2/3/17
No Science Starter
Find your poster and start working (you have 10 minutes)
Take out a sheet of paper for notes 8.5 – Geologic Dating
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 motion, mountain formations, etc
• 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)
• Ex. Carbon-14 decays into Nitrogen
• Ex. 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
Practice
• The half life of carbon-14 is 5,730 years. If only 25% of the original amount of carbon-14
is left in an fossil, how hold 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
Fraction of
Fraction of
# of Halfradioactive
radioactive
lives
isotype Remains isotype Remains
Radiometric Dating
1
1/2
50%
2
1/4
25%
3
1/8
12.5%
4
1/16
6.25%
Practice
• If a sample contains 20g of an isotope that has a half life of 1000
years. How much will be left after 2000years?
• 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