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GEOLOGIC EONS, ERAS and PERIODS
The period is the basic unit of geological time in which a single type of rock
system is formed. Two or more periods comprise a geological Era. Two or
more Eras form an Eon, the largest division of geologic time. Some periods
are divided into epochs.
The major periods in the geologic history of the Earth are (mya=million years
ago):
Era
Period
Quaternary
(1.8 million years ago - Present)
The second period of the Cenozoic
era. It contains two epochs: the
Pleistocene and the Holocene. It is
named after the Latin word "quatern"
(four at a time).
Cenozoic
(65 million years ago
- Present)
"Age of Recent
Life"
The several geologic eras were
originally named Primary,
Secondary, Tertiary, and
Quaternary. The first two names are
no longer used. Tertiary and
Quaternary have been retained but
used as period designations.
An era of geologic time
from the beginning of
the Tertiary period to
the present. Its name is
from Greek and means
"new life."
Epoch
Holocene
(8,000 years ago Present)
An epoch of the
Quaternary period. It is
named after the Greek
words "holos" (entire)
and "ceno" (new).
Pleistocene
(1.8 million - 8,000
years ago)
"The Great Ice Age"
An epoch of the
Quaternary period. It is
named after the Greek
words "pleistos" (most)
and "ceno" (new).
Pliocene
(5.3 - 1.8 million years
ago)
Tertiary
(65 - 1.8 million years ago)
The first period of the Cenozoic
era (after the Mesozoic era and
before the Quaternary period).
Final epoch of the
Tertiary period. It is
named after the Greek
words "pleion" (more)
and "ceno" (new).
Miocene
(23.8 - 5.3 million
years ago)
A epoch of the upper
Tertiary period. It is
named after the Greek
words "meion" (less) and
"ceno" (new).
Oligocene
(33.7 - 23.8 million
years ago)
An epoch of the early
Tertiary period. It is
named after the Greek
words "oligos" (little,
few) and "ceno" (new).
Eocene
(55.5 - 33.7 million
years ago)
An epoch of the lower
Tertiary period. Its name
is from the Greek words
"eos" (dawn) and "ceno"
(new).
Paleocene
(65 - 55.5 million
years ago)
Earliest epoch of the
Tertiary period. It is
named after the Greek
words "palaois" (old) and
"ceno" (new).
Era
Mesozoic
(248 - 65 million years ago)
"Age of Medieval Life"
An era of geologic time
between the Paleozoic and
the Cenozoic. The word
Mesozoic is from Greek
and means "middle life."
Period
Epoch
Cretaceous
Late or Upper
(145 - 65 million years ago)
"The Age of Dinosaurs"
The final period of the Mesozoic era. The
name is derived from the Latin word for
chalk ("creta") and was first applied to
extensive deposits of this age that form
white cliffs along the English Channel
between Great Britain and France.
Jurassic
(213 - 145 million years ago)
Early or Lower
Late or Upper
Middle
The middle period of the Mesozoic era. It
is named after the Jura Mountains
between France and Switzerland, where
rocks of this age were first studied.
Triassic
(248 - 213 million years ago)
The earliest period of the Mesozoic era.
The name Triassic refers to the threefold
division of rocks of this age in Germany.
The Break-up of the continent Pangea
... MORE
Era
Early or Lower
Late or Upper
Middle
Early or Lower
Period
Epoch
Permian
Late or Upper
(286 - 248 million years ago)
The final period of the Paleozoic era. It is
named after the province of Perm, Russia,
where rocks of this age were first studied.
Pennsylvanian*
(325 - 286 million years
ago)
Paleozoic
(544 - 248 million
years ago)
"Age of Ancient
Life"
An era of geologic
time, from the end of
the Precambrian to
the beginning of the
Mesozoic. The
word Paleozoic is
from Greek and
means "old life."
Early or Lower
Late or Upper
Middle
"The Coal Age"
Carboniferous
(360 - 286 million
years ago)
A period of time in
the Paleozoic
era that includes the
Pennsylvanian and
Mississippian
periods.
A period of
the Paleozoic era. It is
named after the state
of Pennsylvania where
rocks of this age are
widespread.
Mississippian*
Early or Lower
Late or Upper
(360 - 325 million years
ago)
A period of
the Paleozoic era. It is
named after the
Mississippi River
valley, which contains
good exposures of
rocks of this age.
Devonian
(410 - 360 million years ago)
Early or Lower
Late or Upper
Middle
Early or Lower
A period of the Paleozoic era. It is named after
Devonshire, England, where rocks of this age
were first studied.
Silurian
(440 - 410 million years ago)
A period of the Paleozoic. It is named after a
Celtic tribe called the Silures.
Ordovician
(505 - 440 million years ago)
The second earliest period of the Paleozoic
era. It is named after a Celtic tribe called the
Ordovices.
Cambrian
(544 - 505 million years ago)
The earliest period of the Paleozoic era. It is
named after Cambria, the Roman name for
Wales, where rocks of this age were first
studied.
Late or Upper
Middle
Early or Lower
Late or Upper
Middle
Early or Lower
Late or Upper
Middle
Early or Lower
Precambrian
(Beginning of earth - 544 million years ago)
All geologic time before the beginning of the Paleozoic era. This includes
about 90% of all geologic time and spans the time from the beginning of the
earth, about 4.5 billion years ago, to 544 million years ago. Its name means
"before Cambrian."
About the geologic time scale
Origins of a geologic time scale
The first people who needed to understand the geological relationships of different rock units were miners.
Mining had been of commercial interest since at least the days of the Romans, but it wasn't until the 1500s and
1600s that these efforts produced an interest in local rock relationships.
By noting the relationships of different rock units, Nicolaus Steno in 1669 described two basic geologic
principles. The first stated that sedimentary rocks are laid down in a horizontal manner, and the second stated
that younger rock units were deposited on top of older rock units. To envision this latter principle think of the
layers of paint on a wall. The oldest layer was put on first and is at the bottom, while the newest layer is at the
top. An additional concept was introduced by James Hutton in 1795, and later emphasized by Charles Lyell in
the early 1800s. This was the idea that natural geologic processes were uniform in frequency and magnitude
throughout time, an idea known as the "principle of uniformitarianism."
About the geologic time scale divisions
The geologic history of the Earth is broken up into hierarchical chunks of time. From largest to smallest, this
hierarchy includes eons, eras, periods, epochs, and ages. All of these are displayed in the portion of the
geologic time scale shown below.
Eon
Era
Period
Epoch
Phanerozoic Cenozoic Paleogene Oligocene
The Phanerozoic Eon represents the time during which the majority of macroscopic organisms — algae, fungi,
plants and animals — lived. When first proposed as a division of geologic time, the beginning of the
Phanerozoic (542.0 million years ago)* was thought to coincide with the beginning of life. In reality, this eon
coincides with the appearance of animals that evolved external skeletons, like shells, and the somewhat later
animals that formed internal skeletons, such as the bony elements of vertebrates. The time before the
Phanerozoic is usually referred to as the Precambrian and is usually divided into the three eons shown.
The Phanerozoic is subdivided into three major divisions: the Cenozoic, Mesozoic, and Paleozoic Eras. The "zoic" suffix comes from the root "zoo," which means animal. This is the same root as in the words zoology and
zoological park (or zoo). "Cen-" means recent, "Meso-" means middle, and "Paleo-" means ancient. These
divisions reflect major changes in the composition of ancient faunas, each era being recognized by its
domination by a particular group of animals. The Cenozoic has sometimes been called the "Age of Mammals,"
the Mesozoic the "Age of Dinosaurs," and the Paleozoic the "Age of Fishes." But this is an overly simplified
view, which has some value for the newcomer but can be a bit misleading. For instance, other groups of
animals lived during the Mesozoic. In addition to the dinosaurs, animals such as mammals, turtles, crocodiles,
frogs, and countless varieties of insects also lived on land. Additionally, there were many kinds of plants living
in the past that no longer live today. Ancient floras went through great changes too, and not always at the same
times that the animal groups changed.