Relative Dating
of Rocks Layers
Age of Earth
• Early 1900s: 6000 years old
• Today: 4.6 billion years old
How did scientist come up
with this number?
1) Relative Age Dating
2) Absolute Age Dating
James Hutton (1700s)
–Developed the principle of
uniformitarianism
•processes occurring today
have been occurring since
Earth formed.
1) Relative Age Dating
- Comparing one rock layer to
another.
- Sequences the order of events
- No exact dates
- Easy comparison of fossil ages
Geologic Principles
a) Original Horizontality –
sedimentary rocks are deposited
in horizontal layers.
b)Superposition – in an
undisturbed rock sequence:
- Oldest rock layers are at the
bottom
- Youngest layers are on top
Law of Superposition
L
M
U
X
•
•
•
•
S
Which layer is the oldest?
Which layer is the youngest?
Would the fossils in rock layer “U” be older
or younger the fossils in rock layer “X”?
How do you know?
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21.2: Relative Dating of Rocks