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Geologic Time Scale
The geologic time scale (GTS) is a system of
chronological dating that classifies geological strata (stratigraphy)
in time. It is used by geologists, paleontologists, and
other Earth scientists to describe the timing and
relationships of events in geologic history. The time
scale was developed through the study of physical rock
layers and relationships as well as the times when
different organisms appeared, evolved and became extinct
through the study of fossilized remains and imprints.
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 (ICS).
Terminology
The primary and largest catalogued divisions of time are
periods called eons. The first eon was the Hadean,
starting with the formation of the Earth and lasting
over 600 million years until the Archean, which is when
the Earth had cooled enough for continents and the
earliest known life to emerge. After about 2.5 billion
years, oxygen generated by photosynthesizing
single-celled organisms began to appear in the
atmosphere marking the beginning of the Proterozoic.
Finally, the Phanerozoic eon encompasses 541 million
years of diverse abundance of multicellular life
starting with the appearance of hard animal shells in
the fossil record and continuing to the present.
Eons are divided into eras, which are in turn divided
into periods, epochs and ages.
The first three eons (i.e. every eon but the Phanerozoic)
can be referred to collectively as the Precambrian
supereon. This is in reference to the significance of
the Cambrian Explosion, a massive diversification of
multi-cellular life forms that took place in the
Cambrian period at the start of the Phanerozoic.
The following five timelines show the geologic time
scale. The first shows the entire time from the
formation of the Earth to the present, but this gives
little space for the most recent eon. Therefore, the
second timeline shows an expanded view of the most
recent eon. In a similar way, the most recent era is
expanded in the third timeline, the most recent period
is expanded in the fourth timeline, and the most recent
epoch is expanded in the fifth timeline. |
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Principles
Evidence from radiometric dating indicates that 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 are thought to have taken
place. Different spans of time on the GTS are usually
marked by corresponding changes in the composition of
strata which indicate 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
non-avian dinosaurs and many other groups of life. Older
time spans, which predate the reliable fossil record
(before the Proterozoic eon), are defined by their
absolute age.
Geologic units from the same time but different parts of
the world often are not similar and contain different
fossils, so the same time-span 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.
Some other planets and moons in the Solar System have
sufficiently rigid structures to have preserved records
of their own histories, for example, Venus, Mars and the
Earth's Moon. Dominantly fluid planets, such as the gas
giants, do not preserve their history in a comparable
manner. Apart from the Late Heavy Bombardment, events on
other planets probably had little direct influence on
the Earth, and events on Earth had correspondingly
little effect on those planets. Construction of a time
scale that links the planets is, therefore, of only
limited relevance to the Earth's time scale, except in a
Solar System context. The existence, timing, and
terrestrial effects of the Late Heavy Bombardment are
still a matter of debate. |
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Early history
In Ancient Greece, Aristotle (384–322 BCE) observed that
fossils of seashells in rocks resembled those found on
beaches – he inferred that the fossils in rocks were
formed by organisms, and he reasoned that the positions
of land and sea had changed over long periods of time.
Leonardo da Vinci (1452–1519) concurred with Aristotle's
interpretation that fossils represented the remains of
ancient life.
The 11th-century Persian polymath Avicenna (Ibn Sina,
died 1037) and the 13th-century Dominican bishop
Albertus Magnus (died 1280) 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 (1027). The Chinese
naturalist Shen Kuo (1031–1095) also recognized the
concept of "deep time". |
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Naming of geologic periods,
eras and epochs
Early work on developing the geologic time scale was
dominated by British geologists, and the names of the
geologic periods reflect that dominance. The "Cambrian",
(the classical name for Wales) 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 an adaptation
of "the Coal Measures", the old British geologists' term
for the same set of strata. The "Permian" was named
after the region of Perm in 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
(Latin trias meaning triad) – 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
Brongniart for the extensive marine limestone exposures
of the Jura Mountains. The "Cretaceous" (from Latin
creta meaning ‘chalk’) 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 (calcium carbonate deposited by
the shells of marine invertebrates) found in Western
Europe. |
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 Kiddle: Geologic Time Scale
Wikipedia: Geologic Time Scale |
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