Geologic Time Scale

Relative Dating

Tells which rock or fossil is older or younger but does not give exact age in numbers

Absolute Dating

Gives the actual age in years and uses scientific measurements to build accurate timelines of Earth’s history.

Geologic Time Scale

A system scientists use to describe Earth’s history based on major events.

Eon

Era

Period

Epoch

Main Divisions (Longest to Shortest):

Precambrian Eon

Covers 88% of Earth’s history.

Precambrian Eon

Life forms are microscopic (bacteria, algae)

Precambrian Eon

Very few fossils due to soft-bodied organisms.

Hadean Eon

Chaotic eon.

Meteorite bombardment.

Severe volcanism.

Planetesimal frequently collide.

The cores and the crust stabilized.

Moon formed.

Archean Eon

Fossil evidence of earliest life form.

Atmosphere contain mostly of methane.

Recorded primitive ocean and atmosphere.

No evidence of glaciation.

Prokaryotic Microbes

What is the earliest life forms?

Proterozoic Eon

Eon of great changes.

Origin of eukaryotic life and appearance of first oxygen dependent animals.

Oxygenation of atmosphere.

Formation and breakup of supercontinents glaciation.

Ediacara fauna

What is the first oxygen dependent animal?

Phanerozoic Eon

Life becomes abundant and complex while fossils of plants, animals, and other organisms became more common and easier to observe.

Paleozoic Era

Known as the “time of ancient life”, it began about 544 million years ago and lasted for around 300 million years. This era witnessed a great diversification of life, starting with the Cambrian Explosion.

Cambrian Period

First marine organisms with shells.

Ordovician Period

Marks the earliest appearance of vertebrates or the jawless fish.

Agnatha

What is the jawless fish calles?

Silurian Period

The emergence of terrestrial life, air-breathing arthropods (scorpions and millipedes).

Devonian Period

Age of Fishes, first amphibians’ appearance.

Carboniferous Period

Giant insects, reptiles, lush swamps.

Permian Period

Reptiles dominate for almost 200 years, ancestors of dinosaurs (Diapsids) and mammals (Synapsids) emerge.

Mesozoic Era

Known as the “time of middle life.” This was the time of dinosaurs and a period of major geological and biological transitions.

Triassic Period

Dominated by therapsids and thecodonts and the appearance of the first mammals.

Morganucudon

What is the first mammal called?

Jurassic Period

Dominated by dinosaurs, first bird.

Archaeopteryx

What is the first bird called?

Cretaceous Period

Dinosaurs continued to dominate and evolved into a wide variety of forms, T-rex, flowering plants (Angiosperms) and ended with a mass extinction event.

Cenozoic Era

“Time of recent life.” Characterized by the dominance of mammals and birds, following the mass extinction that ended the Mesozoic Era.

Tertiary Period

Mammals diversified into various ecological niches, replacing reptiles as dominant land animals, modern continents form.

Quaternary Period

Characterized by cycles of glaciation (ice ages), human evolution, civilization begins.