ROCKS OF ALL AGES
The geologic time scale is one of the most significant
achievements of all of science. It's taken considerable work by numerous
geologists over many decades to come up with a useful geologic time scale for
Earth with accurate boundary dates. Geochronological work is ongoing that
continues to refine the dates of important boundaries.
Earth history is divided into three large time
intervals - eons. From oldest to youngest, they are the Archean
Eon, the Proterozoic Eon, and the Phanerozoic Eon. Some geologists want
to insert a fourth eon at the beginning of Earth's history (variously called
"Hadean" or "Priscoan").
These terms do not have fixed
definitions. Some define them based on the oldest known preserved rocks -
well, someone may find older ones. Note the recent discovery of
4.28 billion year old rocks from Hudson Bay - these are significantly older
than the previous oldest known Earth rock at 4.03 billion years. Also
note the recent indirect discovery of 4.45-4.55 billion years old mantle
rocks below Baffin Island, Canada.
Or, some have suggested that these terms be defined
based on speculative events (e.g., 1 - the collision event that
produced the Moon, which may not have happened according to several lines
of evidence, including 4.4 billion year old Earth zircons indicating a cool
early Earth; 2 - the late heavy bombardment event, which is based on
lunar mare evidence that can be interpreted in other ways).
So, the terms "Hadean" and
"Priscoan" are rejected here.
Each eon can be subdivided into smaller units called eras.
Eras can be subdivided into smaller units called periods. Periods
can be subdivided into epochs (a.k.a. series). Epochs/series can
be subdivided into stages.
Photos of example rocks from each major division of
the geologic time scale are given on the linked pages below.
Precambrian
The "Precambrian" is an informal time term
that combines all geologic time units predating the Cambrian. The
Precambrian includes the Archean Eon and the Proterozoic Eon.
Archean Eon
The term "Archean" simply means
"ancient". The Archean starts at the beginning of Earth, 4.55
billion years ago. It ends arbitrarily at precisely 2.50 billion years
ago. Traditionally, the end of the Archean was intended to represent the
beginning of the rise of free atmospheric oxygen gas. Relatively few Archean-aged
rocks are exposed at the Earth's surface. Many exposed Archean rocks are
metamorphic or igneous. Very few Archean rocks are sedimentary.
Different folks subdivide the Archean in different
ways. In general, geologists seem to want four subdivisions (eras) in the
Archean (from oldest to youngest): Eoarchean, Paleoarchean, Mesoarchean,
Neoarchean. Every time scale you look at usually has different
dates on these boundaries.
Archean-aged
orthogneiss (a metamorphosed granite), Beartooth Highway, Montana, USA.
Proterozoic Eon
"Proterozoic" means "hidden-life",
referring to the presence of rare, microscopic fossils in some rocks of this
age. The term was first proposed by Samuel Emmons in
1888. The Proterozoic starts arbitrarily at 2.5 billion years and ends at
544 million years ago (see additional comments about this date in the Cambrian
section).
The Proterozoic has an abundance of preserved
sedimentary rocks, in constrast to the Archean. This was a time of
considerable continental consolidation, and the first appearance of widespread
continental shelves. Such environments are necessary in order to get
sedimentary rocks preserved for long intervals of time.
The Proterozoic is divided into three parts (eras)
(from oldest to youngest): the Paleoproterozoic, the Mesoproterozoic,
and the Neoproterozoic. The PP-MP boundary is often given as 1.60
billion years. The MP-NP boundary is often given as 1.0 billion years or
900 million years.
Proterozoic-aged quartzite (a metamorphosed quartzose sandstone), Lorrain
Quartzite, Manitoulin Island, Ontario, Canada.
Paleozoic
After the Precambrian, we've got the Phanerozoic
Eon. "Phanerozoic" means "apparent-life",
referring to the abundance of macroscopic body fossils, microscopic body
fossils, and trace fossils in sedimentary rocks of this age. The
Phanerozoic is subdivided into three eras (from oldest to youngest): the
Paleozoic Era, the Mesozoic Era, and the Cenozoic Era.
The Paleozoic has six periods (from oldest to
youngest): Cambrian, Ordovician, Silurian, Devonian, Carboniferous,
Permian. In America, the Carboniferous is considered to be two periods,
the Mississippian and the Pennsylvanian. Outside America, the
Mississippian and Pennsylvanian are considered together and called the
Carboniferous. In order to settle this disagreement, the IUGS has ruled that everyone on Earth has to
use all three terms (a good compromise is one that no one likes). So, the
"Carboniferous Period" is a valid term, and so are
"Pennsylvanian Subperiod" and "Mississippian Subperiod".
Cambrian
Period
The Cambrian was first named in 1835 by Adam Sedgwick.
It dates from 544 to 490 million years. Many consider the base of the
Cambrian to be at 542 million years. Based on available information and
evidence, I'm not convinced that the 544 m.y. date is not accurate (there are
correlation and isotope issues that haven't yet been addressed to my
satisfaction). So, 544 m.y. it is!
The Cambrian begins with the most significant event in
the history of Earth - the Cambrian Explosion. Despite the name, this
does not refer to an impact event or a volcanic eruption or a natural
disaster. The Cambrian Explosion refers to the rapid evolutionary
appearance of abundant macrofossils, microfossils, and trace fossils near the
Precambrian-Cambrian boundary. After years of study, the PC-C boundary
was defined to be at the first appearance of complex trace fossils, as
exemplified by Trichophycus pedum (aka Treptichnus pedum; aka
Phycodes pedum). By the way, almost everyone thinks that T.
pedum is the definition of the PC-C boundary. It is not.
Read the original definition. The PC-C boundary is defined at the first
appearance of complex trace fossils, as exemplified by T. pedum.
Cambrian rocks are often noticeably
fossiliferous. The most common fossils found in Cambrian rocks are trilobites.
Cambrian-aged
fossiliferous sandstone (with abundant trilobite fragments), Eau Claire
Formation, Little Falls Dam, Wisconsin, USA.
Ordovician
Period
The Ordovician was first named in 1879 by Charles
Lapworth. It dates from 490 to 444 million years ago. During
the Ordovician, Earth experienced the highest sea levels that we have evidence
for. This was also a time of a global greenhouse climate.
Ordovician rocks around the world are often richly fossiliferous.
Ordovician-aged fossiliferous limestone (with abundant crinoid stem pieces, plus
trilobite fragments, bryozoans, tentaculites, etc.), Kope Formation, Covington,
Kentucky, USA.
Silurian Period
The Silurian was first named in 1839 by Roderick Murchison.
It dates from 444 to 416 million years ago.
Silurian-aged
dolostone (with a few fossil ostracods - see the oval-shaped structures near
the bottom), Tymochtee Dolomite, Latham Limestone Quarry, Ohio, USA.
Devonian Period
The Devonian was first named by Adam Sedgwick and Roderick Murchison
back in 1839, when they were still friends and working together. The
Devonian dates from 416 to 359 million years.
Devonian-aged
fossiliferous limestone (with abundant brachiopods), Columbus Limestone,
Columbus, Ohio, USA.
Mississippian Period (Subperiod)
The Mississippian (= lower Carboniferous) was first
named in 1891 by Henry
Shaler Williams. It dates from 359 to 318 million years ago.
Mississippian-aged fossiliferous limestone (partly chertified, with abundant crinoid
stem pieces and a camerate crinoid calyx at right-center), Fort Payne Formation,
Lake Cumberland, Kentucky, USA.
Pennsylvanian Period (Subperiod)
The Pennsylvanian (= upper Carboniferous) was first
named in 1891, in the same publication by H.S. Williams that names
and defines the Mississippian. It dates from 318 to 299 million
years. Global sea levels during the Pennsylvanian were wildly
fluctuating, due to the repeated waxing and waning of glacial ice sheets in
Gondwana (= the modern-day southern continents).
Pennsylvanian-aged fossiliferous ironstone (with carbonized fern frond and plant
stems), Nelsonville, Ohio, USA.
Permian Period
The Permian was first named in 1841 by Roderick Murchison.
It dates from 299 to 251 million years ago. The end of the Permian is the
same as the end of the Paleozoic. The end-Permian/end-Paleozoic marks the
greatest mass extinction event in the history of planet Earth. There's
tons of geologic data that help geologists interpret the causative factors
behind the end-Permian mass extinction, but long story short - it's still a bit
of a mystery. There's excellent evidence for one (or more) impact events
(we've got pieces of the original meteorite). There's excellent evidence
of the largest volcanic flood basalt event in history occurring at this time
(the Siberian Traps). There's impressive evidence that Earth's atmosphere
lost much of its free oxygen gas. The list goes on and on.
Permian-aged
limestone, Bird Spring Formation, Arrow Canyon Range, Nevada, USA.
Mesozoic
The Paleozoic Era ended with a severe mass extinction
event. What followed next was the Mesozoic Era, which is subdivided into
three periods (from oldest to youngest): Triassic, Jurassic, Cretaceous.
The Mesozoic was the "Age of Reptiles". Reptiles were at the
top of the food pyramid in the water, on the land, and in the air. The
most famous Mesozoic fossils are the dinosaurs. They first appear
in the Triassic, and they disappear at the end of the Cretaceous.
The Mesozoic dates from 251 to 65 million years ago (a
186 million year duration).
Triassic Period
The Triassic was first named in 1834 by Friedrich August
von Alberti. It dates from 251 to 200 million years.
Triassic-aged
mudshale (redbed), Connecticut Valley, Connecticut, USA.
Jurassic Period
The Jurassic was first named in 1829 by Alexandre
Brongniart. It dates from 200 to 145 million years.
Jurassic-aged
rock gypsum, Arapien Formation, San Pitch Mountains, Utah, USA.
Cretaceous Period
The Cretaceous was first named in 1822 by Jean-Baptiste-Julien
d'Omalius d'Halloy. It dates from 145 to 65 million years.
During the Cretaceous, Earth had the second-highest sea levels of all time
(they were higher during the Ordovician). This was a time of a global
greenhouse climate.
The Cretaceous literally ends with a bang. The
Cretaceous-Tertiary boundary (often referred to as "K-T"; = Mesozoic-Cenozoic
boundary) is placed at one of the most significant mass extinction events in
history. Famous victims of this extinction include the dinosaurs, the
pterosaurs, the plesiosaurs, the mosasaurs, the ichthyosaurs, etc.
Abundant evidence has been accumulated since the late 1970s indicating a
massive impact event occurred in the Yucatan Peninsula of Mexico. The
extinction was a result of the severe climate effects of dust & debris
blocking sunlight and possible acid rain. For more info., see the K-T
clay.
Cretaceous-aged granite, Sierra Nevada Batholith, Lake Tahoe (Nevada side), USA.
Cenozoic
The Cenozoic Era includes the modern day. It
started 65 million years ago, at the catastrophic impact event that killed off
the dinosaurs. Historically, the Cenozoic is divided into a long Tertiary
Period (65 to 1.8 million years) and a short Quaternary Period (1.8
million years to now). More recently, the Cenozoic is divided more or
less evenly into the Paleogene Period (65 to 23 million years) and the Neogene
Period (23 million years to now).
The Cenozoic's period terminology is less important
than the epoch terminology. The Cenozoic is divided into seven epochs
(from oldest to youngest): Paleocene, Eocene, Oligocene, Miocene, Pliocene,
Pleistocene, Holocene.
Paleocene Epoch
The Paleocene was first named in 1874 by Wilhelm
Schimper. It dates from 65 to 55.8 million years. The end of
the Paleocene is marked by a dramatic climate warming event called the Paleocene-Eocene
Thermal Maximum.
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Paleocene-aged oncolitic limestone, Flagstaff Formation, San Pitch Mountains,
Utah, USA.
Eocene Epoch
The Eocene was first named in 1833 by Charles Lyell.
It dates from 55.8 to 33.9 million years. The Eocene starts with a
significant climate warming event. Things gradually cool down from the
Eocene into the Oligocene.
Eocene-aged
fossiliferous marlstone (with complete fossil herring, Knightia eocaena),
Green River Formation, Kemmerer, Wyoming, USA.
Oligocene
Epoch
The
Oligocene was first named in 1854 by Ernst
Beyrich. It dates from 33.9 to 23 million years.
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Oligocene-aged porphyritic phonolite, Cripple Creek Diatreme, Cripple Creek,
Colorado, USA.
Miocene Epoch
The Miocene was first named in 1833 by Charles Lyell.
It dates from 23 to 5.33 million years.
Miocene-aged
basalt, Columbia River Flood Basalt Group, Rt. 90, Washington State, USA.
Pliocene Epoch
The Pliocene was first named in 1833 by Charles Lyell.
It dates from 5.33 to 1.81 million years.
Pliocene-aged
sandstone (diagenetic concretion of alunite- & jarosite-cemented
sandstone), Camp Rice Formation, Picacho Mountain, New Mexico, USA.
Pleistocene Epoch
The Pleistocene was first named in 1839 by Charles Lyell.
It dates from 1.81 million to 10 thousand years. The Pleistocene is
otherwise well known as the last Ice Age. It included four separate
glacial intervals. Each phase of glaciation was separated from the next
by a warm, interglacial interval. The last of the Pleistocene glaciations
ended about 10,000 years ago. Two massive continental ice sheets still
remain from the Pleistocene glaciations - one in Greenland, and one in
Antarctica.
Pleistocene-aged fossiliferous limestone, Grotto Beach Formation, San Salvador
Island, Bahamas.
Holocene Epoch
The Holocene was defined in 1885 by the International
Geological Congress. It dates from 10,000 years ago to now. The 10
ky date is arbitrary, but is traditionally considered to represent the end of
the last Ice Age.
Holocene-aged
scoriaceous basalt, Black Rock Desert, Utah, USA.
For those interested in all things related to the
geologic time scale, the following reference (& its precursor) is highly
recommended:
Gradstein, F., J. Ogg & A. Smith.
2004. A Geologic Time Scale 2004. Cambridge. Cambridge
University Press. 589 pp. 1 poster.
Harland, W.B., R.L. Armstrong, A.V. Cox, L.E. Craig.,
A.G. Smith & D.G. Smith. 1990. A Geologic Time Scale 1989.
Cambridge. Cambridge University Press. 263 pp.