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Radiometric Dating and Half Life
Radiometric Dating
A method of dating rocks and other
materials based on the rate of radioactive
isotope decay.
 Radioactive isotopes emit nuclear
particles at a constant rate.
 As particles are emitted, the number of
protons (atomic number) changes and
therefore the identify of the element
changes.

Radiometric Dating
The original radioactive isotope is called
the PARENT. Different and unique
elements formed after particle emission
are called the DAUGHTER.
 For example, U-238 (Uranium isotope)
decays into Pb-206 (Lead isotope)
 In 4.5 billion years, half of a sample of U238 will have been converted to Pb-206

Radiometric Dating
The emission of radioactive particles and
the resulting change into other isotopes
over time is called RADIOACTIVE
DECAY.
* Radioactive decay occurs independently
of pressure, temperature and weathering
agents.

Radiometric Dating
When 50% of the parent material has
converted to daughter material, the ratio
of PARENT:DAUGHTER is 1:1.
 The time it takes for this ratio to
establish is called HALF-LIFE.

Radiocarbon Dating
Carbon-14’s half life is significantly shorter
than other radioactive parent isotopes
(~ 5730 years)
 Living organisms contain trace amounts of
C-14.
 Radioactive decay of C-14 starts only
when an organism dies

Radiocarbon Dating
1.
2.
Cosmic rays from the Sun strike N-14
and converts it to C-14. This C-14
combines with oxygen creating
radioactive CO2
This radioactive CO2 becomes part of
the carbon cycle starting with
photosynthesis.
Radiocarbon Dating
3. Living things contain a constant ratio of
C-14 (radioactive) to C-12(stable)
◦
4.
5.
(1 : 1 000 000 000 000 )
At death, C-14 exchange stops and the
remaining C-14 begins to decay into N14
The change from C-14 to C-12 is the
basis for radiocarbon dating
Radiocarbon Dating
Assumption:
 The rate of C-14 production (amount
and quality of cosmic rays reaching the
Earth)
________________________________
BOTTOM LINE:
 Radioactive materials break down into
other materials at known and constant
rates.
WHY IS RADIOMETRIC
DATING ONLY USEFUL FOR
METAMORPHIC AND
IGNEOUS ROCK AND NOT
USEFUL FOR SEDIMENTARY
ROCKS?
Other methods of absolute dating

Dendrochronology:
◦ The science or technique of dating events,
environmental change, and archaeological
artifacts by using the characteristic patterns of
annual growth rings in timber and tree trunks
Other methods of absolute dating

Ice Cores
◦ A sample from ice and snow that froze many
years ago and provides clues to the climate at
the time of the freezing.
Other methods of absolute dating

Varves
◦ an annual layer of sediment or sedimentary rock.