There has alsways been an interest in knowing when things happened and putting those events into some sort of time frame. The Geologic Time Scale did not develop all at once, and was developed using relative dating techniques and correlation techniques.

Geologic Time Scale

An arbitrary chronologic arrangement or sequence of geologic events. Arbitrary because certain geologic and evolutionary events and features were used to divide the scale into smaller units - these smaller units are not of equal duration.

Structure of the Time Scale

6 major divisions or time units - Eon, Era, Period, Epoch, Ages, Chrons

Eon is the largest time unit, with each subsequent unit being of a subdivision of the preceeding unit and covering a smaller time duration.

Eons

Four Eons are defined on the Geologic Time Scale

Hadean Eon (Eoarchean)

4.6-3.8 billion years ago
Hades - Greek mythological hell
This is an older term that has more recently been replaced by the Eoarchean Era

Archean Eon

3.8-2.5 billion years ago
Originally called the Archeozoic
archeo- means ancient or primative (Greek)
-zoic is derived from zo- meaning animal (Greek) or life
Ancient life

Proterozoic Eon

2.5 billion - 570 million years ago
proto- meaning before (Greek) or first formed
first formed life or before life

Phanerozoic Eon

570 million years to the present
phanero- meaning visible (Greek)
visible life

Precambrian

Older term used for the time covered by the Hadean, Archean and Proterozoic Eons
before the Cambrian

Originally the division between the Phanerozoic and the Precambrian was based on the presence or absence of fossil evidence. Early geologists did not find any fossils in Precambrian rocks. Since that time fossils have been found in rocks from those Eons.

Eras

Eons of the Precambrian are divided into Early, Middle and Late Eras
The Phanerozoic is divided into 3 Eras

Paleozoic Era

570-245 million years ago
paleo- meaning ancient (Greek)
ancient life

Mesozoic Era

245-66 million years ago
meso- meaning middle (Greek)
middle life

Cenozoic Era

66 million years - present
ceno- meaning recent (Greek)
recent life

Periods of the Paleozoic

Cambrian Period

570-505 million years ago
Named for Cambria - Latin name for Wales in Great Britain
First described and named by Adam Sedgwick in the 1830s

Ordovician Period

505-438 million years ago
Originally part of the Cambrian and Silurian Periods
Separated from the Cambrian and Silurian Periods in 1879 by Charles Lapworth
Named after the Ordivices - Celtic tribe

Silurian Period

438-408 million years ago
Named in 1835 after the Silures - Celtic tribe - which occupied a region of southern Wales where these rocks were first described by Sir Roderick Murchison

Devonian Period

408-360 million years ago
Named for the rocks found in Devonshire, England (southwest) by Sedgewick and Murchison in 1839

Carboniferous Period

360-286 million years ago named for coal-bearing strata in north-central England in 1822 by William Canybeare and William Phillips
This Period is used mainly in Europe and Asia
In North America it has been divided into two Periods

Mississippian Period

360-320 million years ago
Alexander Winchell in1870 named this Period for rocks exposed in the upper Mississippi River drainage basin

Pennsylvanian Period

320-286 million years ago
In 1891, Henry Williams named this Period for coal-bearing rocks in Pennsylvania

Permian Period

286-245 million years ago
Named for the Russian provence of Perm by Muchison in 1841

Periods of the Mesozoic

Triassic Period

248-208 million years ago
Named by German geologist Frederich von Alberti in1834
Based on the 3 division of strata found in this Period

Jurassic Period

208-144 million years ago
Named by Alexander von Humboldt in 1795 for rocks in the Jura Mountains
Redefined in 1839

Cretaceaous Period

144-66 million years ago
Belgian geologist Omalius d'Halloy proposed this Period in 1822
creta- meaning chalk
Most Cretaceous Period rocks are not chalk, but chalk beds are often found

Periods of the Cenozoic

Tertiary Period

66-1.6 million years ago
Based on a time system developed by Giovanni Arduino in 1759
Original system divided the history of the Earth into 4 parts
Primary, Secondary, Tertiary, Quaternary

Quaternary Period

1.6 million years ago to present
Formally applied in 1829 by French geologist Jules Desnoyers

Historic Development of the Geologic Time Scale

1759 - Tertiary and Quaternary
1795 - Jurassic
1822 - Cretaceous and Carboniferous
1830 - Cambrian
1832 - Tertiary (redefined)
1834 - Triassic
1835 - Silurian
1839 - Devonian
1841 - Permian
1854 - Quaternary (redefined)
1870 - Mississippian
1878 - Ordovician (separated from Cambrian and Silurian)
1891 - Pennsylvanian

Additional information may be obtained at: Geologic Time Scale

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