Geologic Time Scale

The Geologic Time Scale

The geologic time scale is like a calendar of Earth’s entire history—but instead of days and years, it shows billions of years! Scientists use it to describe major events in Earth’s 4.5-billion-year past, like the formation of the planet, the rise of life, and mass extinctions.

The time scale is divided into sections, kind of like how a book has chapters. From largest to smallest, the divisions are:

Reasons for the Divisions

Mass Extinctions

Sudden, global die-offs of many species.

Example: The extinction of the dinosaurs marks the end of the Mesozoic Era and the start of the Cenozoic Era.

2. Rapid Evolution or Appearance of New Life

Sudden bursts of new, complex life forms.

Example: The Cambrian Explosion marks the beginning of the Paleozoic Era, when many new types of animals appeared.

3. Major Climate Changes

Ice ages, global warming, or big shifts in weather patterns. These can lead to changes in ecosystems and life forms.

4. Geological Events

Huge events like:

- Formation of supercontinents
- Massive volcanic eruptions
- Changes in sea levels
- Mountain building

These reshape Earth’s surface and affect life and climate.

5. Changes in the Chemical Makeup of Earth's Atmosphere or Oceans

Example: The Great Oxygenation Event added oxygen to the atmosphere and changed which organisms could survive.

How do Scientists know where to Divide?

They look at:

Fossils in different rock layers
Chemical signatures in rocks
Radiometric dating to figure out ages
Global changes recorded in rocks all over the world

Click here for an Interactive Version of the Geologic Time Scale

Precambrian Time

Precambrian Time covers roughly 4 billion years of Earth’s history - approximately 88% of Earth’s history. Precambrian Time includes three major eons:

Hadean Eon (4.5 - 4.0 Billion Years Ago)

Earth formed from space dust and rock
Surface was molten, very hot, and unstable
No life existed yet

Archean Eon (4.0 - 2.5 Billion Years Ago)

Earth cooled, solid crust formed
Oceans formed
First life appeared: simple bacteria and cyanobacteria
These organisms began producing oxygen through photosynthesis

Proterozoic Eon (2.5 Billion - 541 Million Years Ago)

Oxygen started building up in the atmosphere (Great Oxygenation Event)
First eukaryotic cells (cells with nuclei) evolved
Later, multicellular life appeared (like simple algae and soft-bodied animals)

Why Do We Know So Little About the Precambrian?

Precambrian rocks are very old and often deeply buried or changed by heat and pressure.
Fossils are rare because most life was soft-bodied and microscopic.

Phanerozoic Eon

The Phanerozoic Eon is the current eon we’re living in. It started about 541 million years ago when the Cambrian Explosion ended the Proterozoic Eon, and continues to today.

The word Phanerozoic means "visible life", because this is when complex life forms—like fish, plants, dinosaurs, and humans—became common and left fossils. The Phanerozoic Eon is divided into three main eras:

Paleozoic Era (541 - 252 Million Years Ago)

“Ancient life”

Began with the Cambrian Explosion – sudden burst of diverse life
First invertebrates, fish, land plants, and amphibians
Ends with the largest mass extinction in Earth’s history (Permian-Triassic extinction)

Mesozoic Era (252 - 66 Million Years Ago)

“Middle life”

Known as the Age of Reptiles or Dinosaurs
Dinosaurs, early mammals, and flowering plants appear
Ends with the famous asteroid impact that wiped out the dinosaurs

Cenozoic Era (66 Million Years Ago - Today)

“Recent life”

Known as the Age of Mammals - Mammals and birds take over after dinosaurs go extinct
Modern Humans evolve (Homo sapiens) about 300,000 years ago
Continues to the present day

When Do We Live?

We are living in the Phanerozoic Eon, the Cenozoic Era, the Quaternary Period, and the Holocene Epoch.

Fun Fact:

If the entire history of the Earth were compressed into a single 24 hour day, modern humans would have appeared just before midnight, a mere 3 seconds before the end of the day.

How Do Scientists Know All This?

Scientists study rocks and fossils to understand Earth's history. Deeper rock layers are older, and fossils tell us what kinds of life existed at different times.

In Summary

The geologic time scale helps scientists organize Earth's 4.5 billion years of history into manageable chunks based on big changes in life, climate, and geology.