1. Ch 23.6: Geologic Time & Earth’s Evolution
Piecing together Earth’s history
Relative Dating
Using laws of superposition, cross cutting, etc. to determine a sequence of events (this happened, then that)
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

2. Relative Dating: Index Fossils

3. Ch 23.6: Geologic Time & Earth’s Evolution
Piecing together Earth’s history
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:

4. 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.

5. ERAS 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.

6. CH 23.6 Geologic Time and Earth’s Evolution
Complex multicellular life is a “recent” development.

7. Precambrian
Fossils from this time are not abundant in the rock record because most organisms are “soft bodied” – no skeletons or shells.

8. 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

9. Paleozoic Era

10. Paleozoic Era: 540 – 245 mya MASS EXTINCTION = “GREAT DYING”
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?

11. Mesozoic

12. 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

13. Cenozoic

14. 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… ?????

15. 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)