Earth History
Geologic Time Scale
Geologic Time Scale
• The geologic Time Scale is a record of
Earth’s history
– The Earth is 4.6 Billion Years Old
• The names of the divisions do not change
however the years designating the
beginning and end of these divisions are
often reconsidered
Eons
• The time scale is divided into eons
• An Eon is the longest time unit and is
measured in billions of years
Eons
• The history of the Earth is divided into
4 eons:
–Hadeon(4.6 Bya-3.8 Bya)
–Archaen(3.8 Bya-2.5 Bya)
–Proterozoic(2.5 Bya-542 mya)
–Phanerozoic(542 mya-Present)
Precambrian
• The Precambrian includes the
– Hadeon
– Archaen
– Proterozoic eons.
• Includes 90% of Earth’s History
• The end of the Proterozoic is defined by
the first appearance of organisms with
hard parts(shells, skeletons)
Precambrian
• Up until the end of the Proterozoic all
organisms had soft bodies
• Many of these organisms resemble:
– Sponges
– Snails
– Worms
Eras
• Eras are the next longest span of time.
• Eras are measured in hundreds of millions
of years
Eras
• The names of the eras of the Phanerozoic
are based on their age
– “Paleo” means old
– “Meso” means middle
– “Ceno” means recent
– “Zoic” means life
Eras
• The Phanerozoic Eon is divided into 3
Eras:
– Paleozoic(542 mya-250 mya)
– Mesozoic(250 mya-65 mya)
– Cenozoic(65 mya-Present)
Paleozoic Era
• During the Paleozoic era the oceans had a
wide diversity of plants and animals
• Trilobites were the dominant organism in
the oceans during the early Paleozoic
• All trilobites were extinct by the end of the
Paleozoic
Paleozoic Era
• Land plants and land animals first
appeared
• At the end of the Paleozoic 90% of marine
organisms went extinct
• This is considered to be the largest mass
extinction event in Earth’s History
Mesozoic Era
• The mesozoic era is known for:
– The emergence of the dinosaurs
– Reef Building corals
– Predatory reptiles
– Amphibians living on land and in water
• Dinosaur population began to decline
towards the end and mammals began to
evolve
Mesozoic Era
• Like the Paleozoic the end of the
Mesozoic is marked by a massive
extinction event
Cenozoic Era
• During the Cenozoic:
– Mammals increased in number and diversity
– Human ancestors developed
– Grasses and flowering plants expanded on
land
– Ocean life remained relatively unchanged
however
Periods
• Eras are divided into periods
• Periods are usually defined by life forms
that appeared or went extinct during that
time
• Some periods are named for a geographic
area in which the first rock of that age was
discovered
Periods
• The Mississippian period was named for a
distinctive limestone that formed along the
Mississippi River
Periods
• The Jurassic Period is named for the rocks
discovered in the Jura Mountains in
Europe
Epochs
• Periods are divided into Epochs which are
measured in millions of years to tens of
millions of years
• The fossil record the Cenozoic Era is
relatively complete
• There has been less time for weathering
and erosion to destroy the fossil record
Epochs
• Certain organisms are used to distinguish
the various epochs
– Marine Fossils are used to mark the
Oligocene
– Terrestrial plant fossils are used to mark the
Eocene
Relative Dating
Relative Dating Of Rocks
• Relative Dating estimates the order of past
geologic events by using basic
stratigraphic rules
• Relative dating does not give us the ages
of past events but can tell us the order in
which they happened
Relative Dating
• Uniformitarianism states that “The Present
is the Key to the past”.
• In other words processes that are
occurring today were more than likely
occurring throughout Earth’s history
Geologic Principles
• Principle of Superposition
• Principle of Original Horizontality
• Principle of Cross Cutting Relationships
Principle of Superposition
• The principle of superposition states that
in an undisturbed rock sequence the
oldest rocks are on the bottom and going
up they progressively get younger
• faculty.icc.edu/easc111lab/labs/labf/prelab
_f.htm
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Principle Of Original Horizontality
• The Principle of Original Horizontality
states that sedimentary rocks are
deposited in horizontal or nearly horizontal
layers
Principle Of Original Horizontality
• If the layers are not horizontal then there
was an event that caused the layers to fold
or tilt
Principle Of Cross Cutting Relationships
• The Principle of Cross Cutting
Relationships states that a rock that
intrudes into another rock is going to be
younger than the rock that it is cutting into
Principle Of Cross Cutting Relationships
• The Principle of Cross
Cutting Relationships allows
us to say that the fault
formed after the rock layers
formed
• A fault is always going to be
younger than the feature
that it cuts through because
of the Principle of Cross
Cutting Relationships
Inclusions
• Inclusions are pieces of one rock that are
contained within another
• If a rock layer contains particles of another
rock material from the layer beneath it
then the layer beneath it is older
Inclusions
• The bottom layer became eroded and the
loose material became incorporated in the
new top layer
• A newly formed rock that is Jurassic in age
may contain particles that are Ordovician
in age
Unconformities
• Erosion and flooding can often destroy
evidence of past geologic events
• Erosional surfaces may be later buried by
a younger rock
• When a buried erosional surface results in
a gap in the rock record it is called an
unconformity
Unconformities
• There are 3 types of unconformities:
– Disconformity
– Nonconformity
– Angular Unconformity
Disconformities
• A disconformity is an unconformity
between parallel layers of sedimentary
rock which represent a period of erosion or
non-deposition
Nonconformities
• A nonconformity is a break that separates
older metamorphic or intrusive igneous
rocks from younger sedimentary rocks
Angular Unconformity
• An angular unconformity consists of a
tilted sedimentary rock that is overlain by a
younger more flat lying strata
Correlation
• Correlation is the matching of outcrops of
one geographic region to another
Absolute Dating
Absolute Dating Techniques
• Absolute dating allows scientists to
determine the actual age of a rock, fossil
or other object
• Scientists use radioactive decay to
determine the ages of rocks
Radioactive Decay
• Radioactive substances emit particles at a
set rate
• As they emit particles the number of
protons and neutrons change and the
element is converted into a different
element
• Radioactive dating is the emission of
radioactive particles and the resulting
change into other elements
Radiometric Dating
• Radiometric dating is the process in which
scientists determine the ratio of parent
nuclei to daughter nuclei
• After they determine the ratio of parent to
daughter nuclei they can figure out the
actual age of the object
Radiometric dating
• As this process takes place the “Parent”
decays into the “daughter”
• The parent isotope is what the element
originally was
• The daughter isotope is what the parent
isotope is turning into
Radiometric Dating
• Example:
– Uranium-238 will decay into Lead-206 during
a specific span of time
• The rate at which these particles decay
remains constant making them good
indicators of the actual age of the object
Half Life
• Half life-The period of time it takes for one
half of the isotope to decay
Useful Isotopes
Radioactive Isotope
(Parent)
Approximate Half Life
Decay Product
(Daughter)
Rubidium-87
48.6 Billion Years
Strontium-87
Thorium-232
14 Billion Years
Lead 208
Potassium 40
1.3 Billion Years
Argon 40
Uranium 238
4.5 Billion Years
Lead 206
Uranium 235
700 Million Years
Lead 207
Carbon 14
5730 Years
Nitrogen 14
Useful Isotopes
• Carbon 14-Useful for finding the age of
materials that are of organic origin
– Amber
– Humanoid Bones
– Papyrus
– Charcoal Fragments
• The half life of Carbon 14 is 5730
therefore it would be used to date rocks
that are thousands of years old
Half Lives
Number Of
Half Lives
Percent
Parent
Isotope
Percent
Daughter
Isotope
Elapsed
Years
0
100%
0%
0
1
50%
50%
5730
2
25%
75%
3
12.5%
87.5%
11560
(5730 X 2)
17090
(5730 X 3)
Half Lives
• The half life of U-238 is 4.5 Billion Years
old so therefore it would not be useful to
date something that is thousands of years
old
• The isotope used is based on the age
range of the rock
Practice
• How old are the following rocks:
– Contains 50% U-235, 50% Pb-207
– Contains 25% K-40, 75% Ar-40
– Contains 12.5% C-14, 87.5% N-14
– Contains 50% U-238, 50% Pb-206
Other Absolute Dating Techniques
• Dendrochronology-The science of
comparing annual growth rings in trees to
date event and changes in the past
environments
• During the spring a tree experiences it’s
greatest growth
• During the winter it’s growth is less
Dendrochronology
• The widths of a trees rings are related to
the climate conditions during growth
periods
Varves
• Vares are bands of
alternating light and dark
colored sediments of
sand, clay and silt
• During the Pleistocene
(11,000 years ago) there
was mass glaciation
• When the glaciers melted
the glacial sediments
were deposited in lakes
Varves
• The glacier sediment is dark in color
• When they mix with sediments that
accumulated during warm temperatures
varves are formed
Key Beds
• Key Beds-Sediment layers that serves as
a time marker in the rock record.
• Key beds result from:
– Volcanic Ash
– Meteorite Impact debris
• Found in rocks deposited during the
Cretaceous Period
Key Beds
• Key Beds are released by volcanic
eruptions as well
• Will be used to date rocks back to the
eruption of Mount St. Helens(1980)
Fossils: Indicators Of Past
Life
Fossils
• Fossils are remains or traces of prehistoric
life
• Fossils are important tools in interpreting
the geologic past
Fossils
• Fossils are important in:
– Providing evidence that organisms have
changed through time
– Providing information about past
environmental conditions
– Useful in correlating rock layers
Fossils
• The following is a trilobite. A useful fossil
for dating back to the Cambrian period
Fossils
• A fossil is said to have original
preservation if the remains have not
undergone any change
• These fossils require conditions such as:
– Freezing
– Drying Out
– Oxygen Free Environments
Fossils
• Some environments
that can provide
original preservation
are:
– Permafrost in Alaska
– Sticky ooze in La Brea
Tar Pits in California
– Tree sap that hardens
into amber
Altered Hard Parts
• Altered hard parts are fossils whose
organic material has been removed
• Permineralization is the process by which
minerals fill in the empty pore spaces
• Petrified Wood
Index Fossils
• Index Fossils are remains of plants or
animals that can be used by geologists to:
– Correlate rock layers over large areas
– Date a particular rock layer
Index Fossils
• Index Fossils must be:
– Easily Identifiable
– Widespread Geographically
– Short Time Range
Index Fossils
• Mollusk Ecphora-Excellent index fossils for
the Mesozoic era.
• It has a:
– Distinct Shape
– Widespread Abundance
Molds
• A mold is formed when a shell or other
structure is buried in sediment and then
dissolved by underground water
• If the mold later becomes filled with
minerals it is known as a cast
Trace Fossils
• Trace fossils are evidence of an animals
past activity
• Trace fossils include:
– Footprints
– Worm Trails
– Burrows
Trace Fossils
• Dinosaur track ways in Texas and
Connecticut
• Gastroliths-Rocks that dinosaurs had in
their stomachs. Helped them digest their
food
• Caprolites-Remains of solid waste
material. Helps us learn about eating
habits
Fossils
• Fossils have helped scientists determine:
– How organisms have changed through time
– Early environmental conditions
– Past patterns and cycles than can help predict
future climate changes
The Precambrian Earth
The Precambrian Earth
• The Precambrian includes the:
– Hadeon Eon(4.6 Ga-3.8 Ga)
– Archaen Eon(3.8 Ga-2.5 Ga)
– Proterozoic Eon(2.5 Ga-542 mya)
Earth’s Birth
• The Earth must be at least as old as the
oldest rocks.
• The oldest rocks are around 3.9 Billion
years old
• Rocks older than this have been eroded
How Old Is The Earth?
• Scientists Agree that the Earth is about 4.6
Billion Years old due to meteorites found
that date back to 4.6 Billion Years
Heat Sources
• It is believed that the Earth was very hot
when it first formed
• Earth’s heat sources destroyed much of
the evidence for this time period
Formation Of The Crust
• Iron and Nickel melted and concentrated
into the core due to their high densities
• Granite, a less dense material composes
the crust
• Denser minerals compose the mantles
Differentiation
• Differentiation is the process where the
heavier materials sink towards the center
and lighter materials accumulate towards
the surface
Formation Of The Crust
• The formation of the crust is believed to
have been completed about 2.5 Billion
Years ago (The boundary between the
Archaen and the Proterozoic Eons)
Growth Of The Continents
• By 1.8 Ga Laurentia formed
• Laurentia is an ancient continent that was
composed of most of modern day North
America
Grenville Orogeny
• 1.8-1.6 Ga-The Grenville Orogeny
• The Grenville Orogeny was a collision of
Laurentia with an Island Arc
• By the end of the Proterozoic 75% of
North America had formed
Rodinia
• During the Proterozoic all land came
together to form a supercontinent known
as Rodinia which broke apart during the
Early Phanerozoic
Earth’s Early Atmosphere
• Hydrogen and Helium composed Earth’s
Early Atmosphere
• These gases escaped while more massive
gasses such as Nitrogen and CO2 did not
• Massive volcanic activity released gases
such as H2O, CO2, N2, and CO which is
believed to have formed Earth’s early
atmosphere
Cyanobacteria
• Stromatolites are mounds of billions of
cyanobacteria
• Cyanobacteria are oxygen producing
bacteria
Stromatolites
• Stromatolites formed:
– During the Proterozoic
– In Shallow oceans
BIF’s
• BIFs(Banded Iron Formations) are
deposits which consist of alternating
bands of chert and iron oxides
• BIF’s formed from 2.5 Ga-1.8 Ga
BIF’s
• BIF’s were formed by Stromatolites which
were giving off Oxygen in shallow marine
environments
• BIF’s stopped forming when Oxygen levels
rose
Red Beds
• Red Beds are sedimentary rock deposits
that contain oxidized iron
• Red Beds are evidence that the
Proterozoic contained oxygen
Formation of the Oceans
• As the Earth cooled the Water Vapor in
the air condensed and liquefied
• The water filled the low lying, basalt
floored basins
• Another hypothesis suggests that Earth’s
water came from comets made of frozen
gas and water
Proterozoic Life
• It is believed that the only life forms that
existed during the Proterozoic were
Prokaryotes
– Prokaryotes are single celled organisms.
They have no nucleus
– Eukaryotes are organisms that are composed
of cells that contain a nucleus
Snowball Earth
• 800-700 mya it is believed that there was
a widespread glaciation called “Snowball
Earth”
• Glaciers extended all the way down to the
equator
The Paleozoic Era
Periods Of The Paleozoic
•
•
•
•
•
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
– Mississippian
– Pennsylvanian
• Permian
• The Paleozoic runs from 542 mya-250 mya
Paleozoic
• The Paleozoic Era lasted from 542 mya to
250 mya
• By the Cambrian Laurentia had split off
from Rodinia
– Was located near the equator
– Surrounded by tropical seas
Change In Sea Level
• A transgression is a rise in sea level
• A regression is a fall in sea level
Sea Level
The Grand Canyon
shows the sandstoneshale-limestone
deposits of the
Cambrian
Sea Level
• The sandstone-shale-limestone sequence
was deposited side by side however were
found stacked one on top of the other in
the grand canyon
• This was due to a change in sea level
Cambrian Explosion
• The Cambrian Explosion in an increase in
diversity and abundance of life forms at
the beginning of the Cambrian Period
• The beginning of the development of hard
parts such as skeletons mark the
beginning of the Cambrian
• Trilobites are a very good index fossil for
the Cambrian
Middle Paleozoic
• Sea Level rose during the Ordovician
• In the middle Ordovician Corals and
sponges became common. Began to build
reefs
• Reefs protect the environments behind
them from wave energy
• The area behind a reefs is called a Lagoon
Landmasses
• Baltica-Consisted of modern day Northern
Europe and Russia
• Laurentia-Most of Present Day North
America
• Avalonia-Present day Newfoundland
Taconic Orogeny
• The Taconic orogeny was a collision
between Laurentia and an Island arc
• Occurred in the Middle Ordovician
• Named for the Taconic Mountains of
Eastern New York
Other Orogenies
• Caladonian Orogeny-Silurian event where
Laurentia and Baltica collided forming a
continent known as Laurasia
• Acadian Orogeny-Mid-Devonian event
where Avalonia and Laurentia collided
• Antler Orogeny-Late Devonian collision of
Laurentia and an Island Arc
Mid Paleozoic Life
• The Middle Paleozoic marked the first time
that there was life on land
• There is evidence that plants lived on land
during this time
Pangaea
• Pangaea formed during the Late Paleozoic
• Pangaea was a massive supercontinent
that contained all landmasses.
• The Southern portion was known as
Gondwana and consisted of South
America, Africa, India and Antarctica
• Pangaea broke apart in the Early
Mesozoic
Sea Level
• Pennsylvanian rocks in North America
show a repeating change in sediments
stacked on top of each other
• This is called a cyclothem
• 50 Changes in Sea Level
Late Paleozoic Mountain Building
• Alleghanian Orogeny-Carboniferous
collision between Gondwana and
Laurasia.
– Responsible for building the Appalachian
Mountains
• Ouachita Orogeny-Carboniferous collision
between Gondwana and Laurasia
Permian Mass Extinction
• The Permian Mass Extinction marks the
end of the Paleozoic.
• Also called the Permo-Triassic Extinction
Event
• 95% of all species including trilobites went
extinct
Permian Mass Extinction
• Affected land organisms as well
• 65% of all amphibians and reptiles did not
survive
Permian Mass Extinction
• A possible cause for this was a major
marine regression
• Not a lot of room for shallow marine
organisms to live
• Life changed significantly at this time
Mesozoic Era
Periods Of The Mesozoic
•
•
•
•
Triassic
Jurassic
Cretaceous
The Mesozoic runs from 250 mya-65 mya
Breakup Of Pangaea
• By the late Triassic the plates of Pangaea
began to separate
• This was due to high heat beneath the
surface
• Resulted in the formation of the Atlantic
Ocean and the Mid Atlantic Ridge
Tectonic Activity
• Active tectonic activity in Western North
America
• Formed mountain ranges known as the
Cordillera
Mountain Building
• Nevadan Orogeny-Jurassic. Caused by
subduction of Oceanic Plate
• Sevier Orogeny-Cretaceous. Caused an
eastward shift in volcanism
• Laramide Orogeny-Cretaceous. Helped
create the Rocky Mountains
• All of these occurred in Western North
America
Sea Level
• Sea level dropped during the Triassic
• Western North America became a lot more
arid
Sea Level
• Sea Level rose again in the Jurassic
• The Appalachians began to rise as did the
Cordilleras
• Massive flooding in North America
• The sea covered North America from
Texas to Alaska
Mesozoic Life
• The Mesozoic is referred to as the “Age of
the Reptiles”
• New marine organisms that diversified
during the Mesozoic are:
– Crabs
– Lobsters
– Shrimps
– Snails
Life
• Reef builders were wiped out during the
Permo-Triassic Extinction
• During the Triassic a group of clams called
rudists developed the ability to build reefs
in the Cretaceous
Ammonites
• Ammonites were abundant during the
Mesozoic. Excellent Index Fossils
Other Life Forms
• Ichthyosaurs-Resembled Modern Dolphins
• Plesiosaurs-Walrus like creatures
• Mosasaurs –Sharks
Life on Land
• There was a dramatic change in life on
land due to a climate shift
• Some fossils from the Paleozoic were
present in Mesozoic rocks but not as
prominent
• Cycads were the dominant plant
Angiosperms
• Angiosperms were another dominant plant
• Angiosperms are seed bearing plants that
have flowers. First appeared in the
Cretaceous
• Before the Cretaceous plants did not have
flowers
Mammals
• Mammals evolved during the Triassic
• Pterosuars were flying reptiles that lived
during the Mesozoic
Other Reptiles
• Crocodiles and Turtles also first appeared
during the Mesozoic
• It is unknown why they survived the mass
extinction at the end of the Mesozoic
Dinosaurs
• Dinosaurs were a group of reptiles that
developed around 228 Million Years ago
(Triassic)
• They have an upright posture while other
reptiles have a sprawling posture (Their
legs are not directly beneath them)
Ectothermic/Endothermic
• Ectotherms-Animals whose body
temperature is regulated by the
temperature of it’s surroundings. All living
Reptiles are ectotherms
• Endotherms-Animals that maintain a
relatively constant body temperature
regardless of it’s surroundings. All living
Mammals are Endotherms
Ectothermic/Endothermic
• It is controversial as to whether Dinosaurs
were Ectothermic or Endothermic but
there is evidence that some dinosaurs
were endothermic
Mass Extinction
• Another major mass extinction occurred at
the end of the Mesozoic Era.
• The following groups of organisms died:
– Dinosaurs
– Pterosaurs
– Ammonites
– Mosasaurs
– Plesiosaurs
Meteorite Theory
• Geologists believe that a meteroite
slammed into the Yucatan Peninsula at
240,000 kph
• Any organism that actually survived would
have faced 1000 years of greenhouse
warming and excessive UV radiation from
the sun
Evidence
-Iridium in Cretaceous Aged Rocks
-Crater in the Gulf Of Mexico that
also contains Iridium. Dated
approximately 65 Million Years Old
Cenozoic Era
Periods/Epochs Of The Cenozoic
• The Cenozoic is divided up as follows:
– Paleogene
• Paleocene
• Eocene
• Oligocene
– Neogene
• Miocene
• Pliocene
– Quaternary
• Pleistocene
• Holocene
• The Cenozoic runs from 65 mya to the present
The Cenozoic
• By the beginning of the Cenozoic Pangaea
had completely broken up
• The Cenozoic was a time of major climate
change
Ice Age
Ice Age
-In the Eocene the climate
started to get colder
-Glaciers began to form in
Antarctica
Ice Age
• In the Early Miocene the glaciers began to
melt
• Sea level rose
• However in the Late Miocene the glaciers
returned
Ice Age
• During the Pliocene the Arctic Ocean
began to freeze to form an ice cap
• Late Pliocene-Pleistocene the Northern
Hemisphere had extensive glaciation or an
“Ice Age”
• Some glaciers were up to 3 km thick
Tectonic Events
• The Rocky Mountains were formed during
the Mesozoic
• Eroded sediment filled the basin and
preserved fish, insects, frogs, plants, and
bird fossils
• This was known as the Green River
Formation
Volcanism
• The Cascade Mountains formed at the end
of the Eocene
• This was the result of the subduction of an
oceanic plate beneath the western coast
of North America
• During the Miocene the San Andreas Fault
was formed
• Most of the volcanoes are inactive
Hot Spots
• Hot Spots are hot areas in Earth’s mantle
that is stationary for long periods of time
where high temperature plumes of mantel
material rise towards the surface
• Yellowstone National Park is situated over
a hot spot.
• The hot spot is still active
Mountain Building
• The Himalayan Orogeny occurred during
the Eocene
• The Alps were formed as well
• Tethys Sea
Life
• Most of the currently living organisms
evolved during the Eocene
• During the Paleocene and Eocene the
Earth was predominately forests
Life
• By the late Oligocene grassy savannas
supported mammals such as dogs, cats,
rodents, rabbits, camels, and horses
• As the Pliocene ice age began the land
became more arid
Life
• New animals came along as the Late
Pliocene-Pleistocene Ice Age began:
– Woolly Mammoths
– Sabre-Tooth Cats
Primates
• Primates are mammals with a grasping
hand and an opposable thumb
• Homo Sapiens fall under Hominids
• Hominids fall under Hominoids
Hominids
• A hominid is a hominoid that:
– Is Bipedal
– Has a larger brain
– Has smaller canine teeth
– Uses sophisticated tools
Neanderthals
• Neanderthals inhabited the Earth between
200,000 and 30,000 years ago
• Neanderthals differ from humans by:
– Differences in skulls
– Have short, thick limbs
– Have more muscular bodies
– Have slightly larger brains
• We do not have a lot of information on
hominids due to gaps in the fossil record