The Geological Time Scale

4.5 billion years of Earth history, organized

What Is the Geological Time Scale?

The geological time scale is the system scientists use to describe the timing and relationships of events in Earth's history. It divides deep time into a hierarchy of intervals, eons, eras, periods, and epochs, each defined by distinctive changes in the rock record: mass extinctions, the appearance of new life forms, or major shifts in climate and geography.

The boundaries between these divisions are not arbitrary. They mark real, often dramatic events in Earth's story, the end of the Cretaceous period, for example, is defined by the asteroid impact that wiped out the non-avian dinosaurs 66 million years ago.

The Phanerozoic Eon (541 Ma – Present)

The Phanerozoic is the current eon, beginning with the Cambrian explosion, a rapid diversification of complex animal life. The map covers this entire eon.

Cambrian (541–485 Ma)

Life exploded in complexity. Most major animal body plans appeared in just a few tens of millions of years. The continents were clustered near the equator and southern hemisphere, with a vast ocean covering much of the globe.

Ordovician (485–444 Ma)

Marine invertebrates dominated. The period ended with the first major mass extinction, likely caused by a glaciation that locked up sea water and caused dramatic sea level drops.

Silurian (444–419 Ma)

Life recovered and colonized land. The first vascular plants appeared, and early fish began to diversify. Sea levels were high and shallow warm seas covered much of the continents.

Devonian (419–359 Ma)

Known as the Age of Fishes. The first forests emerged, the first four-limbed vertebrates (tetrapods) crawled onto land, and Gondwana and Laurussia were moving toward each other.

Carboniferous (359–299 Ma)

Vast coal swamp forests covered the equatorial regions. Reptiles evolved and colonized the interior of continents. The continents were assembling into Pangea.

Permian (299–252 Ma)

Pangea was fully assembled. The period ended with the Great Dying, the most severe mass extinction in Earth's history, wiping out an estimated 96% of marine species and 70% of terrestrial vertebrate species.

Triassic (252–201 Ma)

Life slowly recovered. The first dinosaurs and early mammals appeared. Pangea began its breakup, with rifts forming between what would become Laurasia and Gondwana.

Jurassic (201–145 Ma)

Dinosaurs dominated. The Atlantic Ocean began to open. Laurasia (North America, Europe, Asia) and Gondwana (South America, Africa, India, Antarctica, Australia) were now clearly distinct landmasses.

Cretaceous (145–66 Ma)

The peak of dinosaur diversity. Flowering plants appeared. The period ended abruptly with an asteroid impact at Chicxulub, Mexico, triggering a mass extinction that killed three-quarters of all species.

Paleogene (66–23 Ma)

Mammals rapidly diversified to fill the niches left by the dinosaurs. India collided with Asia, beginning the uplift of the Himalayas.

Neogene (23–2.6 Ma)

Grasslands spread across the world. Early hominids appeared in Africa. The climate gradually cooled, setting the stage for the ice ages to come.

Quaternary (2.6 Ma – Present)

Characterized by repeated glacial cycles. Modern humans evolved and spread across the globe. The continents are in roughly their current positions, though they continue to move.

Before the Phanerozoic

The Precambrian, everything before 541 million years ago, represents about 88% of Earth's entire history. During this vast span, Earth cooled from a molten ball, the first oceans and atmosphere formed, life first appeared (around 3.5 billion years ago), and several earlier supercontinents assembled and broke apart.