The Rules for Relative
Dating
Chapter 10: Geologic Time
Background Information:
• In the 1800’
1800’s and early 1900’
1900’s
– Scientists attempted to determine the age of
the earth
• Absolute date = a numerical date that pinpoints a time
in history (ex. extinction of the Dinosaurs 65 million
years ago)
– They did not have any reliable methods (today
radioactive dating is used)
– Instead they had to rely solely on Relative
Dating
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What is Relative Dating?
• Relative Dating = placing rocks in their
proper sequence of formation
(which formed first, second, third etc.)
• It can’t tell us how long ago something formed,
only in what order
• Relative Dating is still used today along with
absolute dating
The Four Rules of Relative Dating:
1) The Law of Superposition
2) The Principle of Original Horizontality
3) The Principle of Cross-Cutting
Relationships
4) Inclusions
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The Law of Superposition:
– Nicolaus Steno (Danish anatomist, geologist and priest):
Developed the law while working in the mountains of
Western Italy in 1669
– The Law of Superposition = in an
undeformed sequence of sedimentary rock,
each bed is older than the one above it and
younger than the one below it. (also applies to
lava flows, and ash beds)
Superposition Example:
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The Principle of Original
Horizontality:
• Also developed by Steno
• The Principle of Original Horizontality =
layers of sedimentary rock a generally deposited
in a horizontal position.
– If the rock layers are flat, it means they have not been
disturbed and still have their original horizontality
– If the rock layer are folded or inclined, it means they
must have been moved by some crustal disturbances
after their deposition
Original
Horizontality
Example:
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The Principle of Cross-Cutting
Relationships
• The Principle of Cross-Cutting Relationships
= when a fault cuts through other rocks or when magma
intrudes and crystallizes, we know that the intrusion is
younger than the rocks affected.
Cross-Cutting Example:
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Inclusions
• Inclusions = pieces of one rock unit that
are contained within another
– The inclusions within a rock are older than the
rock containing the inclusion because the
inclusions had to have been there in order to
be “included” in the rock
Inclusions Example Step 1:
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Inclusion Example Step 2:
Inclusion Example Step 3:
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