Relative Age Dating Vs.
Exact Age Dating
How do geologists know how old
something is?
How old is the earth?
Relative Age Dating
WITHOUT TALKING!
With the students in your row
Line up from youngest to oldest
WITHOUT TALKING!
You have 5 min
WITHOUT TALKING!
Relative Age Dating
Analyze: Without moving, determine
your group’s accuracy. (You can talk
now!) How accurate were you?
Exact!
Nearly.
Yikes! Not at all.
EXACT AGE DATING: 2nd attempt,
talking allowed, fix the age line up.
Relative Dating Laws review
1.
2.
3.
4.
Law of superposition
Law of original horizontality
Law of lateral continuity
Principle of cross cutting
relationships.
5. Principle of fossil
succession/correlation
 All of these give us relative ages.
 This rock is older than that one, but
younger than this other one.
Law of original horizontality
Layers have to be formed before anything
can be done to them. Any layer that has
folds, tilts, or erosion can use this law to
help date it.
Law of lateral continuity
Layers
continue
until they
reach an
edge or run
out.
Relative Play Dough
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Relative dating practice problem #1
 B-superposition
 A-cross cutting
 C-superposition
 D- superposition
 K-original horizontality
 L- cross cutting
 J- superposition/lateral
continuity
 Folding - original
horizontality
 I- superposition
 H- superposition
 G- superposition
 F- superposition
 E- superposition
Relative dating practice #2
Tilt may have occurred
before N & L or after.
 Tilt- original horizontality
 F, H, K, A, Q- (in order)
superposition
 Tilt? And erosion-original
horizontality
 L-cross cutting
 N-cross cutting
 M-superposition
 J-superposition
 I-superposition
 Erosion of C/B
 B-cross cutting
 E/G?-Cross cutting
 C-superposition
 D-superposition
Exact Age Dating
Measurable! Like lining up by height.
Radiometric Dating
Uses radioactive isotopes and decay
rates to determine exact ages of rocks
and fossils.
Half life- the amount of time it takes
for one half of a radioactive isotope to
decay.
Radioactive Decay
For example,
uranium decays into
lead.
A sample starts off
with all uranium
and no lead.
Over time, the ratio
of lead to uranium
increases.
Radioactive Decay
Different elements have different decay
rates
Must use different elements for different
ages of rocks
Limitations:
Presence of radioactive isotopes (some types
of rocks don’t have any)
Age of rock (too young/old for the isotope
present)
Sedimentary rock is made up of sediments or
pieces of other rock.
Elements for Radiometric
Dating
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How do they do it?
Radiometric dating process.
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