RADIOMETRIC DATING
Pages 282-284
AGE OF EARTH
The earth is about 4.6 billion years old
How did we measure that?
AGE OF EARTH
• The age of the Earth is measured through
Radiometric Dating
• calculate age of an object by measuring amount of
radioactive parent element and amount of stable
daughter elements
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
CARBON-14 DATING
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.
The half-life is so short (5730 years) that this
method can only be used on materials less than
70,000 years old.
Archaeological dating uses this method. Also useful for dating the
Pleistocene Epoch (Ice Ages).
PRACTICE
If the half-life of a radioactive isotope is 5,000 years, how much
of the radioactive isotope in a specimen will be left after 10,000
years?
Summary:
1. Half-life 5,000 years
2. How much of radioactive isotope left after 10,000 years?
PRACTICE
Steps to solve:
1. Half life 5000 years
2. How many half-lives have passed?
 5000= 1 half-life, 10,000= 2 half-lives
3. How much is left?
Start: 100% After one half-life: 50% After two half-lives: 25%
MORE PRACTICE
You are determining the age of an organic object using carbon14 dating. You know that the half-life of carbon-14 is 5,730
years. If only 25% of the original amount of carbon-14 is left in
the object, approximately how old is the object?
Summary:
1. Half-life of C-14 is 5,730 years
2. Only 25% of the original C-14 is left in the object
3. How old is it?
MORE PRACTICE
Steps to solve:
1. Determine how many half-lives have elapsed
•½ or 50% = 1 half-life
•¼ or 25% = 2 half-lives
2. Multiply this by the known half life
•(2 half-lives)(5730years)=11,460 years
EVEN MORE PRACTICE
A rock contains 1/32nd of its original Uranium-235. The half-life of Uranium-235 is
704,000,000 years. How old is the rock?
Summary
1. rock contains 1/32nd of its original U-235
2. U-235 half-life 704,000,000 years
3. How old is it?
EVEN MORE PRACTICE
Steps to solve
1. How many half lives does it take to get 1/32nd of the U-235?





1=½
2=¼
3 = 1/8
4 = 1/16
5 = 1/32
2. Half-lives x half-life = age
 5 x 704,000,000 =3,520,000,000 (3.52 billion) years old