Chapter 36
A Brief History of the Earth
Presented by April Senger
Geologic Time
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If the 4.5 by the Earth is estimated to be were compacted into
one year…
The Earth would start forming from matter on January 1
The oldest known rocks would appear at the end of February
Simple bacterial life would appear in the sea at the end of March
Complex plants & animals would appear in late October and
early November
Dinosaurs would rule in mid-December & die out by December
26
Humans would appear at 11:50 p.m. on the evening of
December 31
All human history would occur in the last minute of New Year’s
Eave
Presented by April Senger
Uniformitarianism
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The Earth’s history is recorded in the rocks
The present is the key to the past
Our class will cover the highlights of the history
of the Earth
There are entire classes dedicated to studying the
Geologic Clock and events
The Earth itself is thought to contain 4.5 by of
history
Presented by April Senger
Relative Age of Rocks
5 Principles help us compare layers and geologic
events to determine which came first, second
and so on
 1. Original horizontality – layers are laid
horizontally. If layers are at an angle, it indicates
they were moved into that position by a crustal
disturbance after deposition
 2. Superposition – The oldest layers are on the
bottom and youngest are on top
Presented by April Senger
Principles Continued
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3. Cross-cutting – An igneous intrusion or fault that
cuts through pre-existing rock is younger than the rock
through which it cuts
4. Inclusion – Inclusions are pieces of rock contained
within another. An inclusion is older than the rock
containing it
5. Faunal succession – Fossils follow a set pattern in
their history & development. Studying the fossils in
layers tells us the sequence the rocks were formed
based on the time of the fossil
Presented by April Senger
Unconformities
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Weathering, erosion, crustal uplifts, etc interrupt
the normal sequence of deposition
There are breaks and gaps in the rock record
We find the gaps or unconformities by
observing the relationships of layers and fossils
Presented by April Senger
Angular Unconformity &
Nonconformities
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Angular unconformities is we see tilted or folded
sedimentary rocks that are overlain by horizontal
younger rock layers
Erosion can occur between layers erasing part of the
rock record
Nonconformity is when overlying sedimentary rock are
found on eroded metamorphic or intrusive igneous
This indicates large amounts of uplift & erosion before
sedimentary layers were deposited
Presented by April Senger
Concept Check
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If a granitic intrusion, a dike for example, cuts
into or across sedimentary layers, which is older:
the granite or the sedimentary layers?
The intrusion is new
If a layer is cut, it was there first
Presented by April Senger
Radiometric Dating
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Relative dating can tell us younger and older but
not an absolute age
To get a span of time, we use radiometric dating
More simply we use radioactive isotopes and the
ratios they exist in to estimate an age
Common isotopes are U-238, U-235, K-40 & C14 (fossils)
The oldest material found is 4.4 bya and the
oldest rock is 3.8 bya
Presented by April Senger
Concept Check
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Could carbon-14 be used for dating rocks in the
Precambrian time (see page 644 for Geologic
Time Scale)
Note: the Precambrian is estimated to occurred
3956 mya
Carbon dating is only good for the last 50,000
years due to its short half life
Presented by April Senger
Precambrian
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The Precambrian stretches from 4.5 bya to 544
mya when abundant macroscopic life appeared
This is 85% of the Earth’s history
It is difficult to study because most of the rocks
have been recycles and the life didn’t have hard
parts to leave a good fossil record
The Earth was oceanless, extremely volcanic,
riddled with meteorites until 4 bya when is
slowed
Presented by April Senger
Moving Forward
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The volcanic gases produced a primitive atm and
the ocean
The atm was rich in water vapor but void of O
The first simple fossils were found 3.4 bya
Stromatolites were the remains of wavy layers
of algae that lived in shallow seas
(photosynethic)
The oxygen produced led to an ozone layer
Presented by April Senger
A Very Slow Process
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The first cells reproduced asexually and had no
nucleus. 1. 5 bya the first nucleated cells formed
The first multicellular plants/animals appeared
700 mya
Australian fossil records show soft bodied jelly
fish to worms
The evidence showed us the first animal
communities in marine waters
Presented by April Senger
Continental Precambrian Changes
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The first evidence of plates moving was 2.5 bya
About 1.5 bya Siberia merged into the western
edge of North America
Europe was converging with the eastern region
of North America
Others converged from the south to form the
first super continent long before Pangaea
Presented by April Senger
The Break Down
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We break the remaining time into Eras
(Paleozoic, Mesozoic & Cenozoic), then Periods
& finally Epochs
There are more subdivisions that can be studied
in detail in a more advanced class
See page 644 for quick reference
Presented by April Senger
The Paleozoic Era
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It spanned from 544 mya to 300 mya
Sea levels rose and fell several times
Marine life flourished
Life forms changed from marine invertebrates
to fishes, amphibians & reptiles
Fossils containing shells helped preserve
organisms
There are 6 periods in the Paleozoic
Presented by April Senger
Cambrian Period
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The Cambrian is marked by an explosion of life forms
All most all forms of marine life developed during this
period
Organisms secreted calcium carbonate and calcium
phosphate exoskeletons leaving excellent fossil records
Organisms could withstand more UV rays, moved into
shallower habitats & the skeletal support allowed them
to grow bigger
Example: Trilobite or cockroach of the Cambrian Sea
Presented by April Senger
Ordovician Period
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Life continued to explode
Earliest appearance fo vertebrates with the
jawless fishes (agnatha)
The end of the period brought many extinctions
thought to be a result of widespread cooling and
glaciation
Tropical shallow-water marine groups were the
most affected
High-latitude and deep-water organisms were
relatively unaffected
Presented by April Senger
Silurian Period
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Life begins to emerge on land
Most of North American was at or above sea
level
Shallow seas were vanishing as thick gypsum and
evaporite minerals accumulated
Terrestrial plants appeared including vascular
plants
Terrestrial plants were tied to the wet lands
Land animals included scorpions and millipedes
Presented by April Senger
Devonian Period
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Known as the “Age of the Fishes”
Plants become wide spread –seed ferns, scale
trees and true ferns
Boney fish and lobed fish flourished
The lobed fish developed internal nostril so they
could breath air
The lungfishes and coelacanth (means living
fossil) are similar to these first fish
Presented by April Senger
More Fish
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Lobed fish also had the ability to use their fins
to walk due to jointed appendages, lobed fins
and defined muscles
Evolutionists propose that these animals gave
rise to the land animals
Amphibians (relative to the lobed fish) appeared
at the end of the period
They were significant because they lived on land
but had to return to water to lay their eggs
Presented by April Senger
The Carboniferous Period
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Time of the Great Swampy Forests
It includes the Mississipian and Pennsylvanian
periods
Warm and moist climate to lush vegetation and
swampy forests which are now the source of
coal beds in North America, Europe, and
northern China
Presented by April Senger
Ewe…Giant Bugs
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Insects underwent rapid changes and became
giants
Examples: cockroaches and dragonflies
(wingspan of 80 cm)
Evolution of the first reptiles appeared with the
amniote egg (self contained and porous shell)
Amniotic eggs allowed animals to cut the
dependence on water and fully transition to land
Presented by April Senger
The Permian Period
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The beginning of the Age of Reptiles
They ruled for 200 my (humans have only been
here for 100,000 years)
Two land reptiles groups emerged: diapsids &
synapsids
The diapsids were less dominant and are the
predecessors to the dinosaurs
The synapsids are like the fin-backed pelycosaur
(fin used in temperature regulation)
Presented by April Senger
What A Finale
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The end of this period ended in one of the largest
extinctions in fossil records
95% of marine species and 70% of land species died
out
The cause is still debated and might be linked to
worldwide cooling, glaciations & a drop in sea level for
about 20-25 million years
Terrestrial animals took a less drastic impact that others
feel were a direct result of plate tectonic activity as the
continents formed Pangaea
Presented by April Senger
Pangaea
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During the Devonian and Permian Periods, all major
land masses collided once more to form Pangaea
They hadn’t been joined since 600 my before during the
Precambrian
The Appalachians in NA, the Hercynian and
Caledonian Mountains in Europe, and the Ural
Mountain in Russia formed
The drastic collision was powerful enough to form the
Rocky Mountains so far inland
The southern climate was mainly dominated by glaciers
because it was very close to the South Pole
Presented by April Senger
Mesozoic Era
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When dinosaurs rule the Earth
“The Age of Reptiles”
Made up of the Triassic, Jurassic & Cretaceous
periods
Mammals were small and insignificant compared
to the dinosaurs
Land plants greatly diversified and true pines
and redwoods spread
Presented by April Senger
More of the Mesozoic
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Flowering plants arose in the Cretaceous and
become the dominant plant by the end of the
period
The insects benefited greatly by flowering plants
The Cretaceous ended 65 mya in another mass
extinction where the dinosaurs, flying reptiles,
marine reptiles & many nonreptiles both on land
and sea were wiped out
Presented by April Senger
The End To An Era
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The extinction was thought to have resulted from the impact of
a large meteorite
Large layers of iridium have been found worldwide in this period
The meteorite is thought to have sent up a dust cloud creating a
nuclear winter knocking out the food supplies and chilling the
Earth
A meteorite of this size would have caused acid rain, tsunamis,
wildfires, and delayed greenhouse effect
Some feel the iridium came for massive volcanic eruptions
Last, others propose that the impact came from an
extraterrestrial object
The Cretaceous marked the end of the Mesozoic Era
Presented by April Senger
Mesozoic Tectonics
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Pangaea started its initial break up
The breakup ended at the end of the Triassic period
with the eruption of a basalt flow with into two major
rift zones
One of these rifts separated NA formng the Atlantic
Ocean
During the Jurassic, India stared N, SA/Africa
separated from Aurstralia/Antartica
The plates are still moving today
During the Triassic period, the Farallon Plate and
related accretions to the NA continent started creating
volcanism, grantitic batholiths, and mountain belts sill
rimming te Eastern Pacific Basin
Presented by April Senger
The Cenozoic Era
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Time or Age of the Mammal
It is made of the Tertiary and Quaternary
periods
These periods are broken into the Paleocene,
Eocene, Oligocene, Miocene & Pliocene Epochs
for the Tertiary and Pleistocene and Holocene
Epochs for the Quaternary
We are currently in the Quaternary
Presented by April Senger
Empty Niches
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After the mass extinctions, many niches were vacant
and the mammals quickly took over the habitat left by
their predecessors
Flying bats, large land mammals, & marine animals such
as whales and dolphins were some successful organisms
Climates cooled as characterized by the widespread
glaciations of the Pleistocene
The ice age is considered be continuing today with
alternations between glacial and interglacial conditions
Presented by April Senger
Glacier Impacts
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Glaciers can cover as much as 1/3 of the land
The glaciers left their mark on the continents…literally
Humans were thought to have evolved
The continents were in similar locations to that of
today but lower water levels created land bridges
(Bering Strait)
This bridge allowed migration from Asia to North
America
There were extinctions in large land mammals that
might be explained by the arrival of man in NA, stone
hunters in Africa & varying temperatures
Presented by April Senger
Cenozoic Tectonics
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The Tertiary period had a spreading center off the
western margin of NA, with the Pacific plate on the
west and Farallon plate on the east
This occurred 30 mya and gave birth to the San
Andreas Fault
Baja California was torn away from the Mexican
mainland and the Gulf of California was created
The plates are still moving today and eventually
California and Baja CA will completely detach or find
themselves joined to western Canida
Presented by April Senger
Hot Topic…Rather Hot Spot
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The Hawaiian Island or Emperor Seamount
Chain orginaly was a northbound plate
When the collision of the NA plate and Pacific
ridge system occurred at the same time the
island change started to travel NW
The bend in the chain occurred between 30-40
mya
Presented by April Senger