Earth History
First dating was done in a relative manner…or determining what came first,
second, third,…
Several rules to determine the order that events occur.
1. Law of superposition: the oldest rock layer is the one on the bottom.
2. Principle of Original Horizontality: layers of
sediment are generally deposited in a
horizontal position. Folding or bending
occurs after rocks are deposited.
3. Principle of Cross-Cutting Relationships: Any event that cuts through a rock is
younger than the rock. These events include faults and igneous intrusions.
I
H
G
F
B
A
C
D
E
4. Inclusions – these are pieces of one rock formation that are contained within
another. The rock particles included in another had to be there first.
5. Unconformities:
Rocks deposited without interruption are
called conformable.
A break in the rock record is called an
unconformity.
This represents a long period of time where
erosion rather than deposition occurred.
List the relative order of events.
Relative age
correlations
6. Fossil succession: Index fossils can be used to correlate the relative age of
rocks.
Index fossil – an organism that is widely distributed, and had a relatively short
lifespan before going extinct.
Using radioactive isotopes, it is theoretically possible to determine the number of
years that have passed since an igneous rock was first formed.
Geologic Time Scale – The Earth is about 4.5 billion years old. This scale breaks up
Earth history into 4 Eras: Pre-Cambrian, Paleozoic, Mesozoic and Cenozoic.
Pre-Cambrian era: 4 billion year duration.
Atmosphere lacked oxygen
No life forms eventually changed to 1 cell bacteria colonies, called
stromatolites.
Paleozoic Era: 570 – 225 m.y.a
Early Paleozoic Era:
The only life forms were found in the sea.
Late Paleozoic Era:
Plants, Insects, Reptiles and Amphibians inhabit dry land.
The end of the Paleozoic was marked
by a mass extinction of most species.
Pennsylvanian Period: Ancestoral Rocky
Mountains were uplifted, then eroded
into coalescing alluvial fans, which form
the Fountain Formation…Red Rocks,
Flatirons, Garden of the Gods.
Uplift of the Appalachian Mountains
began during the Ordovician Period and
ended during the Permian Period.
Mesozoic Era: (225 – 65 mya)
Called the Age of the Dinosaur
Survivors of the great extinction diversified and flourished.
The end of the Mesozoic Era was marked
by another mass extinction, including the
dinosaurs.
Cenozoic Era: 65 mya to present day
Called the Age of Mammals.
Laramide Orogeny: Uplift of the present
day Rocky Mountains.
Pangaea – super-continent during
the early Mesozoic.
Pangaea breaks up into Laurasia
and Gondwanaland (Mesozoic)
End of Mesozoic: A diverging
continent : continent plate boundary
forms between North America and
Europe.
Same for South America and
Africa.
Time Line Project:
Cut a strip of cash register tape to be 4.5 meters long.
This paper strip represents all of Earth History, where:
1 meter = 1,000,000,000 years (1 billion years)
1 meter = 1,000 mm
there are 1,000 million years in a billion years, therefore:
1 mm = 1 million years of history.
Plot all of the events listed on your geological timeline onto the correct position of
your cash register tape. List things like: beginning of the Earth, Today, Era’s and
Periods, Events such as “beginning of the Rocky Mountains”,… beginning and
extinction of different animals and plants, dominant life forms,…