?
Ans: Precambrian time makes up 90% of the earth’s history which includes Hadean, Archean, and Proterozoic eons.

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