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Earth’s Evolution
Through Geologic
Time
Why is Earth Unique?
• Life is ubiquitous! – It’s everywhere!
• Just the right size – gravitational forces hold a relatively thin atmosphere
• Earth possesses a metallic core – supports a magnetic field protecting life
from lethal cosmic rays
• Just the right distance from the sun – 93 million miles allowing water
to exist in all three phases (solid, liquid, gas)
• Just the right time – enough time for microorganisms to photosynthesize an
oxygen-rich atmosphere 2.2 billion years ago
• Just the right time – asteroid impact about 65 million years ago creates
mass extinction allowing the proliferation of mammals
• Plate Tectonic Processes – recycling lithospheric material
• External forces vs. Internal forces
So, how did earth become what it is today?
65 my
186 my
88% of
Geologic Time
251 my
291 my
542 my
4600 my
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The Geologic Time Scale.
1. Write down the Geologic Time Scale.
Include in your time scale the following:
Eons
Eras
periods
Epochs for the Cenozoic and
Tertiary periods
2. Write a “mnemonic” phrase for the periods
10
Start of the Precambrian 4.6 by
• explosion from a single point
• universe expanding
• cooling
• more contraction / accretion
planetesimals
Earth
Primitive atmosphere
starts to form during the
Hadean Eon
H, He
• contraction
• fusion begins
prtosun
chemical
differentiation
• contribution of heavy
elements
The Hadean Eon (4.6 b.y.): “hellish” conditions
As the Earth begins to cool…
In Earth’s early formation:
• atmosphere: H, He, CH4, NH3, CO2, H2Ovapor
• weak gravity
H, He is lost to space
• T-Tauri phase (high solar winds)
removed all the other gasses
Earth’s First Enduring Atmosphere:
• produced by outgassing – gasses escape from the Earth’s interior
• outgassing produced by hundreds of active volcanoes
• Earth was in a “fluid state,” releasing high amts of gas
• Earth’s atmosphere from outgassing
• water vapor, CO2, SO2, minor amounts of other gasses
So, where is the oxygen?
Oxygen in the Earth’s Atmosphere
• Increasing oxygen-generating organisms
• oxygen increased steadily to stable concentrations
• “the oxygen explosion” forms ozone (O3)
• formed in the stratosphere
• protects life (DNA) from UV radiation
• Oxygen reacts with iron, creating banded iron
formations (3.5 – 2 b.y. ago)
• iron + oxygen --> RUST
• alternating layers of chert and iron-rich
rocks
• Photosynthesizing bacteria release oxygen into the
water---- 3.5 billion years ago
• CO2 + H2O + Energy sun --> Oxygen
• A cooling earth  Condensing water vapor (clouds)
and rain, producing the oceans (filling in low areas)
Stable O2 levels
by 1.5 b.y.
Banded Iron
Formations
Cynobacteria
(blue-green algae)
4.0 b.y.
Banded Iron Formations
•Deposited during the Precambrian Eon
• 3.5 to 2 billion years ago
Evolution of the Earth’s Oceans
CO2 (major greenhouse gas) readily soluble
in seawater (the oceans)
CO2 + H2O + Ca+2  CaCO3 (Limestone)
Atmospheric
CO2
Organisms extract
Dissolved ions CaCO3 shells and die
in the ocean producing LS-sediment
Earth’s atmosphere rich in H2S , CO2, SO2
• Rain + H2S, CO2, and SO2  ACID RAIN
• Highly acidic rain  accelerated weathering
• Na, K, Ca, Si ions carried into the ocean
• Some dissolved ions ppt  chemical sediment
• Other ions increased ocean salinity
About 90% of the current volume of seawater
was contained in the ocean basins (4.0 b.y.)
White Cliffs of Dover, England
Thick chalk sequence (CaCO3)
deposited during the Precambrian
Eon – 542 million years ago
Making Earth’s Continents
• Partial melting of basaltic
rocks  lower density
continental crust
D= 2.7 g/cm3
2.7 g/cm3
3.0 g/cm3
5.5 g/cm3
• Partial melting of mantle 
basaltic rocks (ocean crust)
D = 3.0 g/cm3
Lithosphere
Continued
Chemical Differentiation
• Formation of the lithosphere (thin crust)
• continental crust
• oceanic crust
• Low density, low silica minerals
move from the mantle toward
surface – lighter material rises
• Formation of the Earth’s
metallic core (Fe, Ni) and
rocky mantle
Oldest
Rocks
Acasta gneiss
NW Canada
4.0 b.y.
Making Earth’s Continents
Collision (convergence) and accretion of various island arc systems
• deformed and metamorphosed sediment
• shortening and thickening of continental crust
• silica-rich magmas (less dense) ascend and intrude rocks above
• continued accretion  cratons
• modern-day exposed cratons are known as stable shields
The crust is on the move through plate tectonic activity. Subduction
of lithospheric material  numerous isolated island arc systems.
The Making of North America
“Accretion of Crustal Provinces”
oldest
youngest
youngest
oldest
Piecemeal assembly into a continent
• Continued plate tectonic activity  accretion of island arc systems known as
crustal provinces
• About 1.9 billion years crustal provinces converged  Trans-Hudson Mt. belt
• Other crustal provinces added over geologic time
I
the early earth .
3. Describe the atmospheric condition during
the Hadean Eon.
4. Describe the significance of the banded iron
formations.
5. Explain how abundant concentrations of
limestone (CaCO3) were deposited during the
Precambrian Eon.
6. How would you describe a craton?
Supercontinents of the Precambrian
Supercontinent Cycle:
• cyclic rifting and dispersal of one supercontinent followed by
a long period of gradual reconstruction  a new supercontinent
GONDWANA
Between 800-600 m.y.
fragments of Rodinia become
Gondwana (Southern Hemisphere)
“Future Pangaea”
Continents that will form
Pangaea during the
Phanerozoic Eon
RODINIA:
• Supercontinent
dominating the
Precambrian Eon
• Breaks apart by the
end of the PC
Geologic History of the Phanerozoic Eon
The Formation of Earth’s Modern Continents
Phanerozoic encompasses
approximately 542 million years
of geologic time.
The Phanerozoic Eon:
• Marks the appearance of first life forms
• Increased availability of fossils  improved age accuracy
• Abundant organisms associated with various niches 
invaluable information to ancient environments
Phanerozoic Eon is divided into 3 main eras.
65+ m.y.
Cenozoic Era
186 m.y.
Mesozoic Era
291 m.y.
Paleozoic Era
The Phanerozic Eon
represents about
12% of the geologic
time scale
Evolution of the supercontinent
Pangaea during the Paleozoic
Laurasia + Gondwana = Pangaea
EQ
Laurasia
• warm, wet tropical conditions
• abundant swampy conditions
• future coal deposits (Mississippian)
B
A
C
Pangaea
• The accretion of Pangaea resulted in:
• collision of northern Europe with Greenland  Caledonian Mountains-A
• joining of northern Asia (Siberia) and Europe  Ural Mountains- B
• Joining of North Africa and Eastern U.S.  Appalachian Mountains- C
During the formation of the Appalachian Mountains, Pangaea was at
its maximum size.
Mesozoic History – 186 million years (Triassic, Jurassic, Cretaceous)
Cretaceous Period
• Continued break-up of Pangaea  forming the Atlantic Ocean
• Westward-moving North American plate converging with the Pacific
basin
• Subduction of the Farallon plate (Pacific plate) producing
coast ranges, Sierra Nevada Mts, Idaho batholith
• Laramide orogeny  Formation of the Rocky Mountains
Jurassic Period
• Regressive / Transgressive seas deposit thick sequences
of sedimentary rocks
• Colorado Plateau (Grand Canyon, Bryce Canyon) stratigraphy
The Navajo Sandstone – 300m thick (1000 feet)
• Middle Jurassic – enormous desert (American-Southwest)
evidenced by ancient sand dune remnants
• Steven Spielberg makes the movie Jurassic Park???
Triassic Period
• Breakup of Pangaea  modern day continents
• Much of the current continents above sea level
evidenced by massive terrestrial sandstone, mudstone
deposits
Massive cross-bedded sandstones
deposited during Middle Jurassic Period
I
The Paleozoic Era.
7. Distinguish between the following tectonic
landmasses (when they occurred geologically):
Rodinia, Gondwana, Laurasia, Pangaea
8. How did the Appalachian Mountains form?
9. Describe at least one significant geologic event
that has taken place during the Triassic,
Jurassic, and Cretaceous periods.
Continent Configuration
Cenozoic Era
Cenozoic History – 65 million years (Tertiary, Quaternary Periods)
Cenozoic Era – “era of recent life”
• Only a small amount of geologic time, but more is known about
the Cenozoic than other eras (WHY?)
• rocks units widespread and less disturbed
• higher levels of fossil preservation
Eastern North America (N.A.) during the Cenozoic Era
• Most of N.A. above sea level
• Eastern N.A. “passive” tectonic boundary
• tectonically stable – considered a tectonic trailing edge
• erosional processes > tectonic processes
• abundant marine deposition (transgression of seas)
along the Gulf of Mexico  numerous petroleum traps
• early Cenozoic --- Most of the Appalachian Mountains
eroded  the eastern seaboard
United States Geologic Map
Western United States
Eastern United States
Cenozoic geology
“passive tectonic margin”
trailing edge
Erosion of the Appalachians
Transgression of seas
during the Cenozoic
Western N.A. during the Cenozoic Era
• Laramide Orogeny  the Rocky Mountains coming to an end.
• erosion of the Rocky Mountains  sediments deposited
(clastic-wedge), making the Great Plains
• Miocene Epoch (20 m.y. ago):
•Nevada into Mexico experienced crustal extension 
Basin and Province Range
• Faulted blocks (horst and grabens) extending from
Nevada into Utah and portions of Mexico
• Rocky Mountains re-uplifted
• creating the Grand Canyon, Colorado
• creating the Grand Canyon, Snake River, Idaho
• Flood basalts in Oregon-Washington (CRB’s)
• Flood basalts range in thickness up to 1 mile
• Continued convergence producing the Cascade Volcanoes
• subduction of the Farallon plate  stratacomposite volcanoes
• Sierra Nevada batholith, Idaho batholith faulted and uplifted
• Mesozoic batholiths exposed to the surface
• The onset of the San Andreas Fault
• A portion of California (North American Plate) begins
to “slide” northwest against the Pacific plate.
United States Geologic Map
Western United States Cenozoic Geology
Cascade Volcanoes
Larmide Orogeny ends
Erosion of the Rocky Mountains 
The Great Plains
Crustal Extension
Sierra Nevada
batholith
Onset of the
San Andreas Fault
I
the Cenozoic Era.
10. Describe at least 3 geologic events taking place
in Eastern U.S. and 3 geologic events in Western
U.S.
11. Why is the Eastern section of the U.S. less
tectonically active than the Western U.S.?
12. What is significant about the Laramide
Orogeny?
The Geologic Time Scale
and the association of life
will continue in the next
section.