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Geological structure and Time

Geologic time is divided into four large segments called Eons: Hadean, Archean, Proterozoic, and Phanerozoic. The Phanerozoic Eon is divided into Eras: Paleozoic, Mesozoic, and Cenozoic. The divisions among Eras reflect major changes in the fossil record, including the extinction and appearance of new life forms.Geological time with major evolutionary events in the
fossil record

Geological time scale


The vast expanse of geological time has been separated into eras, periods, and epochs. The numbers included below refer to the beginnings of the division in which the title appears. The numbers are in millions of years. The named divisions of time are for the most part based on fossil evidence and principles for relative dating over the past two hundred years. Only with the application of radiometric dating have numbers been obtained for the divisions observed from field observations.

 














































































Era
Period
Epoch
Plant and Animal
Development
Cenozoic
Quaternary
Holocene (.01)
Humans develop
"Age of mammals"

Extinction of dinosaurs and
many other species.

 
Pleistocene (1.8)
Tertiary
Pliocene (5.3)
Miocene (23.8)
Oligocene (33.7)
Eocene (54.8)
Paleocene (65.0)
Mesozoic
Cretaceous (144)
"Age of Reptiles"
First flowering plants
First birds

Dinosaurs dominant.

 
Jurassic (206)
Triassic (248)
Paleozoic
Permian (290)
"Age of Amphibians"
Extinction of trilobites and many other marine animals
First reptiles

Large coal swamps

Large Amphibians abundant.

 
Carboniferous: Pennyslvanian (323)
Carboniferous: Mississippian (354)
Devonian (417)
"Age of Fishes"
First insect fossils
Fishes dominant

First land plants

 
Silurian (443)
Ordovician (490)
"Age of Invertibrates"
First fishes
Trilobites dominant

First organisms with shells

 
Cambrian (540)
Precambrian - comprises about 88% of geologic time (4500)
First multicelled organisms
First one-celled organisms

Origin of Earth

 

Adapted from Lutgens and Tarbuck. They cite the Geological Society of America as the source of the data.

There is another kind of time division used - the "eon". The entire interval of the existence of visible life is called the Phanerozoic eon. The great Precambrian expanse of time is divided into the Proterozoic, Archean, and Hadean eons in order of increasing age.

The names of the eras in the Phanerozoic eon (the eon of visible life) are the Cenozoic ("recent life"), Mesozoic ("middle life") and Paleozoic ("ancient life"). The further subdivision of the eras into 12 "periods" is based on identifiable but less profound changes in life-forms. In the most recent era, the Cenozoic, there is a further subdivision of time into epochs.
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