Absolute Dating
Absolute Dating
• Absolute Dating determines the actual
age of a rock or fossil in years.
• The main method of absolute dating is
RADIOMETRIC DATING.
How Radiometric Dating Works
• Radioactive elements are elements with
atoms that give off particles (radiation)
from their nuclei. This is called
Radioactive Decay.
– Atoms give off particles to become new
lighter, more stable elements. Decay will
continue until the new element is stable and
not radioactive.
Rate of Decay
• Radioactive elements decay at
CONSTANT RATES. As soon as a
rock forms with radioactive elements in it,
decay begins. We use this to find the age
of a rock.
• The rate at which an element decays into
a new element is measured as a HALF
LIFE.
Half Life
• A HALF LIFE is the amount of TIME it takes
for HALF of atoms of a radioactive element
to decay into a new, stable element.
– Terms used to describe material:
• Parent Material: the original, radioactive element
• Daughter Material: the new, stable element
Example
100 % Parent Material Time= 0 The rock formed
50 % Parent, 50% Daughter
1 Half Life
25% Parent, 75% Daughter
2 Half Lives
12.5 % Parent, 87.5 % Daughter
3 Half Lives
6.25% Parent, 93.75 % Daughter
4 Half Lives
Graph of data: Decay Curve
100
% Parent
50
25
12.5
6.25
1
2
3
Half Lives
4
5
Using Half Life to find Age
• If you know the number of years there
are in an element’s half-life, you can
figure out the age of a rock.
– The number of years in an element’s half-life
would be the number of YEARS it takes for
HALF of the radioactive elements in the rock
to turn into stable elements.
Example
• The number of years in a half-life for
Carbon-14 is 5,730.
– So….. If one half-life has passed, the rock is
5,730.
If two half-lives have passed, the rock is
11,460 years old.
Samples of Half-Lives
Questions
• If a rock contains 50% parent material, how
many half-lives have passed?
• If a rock contains 12.5 % parent material, how
many half-lives have passed?
• If a rock’s half-life is 25 years, and two half-lives
have passed, how old is the rock?
• If a rock’s half-life is 50 years, and three halflives have passed, how old is the rock?