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SOL ES9 Paleontology
Absolute Dating
What is Absolute Age?
• Absolute Age is determining the exact age of
an object or event.
• Note that NUMBERS MUST BE
ASSIGNED TO THE OBJECT OR EVENT
What is radioactive decay?
• Rocks can often contain radioactive isotopes
– radioactive isotopes emit particles and energy at a constant
rate
– atoms change from one type to another until they become
stable
– provides accurate method for finding the exact age of a rock
• Scientists measure and compare the ratios of radioactive
and stable isotopes to determine absolute age.
– radioactive = parent material
– stable = daughter material
What is a half-Life?
• The rate of radioactive decay is constant and predictable
for each element
– not affected by temperature, pressure, or other environmental
factors
• Scientists must know the half-life of an element to
calculate age
– Half-life: is the amount of time required for ½ of radioactive
material to decay into stable material.
• note: ALWAYS take ½ of the radioactive material
What is radiometric dating?
• Radiometric Dating is the process of comparing the
amount of parent material to daughter material to
determine age
• Any daughter material found in a sample had to form
after the sample solidified.
• As time passes, the amount of daughter material
increases and the amount of parent material decreases
at a predictable rate.
What is the uranium – lead method?
• Uranium-238 (U-238) will decay into Lead-206 (Pb-206)
• Half life = 4.5 billion years
• Used for rock samples over 10 million years old
What is the potassium – argon method?
• Potassium-40 (K-40) will decay into Argon (Ar) and
Calcium (Ca)
• Half life = 1.3 billion years
• Used for rock samples between 50,000 and 4.6 billion
years old
What is the carbon 14 method?
• Carbon exists in two forms:
– carbon-14 (radioactive)
– carbon-12 (stable)
• Both occur in the atmosphere combined with oxygen to form carbon
dioxide
– most CO2 is made with carbon-12
– living plants take in CO2 during photosynthesis and the carbon moves through
the food chain
• Half life = 5,734 years
• Used for ORGANIC samples less than 70,000 years old