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

 

Period

MYA

Life Forms

Cambrian

600-500

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

Ordovician

500-440

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

Silurian

440-395

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

Devonian

395-345

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

Carboniferous

345-280

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

Permian

280-225

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

 

Mesozoic Era

 

Period

MYA

Life Forms

Triassic

225-190

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

Jurassic

190-136

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

Cretaceous

136-65

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

 

Cenozoic Era

 

Period

MYA

Life Forms

Tertiary

65-2

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

Quaternary

2-Present

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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