Geologic Time
Note : There are many different ideas on the division of geologic time. Please use the
geologic time scale found in your book for testing purposes. Some pictures in this
slideshow may show a slightly different version. Neither is right or wrong, just different
interpretations.
Geologic time is divided into
giant chunks of time called eons.
There have been two eons in
Earth’s history: the Precambrian
and the Phanerozoic.
The Precambrian eon is divided
into sub-eons called the Hadean,
the Archean, and the Proterozoic.
Eons are broken into large chunks of time called eras.
Eras are broken up into
smaller chunks of time called
periods.
Periods are broken up into
even smaller chunks of time
called epochs.
Today, we are in the:
• Phanerozoic eon
• Cenozoic era
• Quaternary period
• Holocene epoch
Precambrian Eon
The Precambrian is an immense
chunk of time in Earth’s history
beginning 4.6 billion years ago until
the start of the Cambrian period (in
the Phanerozoic eon), about four
billion years later.
The Precambrian spans about 87% of Earth’s history.
The Precambrian Eon saw the formation of the earth, the formation of an
atmosphere, and the first one-celled and multi-celled organisms.
Very little is known about this ancient time because early rock history has
been disrupted by plate tectonics, erosion, and deposition.
Untangling the long, complex Precambrian rock record is a large task that is
far from done.
Most Precambrian rocks are devoid of fossils, which makes correlation of rock
difficult.
Rocks of this time are metamorphosed and deformed, extensively eroded,
and obscured by overlying strata.
Consequently, this span of time has not been successfully divided into briefer
time units and is missing many parts.
Shields
Each continent has large “core areas” of Precambrian rocks dominate on the
surface (mostly deformed as metamorphic rock). These areas are called
shields.
North American Precambrian Shield
The North American
shield encompasses
about 7.2 million
square kilometers (2.8
million square miles) –
the equivalent of ten
states of Texas.
Formation of Earth’s Atmosphere
Rocks from the middle Precambrian (1.2 – 2.5
billion years ago) contain most of Earth’s iron
ore, mainly as the mineral hematite (Fe2O3).
These iron-rich sedimentary rocks probably
represent the time when oxygen became
sufficiently abundant to react with iron
dissolved in shallow lakes and seas.
Later, after much of the iron was oxidized
and deposited on lake and sea bottoms,
formation of these iron-rich deposits
declined and oxygen levels in the ocean and
atmosphere began to increase.
Because most of Earth’s free oxygen results from plant photosynthesis, the
formation of extensive Precambrian iron ore deposits is linked to life in the sea.
Earth’s atmosphere formed about 2 billion years ago.
Absent in the Precambrian are fossil fuels (coal, natural gas, oil). The reason
is because this time was virtually absent of land plants to form coal swamps
and of certain animals to form petroleum.
Precambrian Fossils
Today, our knowledge of Precambrian life, although
far from complete, is quite extensive.
Precambrian fossils are disappointing if you expect plants or large animals.
The most common Precambrian fossils
are stromatolites which are mounds or
columns of calcium carbonate.
Stromatolites are not the remains of
actual organisms, but are material
deposited by algae. They are indirect
evidence of algae because they closely
resemble similar deposits made by
modern algae.
Stromatolites did not become common until the middle Precambrian (2 billion
years ago).
Most Precambrian ancient fossils are preserved in chert, a hard, dense
chemical sedimentary rock.
Chert must be very thinly sliced and studied under
powerful microscopes to observe bacteria and algae
fossils within it.
Chert
Precambrian filamentous cyanobacteria
Cyanobacteria (Nostocales) from the Bitter
Springs Chert of Central Australia, 850 million
years old.
Microfossils have been found at several locations worldwide. Two notable areas
are in southern Africa, where the rocks date to more than 3.1 billion years, and
in the Gunflint Chert of Lake Superior, which dates to 1.7 billion years.
Microfossils are perhaps the most important group of all fossils — they are
extremely useful in age-dating, correlation and paleoenvironmental
reconstruction.
In both places, bacteria and blue-green algae have been discovered.
The fossils are of the most primitive organisms, prokaryotes (cells that lack
organized nuclei, and reproduce asexually).
Phanerozoic Eon
The Phanerozoic eon consists
of the Paleozoic, Mesozoic, and
Cenozoic eras.
Approximately 540 million
years ago to the present.
Paleozoic Era
The Paleozoic era includes the Cambrian,
Ordovician, Silurian, Devonian, Carboniferous
and Permian Periods.
It extends from 540-248 million years ago.
Paleozoic Era
Cambrian Period – “Age of Trilobites”
(540-500 million years ago)
"Age of Trilobites“ - Many marine invertebrates (marine animals with mineralized shells)
- First vertebrates - Earliest primitive fish - Mild climate - The supercontinent Rodinia
began to break into smaller continents (no correspondence to modern-day land masses) Mass extinction of trilobites at the end of Cambrian (50% of all animal families went
extinct), probably due to glaciation.
Paleozoic Era
Ordovician Period
(505-438 million years ago)
Primitive plants appear on land - First corals - Primitive fishes, seaweed and fungi High sea levels at first, global cooling and glaciation, and much volcanism - North
America under shallow seas - Ends in huge extinction, due to glaciation.
Paleozoic Era
Silurian Period
(438-408 million years ago)
The first jawed fishes and
uniramians (largest group of
anthropods - like insects,
centipedes and millipedes)
appeared during the Silurian
(over 400 million years ago) First vascular plants (plants
with water-conducting tissue
as compared with non-vascular
plants like mosses) appear on
land - High sea levels
worldwide - Brachiopods,
crinoids, corals.
Paleozoic Era
Devonian Period – “Age of the Fish”
(408-360 million years ago)
Fish and land plants become abundant and diverse - First tetrapods appear toward the
end of the period - First amphibians appear - First sharks, bony fish - Many coral
reefs, brachiopods, crinoids - New insects, like springtails, appeared - Mass extinction
(345 mya) wiped out 30% of all animal families) probably due to glaciation or meteorite
impact.
Paleozoic Era
Carboniferous Period
(360-280 million years ago)
Wide-spread coal swamps, corals, bryozoans, brachiopods, seed ferns, and other plants Amphibians become more common.
Mississippian sub-period (360 to 325 mya)
First winged insects.
Pennsylvanian sub-period (325 to 280 mya)
First reptiles - Many ferns - The first mayflies and cockroaches appear.
Paleozoic Era
Permian Period – “Age of Amphibians”
(280-248 million years ago)
"The Age of Amphibians" - Amphibians and reptiles dominant - Gymnosperms
dominant plant life - The continents merge into a single supercontinent, Pangaea Phytoplankton and plants oxygenate the Earth's atmosphere to close to modern levels The first stoneflies, true bugs, beetles - The Permian ended with largest mass extinction
- Trilobites go extinct, as do 50% of all animal families, 95% of all marine species, and
many trees, perhaps caused by glaciation or volcanism.
Mesozoic Era
The Mesozoic era consists of the Triassic, Jurassic and
Cretaceous periods.
It spans from 248-65 million years ago.
This era is known as the “Age of the Reptiles”.
Mesozoic Era
Triassic Period
(248-208 million years ago)
The first dinosaurs and mammals appear - Mollusks are the dominant invertebrate Many reptiles, for example, turtles - True flies appear - Triassic period ends with a
minor extinction 213 mya (35% of all animal families die out - This allowed the
dinosaurs to expand into many niches.
Mesozoic Era
Jurassic Period
(208 - 146 million years ago)
Continental drift, Pangaea begins to break apart into Laurasia and Gondwana toward the
end of the Jurassic Period.
Apatosaurus (formerly known as Brontosaurus), large, plant-eating dinosaurs of the late
Jurassic
Many dinosaurs - The first birds appear (Archaeopteryx) - The first flowering plants
evolve - Many ferns, cycads, ginkgoes, rushes, conifers, ammonites - Minor extinctions
at 190 and 160 mya.
Mesozoic Era
Cretaceous Period
(146 - 65 million years ago)
Lower epoch - (146-98 mya)
The height of the dinosaurs - The first crocodilians, and feathered
dinosaurs appear - The earliest-known butterflies appear (about
130 million years ago) as well as the earliest-known snakes, ants,
and bees - Minor extinctions at 144 and 120 mya.
100 million years ago: further continental drift, begins formation
of modern continents
Upper epoch - (98-65 mya)
High tectonic and volcanic activity - Primitive marsupials develop - Continents have a
modern-day look - Minor extinction 82 mya that ended with large extinction of
dinosaurs, ammonites, about 50 percent of marine invertebrate species, etc., probably
caused by asteroid impact or volcanism.
Cenozoic Era
Most recent era
Began 65 million years ago, continues today
The “Age of Mammals”
The Cenozoic era is divided into two periods: Quaternary and Tertiary
Some of the highest mountains (Alps, Andes, Himalayan) were lifted during this
time.
Cenozoic Era
Tertiary Period
(65-1.8 million years ago)
The tertiary period includes Paleocene, Eocene, Oligocene, Miocene and Pliocene Epochs.
66 million years ago: asteroid (10-20 mile diameter) impact? - causing extinction of the
dinosaurs and other land animals
34 million years ago: separation of Antarctica from South America, begins formation of
Antarctic ice
Paleocene epoch (65-54 mya)
First large mammals and primitive primates
Eocene epoch (54-38 mya)
Mammals abound - Rodents appear - Primitive whales appear
Oligocene epoch (38-24 mya)
Starts with a minor extinction (36 mya) - Many new mammals (pigs, deer, cats,
rhinos, tapirs appear) - Grasses common.
Miocene epoch (24-5 mya)
More mammals, including the horses, dogs and bears - Modern birds
Pliocene epoch (5-1.8 mya)
First hominids - Modern forms of whales.
Cenozoic Era
Quaternary Period – “Age of Man”
(1.8 million years ago - Present)
The quaternary period includes the Pleistocene and Holocene Epochs.
This period is known as the “Age of Man”.
Pleistocene epoch - The Last Ice Age (1.8 - .011 mya)
The first humans (Homo sapiens) evolve - Mammoths, mastodons, saber-toothed
cats, giant ground sloths - A mass extinction of large mammals and many birds
happened about 10,000 years ago, probably caused by the end of the last ice age.
Holocene epoch - (11,000 years ago to today)
Human civilization