Geologic Time
Geologic Time Scale
• Divisions of Earth’s
history based on life
forms that were living
during certain periods
• Separation =
Change.
• Correlate geologic
events,
environmental
changes, and
changes among life
forms
The Geologic Time Scale
• Record of Earth’s history
• Names of the time periods don’t
change but their boundaries are
always being changed.
Scale is cut into different sections:
1) Eon – longest time unit on the scale
Hadean (oldest)
Archean
Proterozoic
Phanerozoic (youngest)
2) Era – next longest span of time
• Defined by the different life forms
that are found in the rocks. Marked by
major, striking, worldwide changes like mass extinction
 Precambrian (oldest)
 Paleozoic
 Mesozoic
 Cenozoic (youngest)
- Precambrian Era – microscopic life
- Paleozoic Era – Age of Trilobites
- Mesozoic Era – Age of Reptiles
- Cenozoic Era – Age of Mammals
3) Periods – defined by the life forms
that were abundant or became
extinct during the time in which the
rock was deposited. Third longest
division of time.
4) Epochs – smaller division of
geologic time.
Trilobites
• Make excellent index
fossils b/c their
characteristics changed
every period
• Paleontologists uses
changes to determine
the environment
Movement of Pangea
• Closed seas
• Formed mountains
• Not a completely acceptable explanation for
extinctions
Precambrian Time
• 4.5 billion yrs – 544 million yrs
• Longest part of Earth’s history
• Little known about org. in this time period b/c:
– 4 million yr old rock buried deeply and
experiences enough heat & pressure to destroy
fossils
– Most org. had soft parts
Precambrian Time
• How life changed:
– Cyanobacteria – algae that photosynthesized
producing O2
– O2 created ozone layer allowing the evolution of
more complex organisms
– Invertebrates – (no backbone) appeared towards
end of Precambrian Time
• Ex: jellyfish, worms
Precambrian Time
• End:
– Extinctions
– New org. out-competed old org.
– Scientists not completely sure
Paleozoic Era – Era of Ancient Life
• 544 million yrs – 248
million yrs
• How life changed:
– mostly marine org. so
large seas
– Trilobites and shelled
org. common so
abundance of fossils
Paleozoic Era
– Vertebrates (backbone) evolved
– Forest appeared – gymnosperms – plants
producing seeds only
– Vertebrates began to adapt to land = amphibians
ex. Frog
• Purpose was org. needed to survive in oxygen poor
waters
• Can’t keep moisture in their bodies and need water to
lay their eggs
Paleozoic Era
– Several mtn. building episodes – Appalachian
Mountains formed
• End:
– 90% of marine, 70% land species died
– Volcanoes?
– Asteroid/comet collision?
Mesozoic Era – Middle Life
• 248 million yrs – 65 million yrs
• How life changed:
– Pangea seperated into 2 landmasses Laurasia,
Gondwanaland
– Reptiles
– Dinosaurs
Mesozoic Era
– Birds appeared during Jurassic Period
– Mammals appeared during Triassic Period
– Angiosperms – flowering plants – started evolving
during Cretaceous Period
Mesozoic Era
• End:
– Mass extinction
– Comet/asteroid collision causing dust/smoke to
block out sun
– Some survivors
Cenozoic Era – Recent Life
• 65 million yrs – present
• How life changed:
– Mountain building episode: Alps, Andes,
Himalayas
– Grassland expansion – grazing mammals become
larger
Cenozoic Era
– Continents split further, org. became isolated and
began to evolve
• Marsupials – koala, wombat, kangeroos
– HUMANS