Transcript Precambrian - Ramsey Public School District

Ch 23.6: 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.
ERAS
1.
2.
3.
4.
Cenozoic
Mesozoic
Paleozoic
All time
before that =
Precambrian
ERAS are NOT
EQUAL in TIME.
The Precambrian
was the longest
ERA. It took Earth
a LONG time to be
suitable for
complex life.
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.
•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)
•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? Asteroid impact? Combination?
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… ?????
Study Guide
• Know what units geologists use to break up
Earth’s history (Eons, eras, periods, epochs)
• List eras in order w/ dates
• Major events of each era
• Cause & effect relationships (pink sheet
questions)