Relative Dating
I. Relative Dating
• Relative Dating is when you give the age
of a rock or fossil compared to another
rock or fossil.
– Example: Rock A is OLDER than Rock B.
– An actual age in years is not determined.
II. Rules of Relative Dating
1. Law of Superposition: When sedimentary
rock layers are deposited, younger layers
are on top of older deposits.
Rules of Relative Dating
2. Law of Original Horizontality: Sedimentary
rock layers are deposited horizontally. If
they are tilted, folded, or broken, it
happened later.
Rules of Relative Dating
3. Law of Cross-Cutting Relationships: If an
igneous intrusion or a fault cuts through
existing rocks, the intrusion/fault is
YOUNGER than the rock it cuts through
III. Correlation
• Rock layers in different places and be
correlated or matched up by matching up
the rocks and fossils in the layers.
IV. Unconformity
• When rock has been eroded, so you have
a missing section of time in the rock
record.
Unconformity
Why are some of the layers tilted?
What happened to the top of the folded rocks?
The missing rock layers is called an __________.
What type of
rocks formed
above?
What events
might have
caused the
rock layers
to become
tilted?
Why is the
top flat?
What caused the
angular unconformity?
Index Fossils
• Some species of
organisms only lived for
a short period of time
before they became
extinct. If you use
radiometric dating to get
an age for the fossil, then
you know that the rock
the fossil is found in is
also that age.
Trilobite: Index fossil for the
Paleozoic Era
Age: 590-250 mya
Graptolite
• Index Fossil of the
Ordovician Period
• Age: 500-335 mya
Fossil B lived 10
million years ago –
2 million years ago.
Fossil A lived 20
million years ago8 million years ago.
What do you know
about the age of
the rock containing
both fossils?
• What is “B”?
• Place the layers in order from oldest to youngest
Put the events in order
Put the events in order
http://reynolds.asu.edu/glg103/relative_age_principles.htm