1. Chapter 8.2 Lecture
Chronological Time

2. Age Estimates of Earth Counting lifetimes in the Bible
Comparing cooling rates of iron pellets.
Determine sedimentation rates & compare
Estimate age based on salinity of the ocean.
all age estimates were off by billions of years
some were more off than others!

3. Absolute Dating Methods
Radioactive
Decay sequences
acts as an atomic clock
we see the clock at the end of its cycle
analogous to starting a stopwatch
allows assignment of numerical dates to
rocks.
>
>
decay
) into
Radioactive isotopes change (
daughter isotopes at known rates.
rates vary with the isotope
e.g., U , K , C, etc. + +
235 40 14

4. unstable nuclei in parent isotope emits
Decay
unstable nuclei in parent isotope emits
subatomic particles and transform into
another isotopic element (daughter).
does so at a known rate, measured in the
lab
Half-life
The amount of time needed for one-half of a
radioactive parent to decay into daughter
isotope. •
Assumptions?-you bet
Cross-checks ensure validity of method.

5. Rate of Decay t All atoms are parent isotope or some1 3
All atoms are parent isotope or some
known ratio of parent to daughter
1 half-life period has elapsed, half of the
material has changed to a daughter
isotope (6 parent: 6 daughter) 2
2 half-lives elapsed, half of the parent
remaining is transformed into a daughter
isotope (3 parent: 9 daughter)
3 half-lives elapsed, half of the parent
isotope (1.5 parent: 10.5 daughter)
We would see the rock at this point.

6. Think pair share: Practice Problem: You find a rock with 7/8 the
daughter & 1/8 the parent. If the parent material has a half-life of 50,000 years, how old is the rock?
Method:
1. Use the ratio of Parent to Daughter to find
out how many half lives have gone by
2. Multiply the number of half lives gone by
times the value of the half life

7. Answer:
3- ½ lives have gone by so rock is 3 x 50,000yr or
150,000 yrs old

8. 100 % parent remaining Parent Parent Daughter Daughter 50 25 13
Radioactive Isotopes
Radioactive Isotopes • •
analogous to sand in an hour glass
analogous to sand in an hour glass - -
we measure how much sand there is
we measure how much sand there is
>
>
represents the
represents the
mass of elements
mass of elements - -
we measure the ratio of sand in the bottom to sand in the top
we measure the ratio of sand in the bottom to sand in the top - -
at the end (present)
at the end (present)
>
>
daughter (b) and parent (t)
daughter (b) and parent (t) - -
we know at what rate the sand falls into the bottom
we know at what rate the sand falls into the bottom
>
>
the half life of the radioactive element
the half life of the radioactive element - -
how long would it take to get the amount sand in the observed
how long would it take to get the amount sand in the observed
ratio starting with all of it in the top?
ratio starting with all of it in the top?
100
Parent
Parent
% parent remaining
Daughter
Daughter 50 25 13
time >

9. Five Radioactive Isotope Pairs Five Radioactive Isotope Pairs
Effective
Dating Range
Minerals and
Isotopes
Half-Life
of Parent
(Years)
Rocks That Can
Parent
Daughter
(Years)
Be Dated
Uranium 238
Lead 206
4.5 billion
10 million to
Zircon
4.6 billion
Uraninite
Uranium 235
Lead 207
704 million
Muscovite
Thorium 232
Lead 208
14 billion
48.8 billion
Biotite
Potassium feldspar
Rubidium 87
Strontium 87
4.6 billion
10 million to
Whole metamorphic
4.6 billion
or igneous rock
Potassium 40
Argon 40
1.3 billion
100,000 to
Glauconite
4.6 billion
Muscovite
Biotite
Hornblende
Whole volcanic rock

10. Carbon-14 dating is based on the Carbon-14 dating is based on the
Radiocarbon and Tree-
Ring Dating Methods
Carbon-14 dating is based on the
Carbon-14 dating is based on the • •
ratio of C-14 to C-12
ratio of C-14 to C-12
in an organic
sample.
sample.
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>
Valid only for samples less than 70,000
Valid only for samples less than 70,000
years old.
years old.
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>
Living things take in both isotopes of
Living things take in both isotopes of
carbon.
carbon.
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>
When the organism dies, the "clock" starts.
When the organism dies, the "clock" starts.
Method can be validated by cross-checking with tree rings

11. Carbon 14 Cycle

12. If you are still having trouble check out these videos:
Radioactive decay
Carbon Dating