The geologic time scale is a system of chronological
measurement that relates stratigraphy to time,
and is used by geologists, paleontologists,
and other earth scientists to describe the
timing and relationships between events that
have occurred throughout Earth's history.
The table of geologic time spans presented
here agrees with the nomenclature, dates and
standard color codes set forth by the International
Commission on Stratigraphy.
Evidence from radiometric dating indicates
that the Earth is about 4.54 billion years
old. The geology 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 GTS are usually
delimited by changes in the composition of
strata which correspond to them, indicating
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–Paleogene
extinction event, which marked the demise
of the dinosaurs and many other groups of
life. Older time spans which predate the reliable
fossil record are defined by the absolute
age.
Terminology
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 in earth's history.
Geologists qualify these units as Early, Mid,
and Late when referring to time, and Lower,
Middle, and Upper when referring to the corresponding
rocks. For example, the Lower Jurassic Series
in chronostratigraphy corresponds to the Early
Jurassic Epoch in geochronology. The adjectives
are capitalized when the subdivision is formally
recognized, and lower case when not; thus
"early Miocene" but "Early Jurassic."
Geologic units from the same time but different
parts of the world often look different and
contain different fossils, so the same period
was historically given different names in
different locales. For example, in North America
the Lower Cambrian is called the Waucoban
series that is then subdivided into zones
based on succession of trilobites. In East
Asia and Siberia, the same unit is split into
Alexian, Atdabanian, and Botomian stages.
A key aspect of the work of the International
Commission on Stratigraphy is to reconcile
this conflicting terminology and define universal
horizons that can be used around the world.
History and nomenclature of the time scale
In Ancient Greece, Aristotle saw that fossil
seashells from rocks were similar to those
found on the beach and inferred that the fossils
were once part of living animals. He reasoned
that the positions of land and sea had changed
over long periods of time. Leonardo da Vinci
concurred with Aristotle's view that fossils
were the remains of ancient life.
The 11th-century Persian geologist Avicenna
and the 13th century Dominican bishop Albertus
Magnus extended Aristotle's explanation into
a theory of a petrifying fluid. Avicenna also
first proposed one of the principles underlying
geologic time scales, the law of superposition
of strata, while discussing the origins of
mountains in The Book of Healing in 1027.
The Chinese naturalist Shen Kuo also recognized
the concept of 'deep time'.
The principles underlying geologic time scales
were later laid down by Nicholas Steno in
the late 17th century. Steno argued that rock
layers are laid down in succession, and that
each represents a "slice" of time. He also
formulated the law of superposition, which
states that any given stratum is probably
older than those above it and younger than
those below it. While Steno's principles were
simple, applying them to real rocks proved
complex. Over the course of the 18th century
geologists realized that:
Sequences of strata were often eroded, distorted,
tilted, or even inverted after deposition;
Strata laid down at the same time in different
areas could have entirely different appearances;
The strata of any given area represented only
part of the Earth's long history.
The first serious attempts to formulate a
geological time scale that could be applied
anywhere on Earth were made in the late 18th
century. The most influential of those early
attempts divided the rocks of the Earth's
crust into four types: Primary, Secondary,
Tertiary, and Quaternary. Each type of rock,
according to the theory, formed during a specific
period in Earth history. It was thus possible
to speak of a "Tertiary Period" as well as
of "Tertiary Rocks." Indeed, "Tertiary" and
"Quaternary" remained in use as names of geological
periods well into the 20th century.
The Neptunist theories popular at this time
proposed that all rocks had precipitated out
of a single enormous flood. A major shift
in thinking came when James Hutton presented
his Theory of the Earth; or, an Investigation
of the Laws Observable in the Composition,
Dissolution, and Restoration of Land Upon
the Globe before the Royal Society of Edinburgh
in March and April 1785. It has been said
that "as things appear from the perspective
of the 20th century, James Hutton in those
readings became the founder of modern geology".
Hutton proposed that the interior of the Earth
was hot, and that this heat was the engine
which drove the creation of new rock: land
was eroded by air and water and deposited
as layers in the sea; heat then consolidated
the sediment into stone, and uplifted it into
new lands. This theory was called "Plutonist"
in contrast to the "Neptunist" flood-oriented
theory.
The identification of strata by the fossils
they contained, pioneered by William Smith,
Georges Cuvier, Jean d'Omalius d'Halloy, and
Alexandre Brogniart in the early 19th century,
enabled geologists to divide Earth history
more precisely. It also enabled them to correlate
strata across national boundaries. If two
strata contained the same fossils, chances
were good that they had been laid down at
the same time. Detailed studies between 1820
and 1850 of the strata and fossils of Europe
produced the sequence of geological periods
still used today.
The process was dominated by British geologists,
and the names of the periods reflect that
dominance. The "Cambrian", and the "Ordovician",
and "Silurian", named after ancient Welsh
tribes, were periods defined using stratigraphic
sequences from Wales. The "Devonian" was named
for the English county of Devon, and the name
"Carboniferous" was simply an adaptation of
"the Coal Measures", the old British geologists'
term for the same set of strata. The "Permian"
was named after Perm, Russia, because it was
defined using strata in that region by Scottish
geologist Roderick Murchison. However, some
periods were defined by geologists from other
countries. The "Triassic" was named in 1834
by a German geologist Friedrich Von Alberti
from the three distinct layers —red beds,
capped by chalk, followed by black shales—
that are found throughout Germany and Northwest
Europe, called the 'Trias'. The "Jurassic"
was named by a French geologist Alexandre
Brogniart for the extensive marine limestone
exposures of the Jura Mountains. The "Cretaceous"
as a separate period was first defined by
Belgian geologist Jean d'Omalius d'Halloy
in 1822, using strata in the Paris basin and
named for the extensive beds of chalk.
British geologists were also responsible for
the grouping of periods into Eras and the
subdivision of the Tertiary and Quaternary
periods into epochs. In 1841 John Phillips
published the first global geological time
scale based on the types of fossils found
in each era. Phillips' scale helped standardize
the use of terms like Paleozoic which he extended
to cover a larger period than it had in previous
usage, and Mesozoic which he invented.
When William Smith and Sir Charles Lyell first
recognized that rock strata represented successive
time periods, time scales could be estimated
only very imprecisely since various kinds
of rates of change used in estimation were
highly variable. While creationists had been
proposing dates of around six or seven thousand
years for the age of the Earth based on the
Bible, early geologists were suggesting millions
of years for geologic periods with some even
suggesting a virtually infinite age for the
Earth. Geologists and paleontologists constructed
the geologic table based on the relative positions
of different strata and fossils, and estimated
the time scales based on studying rates of
various kinds of weathering, erosion, sedimentation,
and lithification. Until the discovery of
radioactivity in 1896 and the development
of its geological applications through radiometric
dating during the first half of the 20th century
which allowed for more precise absolute dating
of rocks, the ages of various rock strata
and the age of the Earth were the subject
of considerable debate.
The first geologic time scale that included
absolute dates was published in 1913 by the
British geologist Arthur Holmes. He greatly
furthered the newly created discipline of
geochronology and published the world renowned
book The Age of the Earth in which he estimated
the Earth's age to be at least 1.6 billion
years.
In 1977, the Global Commission on Stratigraphy
started an effort to define global references
known as GSSPs for geologic periods and faunal
stages. The commission's most recent work
is described in the 2004 geologic time scale
of Gradstein et al. A UML model for how the
timescale is structured, relating it to the
GSSP, is also available.
Condensed graphical timelines
The following four timelines show the geologic
time scale. The first shows the entire time
from the formation of the Earth to the present,
but this compresses the most recent eon. Therefore
the second scale shows the most recent eon
with an expanded scale. The second scale compresses
the most recent era, so the most recent era
is expanded in the third scale. Since the
Quaternary is a very short period with short
epochs, it is further expanded in the fourth
scale. The second, third, and fourth timelines
are therefore each subsections of their preceding
timeline as indicated by asterisks. The Holocene
is too small to be shown clearly on the third
timeline on the right, another reason for
expanding the fourth scale. The Pleistocene
epoch. Q stands for the Quaternary period.
Millions of Years
Table of geologic time
The following table summarizes the major events
and characteristics of the periods of time
making up the geologic time scale. As above,
this time scale is based on the International
Commission on Stratigraphy. This table is
arranged with the most recent geologic periods
at the top, and the most ancient at the bottom.
The height of each table entry does not correspond
to the duration of each subdivision of time.
The content of the table is based on the current
official geologic time scale of the International
Commission on Stratigraphy, with the epoch
names altered to the early/late format from
lower/upper as recommended by the ICS when
dealing with chronostratigraphy.
Proposed Precambrian timeline
The ICS's Geologic Time Scale 2012 book which
includes the new approved time scale also
displays a proposal to substantially revise
the Precambrian time scale to reflect important
events such as the formation of the Earth
or the Great Oxidation Event, among others,
while at the same time maintaining most of
the previous chronostratigraphic nomenclature
for the pertinent time span.
Hadean Eon - 4600–4030 MYA
Chaotian Era - 4600–4404 MYA - the name
alluding both to the mythological Chaos and
the chaotic phase of planet formation
Jack Hillsian or Zirconian Era - 4404–4030
MYA - both names allude to the Jack Hills
Greenstone Belt which provided the oldest
mineral grains on Earth, zircons
Archean Eon - 4030–2420 MYA
Paleoarchean Era - 4030–3490 MYA
Acastan Period - 4030–3810 MYA - named after
the Acasta Gneiss
Isuan Period - 3810–3490 MYA - named after
the Isua Greenstone Belt
Mesoarchean Era - 3490–2780 MYA
Vaalbaran Period - 3490–3020 MYA - a portmanteau
based on the names of the Kapvaal and Pilbara
cratons
Pongolan Period - 3020–2780 MYA - named
after the Pongola Supergroup
Neoarchean Era - 2780–2420 MYA
Methanian Period - 2780–2630 MYA - named
for the inferred predominance of methanotrophic
prokaryotes
Siderian Period - 2630–2420 MYA - named
for the voluminous banded iron formations
formed within its duration
Proterozoic Eon - 2420–541 MYA
Paleoproterozoic Era - 2420–1780 MYA
Oxygenian Period - 2420–2250 MYA - named
for displaying the first evidence for a global
oxidizing atmosphere
Jatulian or Eukaryian Period - 2250–2060
MYA - names are respectively for the Lomagundi-Jatuli
δ13C isotopic excursion event spanning its
duration, and for the first fossil appearance
of eukaryotes
Columbian Period - 2060–1780 MYA - named
after the supercontinent Columbia
Mesoproterozoic Era - 1780–850 MYA
Rodinian Period - 1780–850 MYA - named after
the supercontinent Rodinia, stable environment
Neoproterozoic Era - 850–541 MYA
Cryogenian Period - 850–635 MYA - named
for the occurrence of several glaciations
Ediacaran Period - 635–541 MYA
Shown to scale:
Compare with the current official one:
See also
Notes and references
Further reading
Aubry, Marie-Pierre; Van Couvering, John A;
Christie-Blick, Nicholas; Landing, Ed; Pratt,
Brian R; Owen, Donald E; & Ferrusquia-Villafranca,
Ismael, "Terminology of geological time: Establishment
of a community standard", Stratigraphy 6:
100–105, retrieved 18 November 2011 
Gradstein, F.M & Ogg, J.G, A Geologic Time
scale 2004 - Why, How and Where Next!, retrieved
18 November 2011 
Gradstein, Felix M., Ogg, James G. & Smith,
Alan G, A Geologic Time Scale 2004, New York;
Cambridge, UK: Cambridge University Press,
ISBN 0-521-78142-6, retrieved 18 November
2011 Paperback ISBN 0-521-78673-8 
Gradstein, Felix M., Ogg, James G., Smith,
Alan G., Bleeker, Wouter & Laurens, Lucas,
J, "A new Geologic Time Scale, with special
reference to Precambrian and Neogene", Episodes
27: 83–100, retrieved 18 November 2011 
Knoll, Andrew H., Walter, Malcolm R., Narbonne,
Guy M., Christie-Blick, Nicholas, "A New Period
for the Geologic Time Scale", Science 305:
621–622, doi:10.1126/science.1098803, PMID 15286353,
retrieved 18 November 2011 
Levin, Harold L, "Time and Geology", The Earth
Through Time, Hoboken, New Jersey: John Wiley
& Sons, ISBN 978-0-470-38774-0, retrieved
18 November 2011 
External links
NASA: Geologic Time
GSA: Geologic Time Scale
British Geological Survey: Geological Timechart
GeoWhen Database
International Commission on Stratigraphy Time
Scale
Chronos.org
National Museum of Natural History - Geologic
Time
SeeGrid: Geological Time Systems Information
model for the geologic time scale
Exploring Time from Planck Time to the lifespan
of the universe
Episodes, Gradstein, Felix M. et al. A new
Geologic Time Scale, with special reference
to Precambrian and Neogene, Episodes, Vol.
27, no. 2 June 2004
Lane, Alfred C, and Marble, John Putman 1937.
Report of the Committee on the measurement
of geologic time
Lessons for Children on Geologic Time
Deep Time - A History of the Earth : Interactive
Infographic
