Earth’s Rotation: The Day-Night Boundary

Geologic Clock

Time is the foundation of geology.

Actually, if you think about it:

The geologic time scale of Earth’s creation is almost unimaginable to us. This is because humans’ lifespans are so short in comparison.

We work in hours, days, months and years. But the Earth works in decades, centuries and millions of years.

Like a geologic calendar, geologists divide time. From longest to shortest, they are eons, eras, periods, epochs and ages.

So that means that timing is everything when it comes to the geologic time scale.

READ MORE: Earth Timeline: A Guide to Earth’s Geological History and Events [Infographic]

How long is the geologic calendar?

Geologic Timeline

The concept of time is very long in geology. First, Earth’s age is approximately 4.5 billion years. This is why we use billions, millions and thousands of years.

For time markers in geologic time, we typically use abbreviations like ‘Ga’ (giga-annum), ‘Ma’ (mega-annum), and ‘Ka’ (kilo-annum).

  • ‘Ga’ or ‘Gya’ (billion) is 1,000,000,000 years ago
  • ‘Ma’ or ‘Mya’ (million) is 1,000,000 years ago
  • ‘Ka’ or ‘Kya’ (thousand) 1,000 years ago

For example, 2.5 Ga refers to 2.5 billion years ago. Because Earth is about 4.5 billion years old, Earth would be about 2 billion years old at this time.

So instead of working in days, months and years, geologists work in millions and billions of years. Then, we subdivide these long chunks of time into eons, eras, periods, epochs and ages.

Like a geologic calendar, they chronologically order units of time into a geologic time scale. And each division of time identifies a prominent event or characteristic feature based on their record.

What are the units of geologic time?

Earth Age


Eons are the longest division of geologic time. Generally, we measure eons as billions of years ago (Ga) and millions of years ago (Ma). Geologists divide the lifespan of Earth into a total of 4 eons. From origin to now, Earth’s 4 eons are the Hadean, Archean, Proterozoic and Phanerozoic Eon. The Hadean, Archean and Proterozoic eons are sometimes grouped as the Precambrian Eon.


Eras are divisions of geologic time shorter than eons but longer than periods. In terms of geochronological units, there are 10 defined eras which generally span several hundred million years. For example, the Paleozoic, Mesozoic, and Cenozoic eras are within the Phanerozoic Eon.


There are 22 defined periods. Periods are divisions of geologic time longer than epochs but shorter than an era. Each period spans a length of tens to one hundred million years. Next, there are 34 defined epochs which generally last for tens of millions of years. The geologic time scale conceptually consists of periods that we break down into smaller epochs.


These epochs are then divided into ages, which are the shortest division of geologic time. In terms of the number of geochronological units, there are 99 defined which can stretch over millions of years. Epochs contain minor differences between each unit. Some geologists divide ages even further. If you do so, chrons are the smallest working geochronological unit. However, these are less common.

Earth’s Timeline

Read the story of Earth

What’s an example of geologic time scale?

Let’s put what we know into practice for the Triassic period which lasted about 50 million years. The Triassic period has a well-defined start and endpoint because it began and concluded with catastrophic mass extinctions.

Eon Era Period Epoch Age Year (Ma)
Triassic-Jurassic Extinction
Phanerozoic Mesozoic Triassic Upper Rhaetian 201.3-208.5
Norian 208.5-227
Carnian 227-237
Middle Ladinian 237-242
Anisian 242-247.2
Lower Olenekian 247.2-251.2
Induan 251.2-251.9
Permian-Triassic Extinction

As you can see in the table above, the Triassic period started 252 million years ago after Earth’s largest extinction event in history. It’s also known as the “Great Dying” because it killed 96% of all marine species and an estimated 70% of land species. Then, it had an abrupt ending in 201.3 mya during a less severe Triassic–Jurassic extinction event.

If you look closely at the table, you can see that the Triassic period is within the Mesozoic Era. Then, the Mesozoic is part of the Phanerozoic Eon (542 million years ago to now) which is notable for having fossil records. Everything else before this era was in the Precambrian Eon without hard body parts.

The Triassic period has 3 epochs and 7 ages. Each of these shorter divisions of time identify a notable event or characteristic feature based on their record.

Who is the International Commission of Stratigraphy (ICS)?

Grand Canyon Layers

The role of the International Commission of Stratigraphy (ICS) is to define global units as it relates to a geological time scale. ICS is the governing body that chronologically orders Earth’s history into eons, eras, periods, epochs and ages as an International Chronostratigraphic Graph.

From signatures inherent in rocks, geologists can realize the true age of the earth. By tracing back on embedded fossilized organisms in rock strata, we can achieve a context of time.

Because we can date rocks through stratigraphy, we can achieve a better understanding of geological events in Earth’s history. Based on our geological time scale, we can better understand the theory of evolution and origin of life on our planet.

An overview of geologic time

Tick, tock fellow geologists. Now, you have a good understanding that time is foundation if geology.

Also, you have a good background in how to break up time into geological units.

Never stop learning about Earth’s history.

Check out our project section and uncover Earth like you’ve never seen before.

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