NOTES: GEOLOGIC DATING
How are the age of rocks determined?
How old is planet Earth?
Geologic Time Scale
• A system of chronological dating that relates
geologic land forms (strata) to time
• Shows sequence of major evolutionary
events
• Plate motions and mountain building events
can be placed on the geologic time scale
• Geologists have divided Earth’s history into
time units based on the fossil record
Geologic Dating
Two Types
• Relative (qualitative): Used to
determine if one thing is younger or
older than another
• Ex: Law of Superposition, use of index
fossils, correlation of rock layers
• Absolute (quantitative): Determines
how many years old something is
• Ex: radiometric dating
Relative Dating
• Law of Superposition
• Younger rocks are on top, older rocks are on bottom
• Lower layers must be in place before younger rocks can be
deposited on top of them
• Problems: Layers can shift, does not give exact age
Absolute Dating
• Earth is estimated to be 4.6
billion years old
• How did we measure that?
• Radiometric Dating: calculating
the age of an object by measuring
proportions of radioactive
isotopes
Radiometric Dating
• Radioactive Isotope: an unstable form of an
element, decays into stable element, gives off
energy (radiation)
• Different radioactive elements decay at different
rates
• Ex. Radioactive Carbon-14 decays into Nitrogen
• Ex. Radioactive Potassium-40 decays into Argon-40
Radiometric Dating
• Half-life: the time it takes
for half of a radioactive
isotope to decay
• Ex. K-40 half life is 1.3 billion
years
Fraction of
Fraction of
# of Halfradioactive
radioactive
lives
isotype Remains isotype Remains
1
1/2
50%
2
1/4
25%
3
1/8
12.5%
4
1/16
6.25%
Radiometric Dating
RADIOMETRIC DATING EQUATION:
(# of half-lives)(known half-life of radioactive isotope) = Age of object
• All living things contain a constant ratio of radioactive Carbon 14 to Carbon 12.
• At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the
organism begins to decay to Nitrogen 14, and is not replenished by new C-14.
• The change in the Carbon 14 to Carbon 12 ratio is the basis for dating.
Example Half-lives
Practice
• The half life of carbon-14 is 5,730 years. If only 25% of the
original amount of carbon-14 is left in a fossil, how old it the
fossil?
1. Determine how many half-lives have elapsed.
• 25% or ¼ = 2 half lives
2. Multiply this by the known half-life.
• Carbon half life = 5,730 years
• 2(5,730 yrs) = 11,460 years
# of Halflives
More practice
Fraction of
radioactive
isotype Remains
Fraction of
radioactive
isotype Remains
1
1/2
50%
2
1/4
25%
3
1/8
12.5%
4
1/16
6.25%
• A sample contains 20g of an isotope that has a half-life of 1000 years.
How much will be left after 2000 years?
• After 2,000 years, how many half-lives would have passed?
2
• What percent of the radioactive isotopes would be left?
25%
• What is 25% of 20g?
5g