Geologic Time & Earth’s Evolution
Piecing together Earth’s history
1. Relative Dating
A. Using laws of superposition, cross cutting, etc.
to determine a sequence of events (this
happened, then that)
B. Index Fossils – constrain (pin down) a time range
when a rock layer was deposited.
Fossils of extinct organisms, that were wide
spread, and lived during a specific period of
time.
Example: Trilobites
Relative Dating: Index Fossils
Ch 23.6: Geologic Time & Earth’s Evolution
Piecing together Earth’s history
2. Absolute Dating
Radioactive elements in rocks, like
uranium, potassium, carbon,
breakdown at a known rate. If you
know how much was in the initial
sample … you can determine age.
(Isotopic dating)
How Earth was Made, Part 1: http://www.youtube.com/watch?v=-IVBfhbhU-c&feature=related
Ch 23.6: Geologic
Time & Earth’s
Evolution
ERAS & PERIODS
Why all the divisions?
To manage Earth’s history,
geologists have divided Earth’s
history into EONS. Eons are
divided into ERAS. Eras are
divided into PERIODS. Periods
are divided into EPOCHS.
You need to know the major
developments in terms of life forms,
tectonics, atmosphere and climate for
each ERA.
Geo. Time is broken
up into ERAS (based
on life forms and
enviro.)
1.
2.
3.
4.
Cenozoic (now – 65 mya)
Mesozoic (254 ma)
Paleozoic (540 ma)
All time before that =
Precambrian (540 ma –
4.6 billion)
ERAS are NOT EQUAL in TIME.
Each era ends with a mass
extinction followed by a
burst of speciation.
The Precambrian was the longest ERA.
It took Earth a LONG time to be suitable
for complex life.
Mass Extinctions: Punctuate E’s History
Between each Era (and some
periods)
• = High % of species go
extinct
• Species that are better
suited to environment are
successful, reproduce, and
evolve.
• Causes: Meteor impact;
Volcanism; Glaciation;
anything that can change
atmosphere, climate, or
environmental conditions.
Mass
Extinctions
CH 23.6 Geologic Time and Earth’s Evolution
Complex multicellular life is a “recent” development.
Precambrian
http://www.pbs.org/wgbh/nova/link/hist_01.html
Fossils from this time are not abundant in
the rock record because most organisms
are “soft bodied” – no skeletons or shells.
Precambrian: 4.5 billion – 540 ma
•Earth starts off as MOLTEN and VOLCANIC
•Atmosphere = CO2, CH4(methane), H2O= Green
house gases
•Oceans form and first continents form when Earth
cools
•Origin of water = volcanic steam & meteorites
(comets)
•1st life = bacteria (single celled) 3.5 bya - use
chemosynthesis (not sunlight) for energy. They can
withstand extreme conditions.
Precambrian: 4.5 billion – 540 ma
•Cyanobacteria (blue-green algae)
• ~ 3 bya – take in CO2 and release O2
(photosynth)  PLANET CHANGING!
•
Temps. Decrease (take in CO2)
•
Oxygen makes O3 = OZONE LAYER  Shields
surface from UV radiation (UV kills cells)
O2 atmosphere– more complex organisms to
develop!
•Simple multicellular organisms in shallow seas
•1st Supercontinent (Rhodinia)
•MASS EXTINCTION = SNOW BALL EARTH  ICE
AGE! Closes PRECAMBRIAN
Paleozoic Era
Paleozoic Era: 540 – 245 mya
•CAMBRIAN EXPLOSION starts off ERA –
“explosion” in the diversity of life forms
• Snowball Earth is thawed by millions of years
of volcanic activity.
•Warm temperatures
•LIFE IN SHALLOW SEAS
•½ through, life “invades” land (amphibians,
land plants, insects) (Thank you Ozone)
Paleozoic Era: 540 – 245 mya
•Early GIANT FORESTS/Swamps become our
fossil fuels (Buried, but not decomposed
plant and animal matter  fossil fuels)
•Pangaea assembles  Interior of
continent becomes DRY and COOL(favors
reptiles etc)
•Ends with…
MASS EXTINCTION = “GREAT DYING”
Volcanic activity? Lava flows in Siberia for
millions of years change climate?
Mesozoic
Mesozoic Era: 245 - 65 mya
•AGE of DINOs
•WARM and subtropical environment
•Virtually ice-free
•Pangaea rifts apart
•1st Flowering plants; Giant insects; birds
•Mammals appear (small)
•Ends with…
MASS EXTINCTION = ASTEROID IMPACT = K-T
EVENT (+ Lava flows in India)
Meteoroid impact in the Gulf of Mexico
Cenozoic
Cenozoic Era: 65 mya - present
•AGE of Mammals
•ICE AGES – Glaciers advance and retreat
•Ocean currents change due to new continent positions; Mt. Building changes air
circulation
•Hominids (human ancestors) – 2 mya; Modern
humans – 200,000 yrs
•Global Mountain building - affect air circulation 
drier & cooler conditions
•Cooler & Drier  Grasslands (not tropical forests)
•When glaciers grow …
•Sea level drops
•More land is exposed (land bridges)
•Ends with… ?????
What does a mass extinction look like in the Rock
Record?
Black layer = carbon rich; carbon from
organisms that died abruptly and could not all
be decomposed; carbon in organism changes
w/heat and pressure to become oil/coal.
White layer = Debris from asteroid/impact
Meteoroid that hit 65 mya
caused a Mass Extinction
•75% Species on planet go extinct.
•90% of planktonic life in ocean die (plankton
supports the oceanic food chain).
•Takes 10,000 years to recover.
How do we know when Meteoroids have hit Earth
in the Past?
Look for … in the rock
layer
K-T Boundary: This layer is
found on every continent in
rocks 65 my old.
1. Iridium – an element not
common on Earth but common
on meteroids (asteroids or
comets)
2. Shocked Quartz – heat changes
quartz grains
3. Tektites - quartz grains
vaporized and cooled so fast
they turn in to glass beads.
K-T Extinction
When: 65 million years ago (End of CretaceousBeginning of Tertiary Periods)
Where: Chicxulub – Yucatan Pennisula Mexico
What: Asteriod or Comet – 10km in diameter
makes a crater 180 km in diameter.
Impact Event: 100 MillioMegatons TNT (6 x
more energetic than 1980 Mt. St. Helen’s
eruption)
Results– Fires; Magnitude 10 Eqs; Tsunamis;
Climate change; Volcanism; Mass extinction
(collapse of ocean food chain & bye-bye
dinosaurs)
Human Evolution & Hominids
http://www.talkorigins.org/faqs/homs/species.
html
Evolution of Early Humans
www.pbs.org/.../evolution/adaptablehuman.html
Cave paintings
http://www.culture.gouv.fr/culture/arcnat/cha
uvet/en/
Evolutionary Tree
http://wwwtc.pbs.org/wgbh/nova/beta/assets/img/theadaptable-human/image-05-large.jpg
800
Mass Extinctions In Earth's History
700
600
# OF FAMILIES
500
400
300
200
100
0
0
100
200
300
Millions of Years Ago
400
500
600