Geologic Time Scale
Earth Science Spring 2014
The Geologic Column
• Geologic column- arrangement of rock layers
based on the ages of the rocks
– No single area on earth a record of all geologic
time
– Observations from around the world were used to
create the geologic column
– Represents a timeline of the earth’s history
– Rock layers are distinguished from one another
primarily by the fossils they contain and the type
of rock they are made of
Divisions of Geologic Time
• Earth’s history is divided by:
– Major surface or climate changes
– Extinction of various species
– **these types of events are used to divide the
geologic time scale into smaller units
Divisions of Geologic Time
• Eras
– Era- largest unit of geologic time
– 4 geologic eras
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Precambrian
Paleozoic
Mesozoic
Cenozoic
Divisions of Geologic Time
• Eras
– Precambrian Era (Precambrian Time)
• Earliest era
– Oldest layer of the geologic column
• Longest era (4 billion years)
• Very few fossils
– Makes it difficult to divide into smaller units
– Bacteria, algae, primitive worms, sponges, & corals
» Evidence that life started in the ocean
Divisions of Geologic Time
• Eras
– Paleozoic Era
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2nd era
Means “ancient life”
292 million years long
Fossils- wide variety of both marine & land plants &
animals
Divisions of Geologic Time
• Eras
– Mesozoic Era
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3rd era
Means “Middle Life”
183 million years long
Fossils- more complex organisms like reptiles & birds
Divisions of Geologic Time
• Eras
– Cenozoic Era
• Present geologic era
• Means “recent life”
• Fossil- mammals are common
Divisions of Geologic Time
• Periods- subdivision of a geologic era
– Characterized by specific fossils
– Named for the location in which the fossils were
first found
• Epochs – subdivision of a geologic time period
– Contain an extremely detailed fossil record
– 2 period of the cenozoic era are divided into these
Geologic History
• Theory of evolution
– Theory that organisms change over time and that
new organisms are derived from ancestral types.
– First proposed by Charles Darwin by way of
natural selection (survival of the fittest)
• Evidence:
– Fossil record
– Examination of living organisms
Geologic History
• Theory of Evolution cont…
– Geologic changes & climatic changes affect the
survival of organisms
• Example of geologic change is a dramatic decrease in
the amount of the earth’s surface covered by water
• Example climatic change is a decrease in atmospheric
temperature
Geologic History
• Precambrian Time
– Makes up 88% of the earth’s history
– Began with the formation of the earth 4.6 billion
years ago & ended 540 million years ago
– Rocks from this era are difficult to interpret
– Shields of precambrian rock are found on every
continent
– Fossils are rare
• Most common are stromatolites
• Stromatolites
Geologic History
• Paleozoic Era
– Began 540 million years ago & ended about 248
million years ago
– It is believed that Pangea formed by the end of
this era
– Abundant fossil record
• Rich in marine & land animal & plant fossils
– Divided into 7 periods
Geologic History
• Paleozoic Era- Cambrian Period
– Contains the first advanced marine life
– Most of the continents were covered with warm,
shallow oceans.
• Marine invertebrates thrived here
– Invertebrates- animals without backbones
– Most common- trilobites
– Brachiopods- shelled animals
• 2nd most common type of animal to live during this time
– Still no evidence of land-dwelling plants or animals
Geologic History
• Cambrian Period
Geologic History
• Paleozoic Era- Ordovician Period
– Number brachiopod species increased & number
of trilobites decreased
– Large number of coral started to appear
– Graptolite- useful index fossil
– First vertebrate appeared
• Vertebrate- animal with a backbone
– Still no plant life on land
Geologic History
• Ordovician Period
Geologic History
• Paleozoic Era- Silurian Period
– Marine life continued to thrive & evolve during
this time.
• Echinoderms became more numerous
• Eurypterids- scorpion-like sea creatures were abundant
– Near the end, land plants & animals started
showing up
• Ex: spiders, millipedes
Geologic History
• Silurian Period
Geologic History
• Paleozoic Era- Devonian Period
– Many kinds of bony fish
• Lungfish- air breathing fish
• Rhipidistians- land going fish
– 1st true amphibian showed up
• Ichthyostega- like a huge salamander
– Land plants began to develop
• Giant horsetails, ferns, cone-bearing plants.
Geologic History
• Devonian Period
Geologic History
• Paleozoic Era- Carboniferous Period
(Pennsylvanian & Mississippian)
– Climate was generally warm & the humidity was high
all over the world
– Forests & swamps covered much of the land
– Amphibians & fish continued to thrive
– Crinoids were common in oceans
– Insects were common on land
– Reptiles appeared & resembled large lizards
– Many of the coal, oil, & natural gas deposits are found
in rocks from this period
Geologic History
• Carboniferous Period
Geologic History
• Paleozoic Era- Permian Period
– End of the Paleozoic Era
– Mass extinction of many Paleozoic life forms
– Nearly all continents had joined to form Pangea
– Areas of desert & savannah formed in the interior
of the continent
• Shallow seas evaporated
• Many species of marine invertebrates became extinct
Geologic History
• Mesozoic Era
– Began 248 million years ago
– Ended 65 million years ago
– Pangea broke up into the continents
– Tectonic plates drifted & collided forming
mountain ranges
– Shallow seas & marshes covered most of the land
– Climate was warm & humid.
• Favored reptiles
Geologic History
• Mesozoic Era- Triassic Period
– Dinosaurs first appeared
– Dinosaurs varied greatly in size
– Lush forests of cone-bearing trees & cycads
– New forms of marine life appear
– Earliest mammals made their appearance
Geologic History
• Mesozoic Era- Jurassic Period
– Dinosaurs were the dominant form of life
• 2 major groups of dinosaurs evolved.
– Saurischians- both carnivores & herbivores
– Ornithischians- herbivores
– Flying reptiles were common
Geologic History
• Mesozoic Era- Cretaceous Period
– Dinosaurs continued to dominate the earth
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Tyrannosaurus rex
Ankylosaurs-armored bodies
Ceratopsians- horned
Ornithopods- duck-billed
– 1st flowering plants appeared (angiosperms)
– End of this period was marked by a mass
extinction
Geologic History
• Cenozoic Era (Age of the Mammals)
– Began 65 million years ago
– Continents in the beginning looked much like they
do today
– Increased tectonic activity
– Dramatic climate changes
– Mammals became the dominant life form
Geologic History
• Cenozoic Era (Age of the Mammals)
– 2 periods
• Tertiary
– Time before the last ice age
• Quaternary
– Began with the last ice age & includes present time
• ** divided into 7 epochs
– Tertiary- Paleocene, Eocene, Oligocene, Miocene, & Pliocene
– Quaternary- Pleistocene & Holocene
Geologic History
• Cenozoic Era: Paleocene & Eocene Epochs
(Tertiary Period)
– Paleocene Epoch
• Many new mammals evolved
– Small rodents
– 1st primates appeared
– Eocene Epoch
• First flying squirrels, bats, and whales appeared
• Smaller reptiles flourished
• World wide temperature dropped 4 degreed Celsius
Geologic History
• Cenozoic Era: Oligocene & Miocene Epochs (Tertiary
Period)
– Oligocene Epoch
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Climate continued to become cooler & drier
Early mammals became extinct
Larger species of today’s common animals flourished
Himalaya mountains formed
– Miocene Epoch
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Climate still dry & cool
Golden Age of Mammals
Common fossils include: deer, rhinoceros, & pig families
Largest known mammals lived
Modern polar ice caps started to form
Geologic History
• Cenozoic Era: Pliocene Epoch (Tertiary Period)
– Hunting animals became fully evolved
– Fossils of first modern horses
– Great climatic changes occurred & continental ice
sheets began to spread
– Land bridge between Eurasia & North America
appeared
– North & South America became connected by the
central American Land Bridge
Geologic History
• Cenozoic Era: Pleistocene & Holocene Epochs
(Quaternary Period)
– Pleistocene Epoch
• Several periods of glaciation occurred over most of North America
& Eurasia
• Many animals became extinct
• Fossils of early ancestors of humans
– Holocene Epoch
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Began about 11,000 years ago as the last ice age ended
Sea level rose drastically as glaciers & ice sheets melted
Great Lakes were formed
Modern humans developed
Agriculture developed
Use of bronze & iron tools began