Maarg Manthan · Topic 1.13

Geological Time Scale

Geography › Earth's Origin, Interior and Motions · Topic 1.13

The geological time scale divides the 4.6 billion years of Earth history into nested units: eons, eras, periods, epochs and ages. The Phanerozoic eon, the last 539 million years, is divided into the Paleozoic, Mesozoic and Cenozoic eras. The boundaries between units mark major changes in rocks and fossils. Ages are fixed by relative methods, such as the order of rock layers and index fossils, and by absolute methods, mainly radiometric dating, which uses the decay of radioactive isotopes.

5 min read · UPSC Prelims · Geography · Earth's Origin, Interior and Motions

At a Glance

Eons Hadean, Archean, Proterozoic, Phanerozoic
Phanerozoic eras Paleozoic, Mesozoic, Cenozoic
Present epoch Holocene
Relative dating Layers and index fossils
Absolute dating Radioactive decay
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic gives the formal framework for the history of the Earth that was narrated in the topic on geological history. It also leads to the second part of the macro, which turns from time and life to the interior of the Earth. Previous topic: Origin of Life on Earth. Next topic: Interior of the Earth: Sources of Information (Direct and Indirect Evidence).

How Is the Geological Time Scale Organised?

In short: geological time is divided into units of falling size, the largest being the eon and the smallest the age.

Eon: the largest unit. There are four: Hadean, Archean, Proterozoic and Phanerozoic. The first three together are the Precambrian.
Era: a division of an eon. The Phanerozoic has three: Paleozoic (‘old life’), Mesozoic (‘middle life’) and Cenozoic (‘new life’).
Period: a division of an era, such as the Jurassic.
Epoch: a division of a period, such as the Pleistocene.
Age: the smallest division, of an epoch.

The boundaries are set by the International Commission on Stratigraphy, and many of them match big changes in the fossil record such as mass extinctions.

What Are the Eras and Periods of the Phanerozoic?

Era	Period (start, million years ago)	Key events
Paleozoic	Cambrian (539)	Explosion of marine animal groups; trilobites
Paleozoic	Ordovician (485)	First jawless fish; ends with a mass extinction
Paleozoic	Silurian (444)	First land plants with vascular tissue
Paleozoic	Devonian (419)	Age of fishes; first amphibians
Paleozoic	Carboniferous (359)	Coal forests; first reptiles
Paleozoic	Permian (299)	Pangaea; Gondwana ice age; the largest mass extinction ends the period
Mesozoic	Triassic (252)	First dinosaurs and mammals
Mesozoic	Jurassic (201)	Dinosaurs dominate; first birds
Mesozoic	Cretaceous (145)	Flowering plants; dinosaurs end at 66
Cenozoic	Paleogene (66)	Rise of mammals; Himalayas begin to rise
Cenozoic	Neogene (23)	Grasslands; early human ancestors
Cenozoic	Quaternary (2.6)	Ice ages; modern humans

The Cenozoic epochs are the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene. The Pleistocene (2.6 million years to 11,700 years ago) was the main glacial epoch, and the Holocene is the warm epoch of today. The dates are rounded and are revised from time to time, so the latest chart of the International Commission on Stratigraphy should be checked before use in an answer.

How Is the Age of Rocks Found?

Relative dating: by the law of superposition, in undisturbed layers the lower beds are older than the upper beds. Rocks can also be matched between places using index fossils, which are the remains of widespread species that lived for a short time.
Absolute (radiometric) dating: radioactive isotopes decay at a steady rate, measured by the half-life. By comparing the parent and daughter isotopes in a rock, the time since it formed can be found. Uranium-lead dating is used for the oldest rocks, potassium-argon for volcanic rocks, and radiocarbon (carbon-14) for organic remains up to about 50,000 years old.
Other tools: tree rings, ice cores and varves (yearly layers of lake mud) give detailed records of recent times.

The age of the Earth, about 4.54 billion years, was fixed by radiometric dating of meteorites. This number is referred to in The Earth’s Origin: Early Theories and the Modern Big Bang Theory.

How Does This Link to India?

The time scale also helps to organise the geology of India. The Archean rocks form the Peninsular shield, the Gondwana coal beds of the Damodar and Mahanadi valleys belong to the Permian, the Deccan Traps erupted near the end of the Cretaceous, and the Himalayas rose in the Cenozoic. The mountain building of the Himalayas is described in Landforms of the World: Fold Mountains, Block Mountains, Volcanic Mountains. As an analytical point, the scale lets scientists place such separate events into one sequence, and thus compare different regions of the world.

Why Is This Topic Important?

Table Dates and fossils are asked
Methods Radiometric dating
Link India’s geology

What Came Next?

The next topic starts the study of the interior of the Earth and the evidence from which it is known. Next topic: Interior of the Earth: Sources of Information (Direct and Indirect Evidence).

Key People and Terms

Eon Largest unit of time
Half-life Time for half a radioactive isotope to decay
Index fossil Fossil that dates a rock layer
Superposition Lower layers are older
Phanerozoic Eon of visible life
Holocene Present epoch

Exam Corner

Points to Remember

Phanerozoic begins 539 million years ago.
Eras: Paleozoic, Mesozoic, Cenozoic.
Carbon-14 dating works up to about 50,000 years.
Mesozoic: Triassic, Jurassic, Cretaceous.
Earth’s age: about 4.54 billion years.

Do Not Confuse With

Era and period Era is the larger unit
Relative and absolute dating Order of layers versus exact age
Pleistocene and Holocene Ice age epoch versus warm epoch of today

Memory Hook

Precambrian long, then Paleo, Meso, Ceno; fossils and half-lives say how old it goes.

Mains Angle

Questions on dating methods and the use of the time scale are possible. These points add depth.

Method Relative and absolute dating
Use Correlating regions
Limit Revisions of boundaries

A Question You May Face

An original practice question, not a past paper question.

“Explain how the geological time scale is constructed and how the age of rocks is determined.”

How to Answer

Units Eon to age.
Methods Relative and radiometric.
Example Phanerozoic eras.
Use India’s geology.

GS Relevance

Prelims: eras and periods with their dates and life forms, dating methods. GS Paper 1: physical geography; geological history.

Frequently Asked Questions

What are the main divisions of the geological time scale?

The geological time scale is divided, from the largest to the smallest unit, into eons, eras, periods, epochs and ages. The four eons are the Hadean, Archean, Proterozoic and Phanerozoic, and the Phanerozoic has three eras: the Paleozoic, the Mesozoic and the Cenozoic.

What is radiometric dating?

Radiometric dating finds the age of a rock from the decay of radioactive isotopes. Because each isotope decays at a steady rate known as its half-life, the proportion of parent and daughter atoms in a sample shows the time since the rock formed. Uranium-lead and potassium-argon methods are widely used.

What is an index fossil?

An index fossil is the preserved remains of an organism that lived widely but only for a short span of geological time. Finding the same index fossil in rocks of different places shows that those rocks are of the same age, so it is used to match rock layers.

Which period is called the age of dinosaurs?

The Mesozoic era, made up of the Triassic, Jurassic and Cretaceous periods, is called the age of dinosaurs or reptiles. Dinosaurs appeared in the Triassic, dominated in the Jurassic and Cretaceous, and died out about 66 million years ago at the end of the Cretaceous.

In which epoch are we living at present?

At present we live in the Holocene epoch of the Quaternary period in the Cenozoic era. The Holocene began about 11,700 years ago at the end of the last major glacial period. A proposal to name a new epoch, the Anthropocene, was not adopted formally in 2024.

PYQ Practice — Statement Analysis

1 The Mesozoic era includes the Jurassic period.

True

It also includes the Triassic and Cretaceous.

2 Radiocarbon dating can date rocks that are millions of years old.

False

It is useful only up to about 50,000 years.

3 In undisturbed layers of rock the lower layers are older.