The Earth’s Geologic Time Scale



** The geologic time scale is a chronologic schema (or idealized model) relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geological Survey.


Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age. Each era on the scale is separated by a major and/or changing event.


The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem, erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time. Years are expressed as MYA or MA, meaning “million years ago.”


Eon: Half a billion years or more.

Era: Several hundred million years.

Epoch: Tens of millions of years.

Age: Millions of years.



Development of Life During The Eras


Paleozoic Era




Life Forms



Algae and simple invertebrates, like jellyfish & worms. Arthropods, brachiopods, & trilobites. 



Graptolites, orthocerous, & primitive fish. The first vertebrates begin to appear.



The first true plants appear. Crinoids & eurypterids are abundant. The first air breathers.



Fish evolve into more complex animals. Sharks and amphibians multiply.



Plentiful ferns. Reptiles evolve. Spiders, cockroaches, & scorpions appear. Life on dry land.



Reptiles become abundant. Pine-like trees develop. Trilobites become extinct.


Mesozoic Era




Life Forms



The beginning of the dinosaurs. Plant eaters, meat eaters, flying reptiles, and crocodiles.



Giant dinosaurs develop. Abundant plant life & shellfish, like ammonites, lobsters, and shrimp.



The peak of development. Downfall of the great dinosaurs, like triceratops, t-rex, & pterodactyls. Deciduous trees develop.


Cenozoic Era




Life Forms



Mammals develop, such as camels, bears, cats, monkeys, rodents, and dogs. Grasses & fruit like todays appear.



More mammals develop, like the saber-toothed tiger and mastodon. Modern man appears.


**Illustration by the USGS


Early Life Chart

**Illustration by the Nat. Park Service



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