Absolute Dating of Rocks
Absolute Age is
Radioactive Dating is is the method by which to determine the absolute age of
Absolute Age and Radioactive Dating
The Smilodon skull is 5,700 years old.
Isotopes
An element is a substance containing atoms that are
chemically alike.
Within the atoms of elements are subatomic particles –
including protons and neutrons -- found within the nucleus of an atom. These subatomic
particles determine the mass of the atom.
Most elements have equal numbers of protons and
neutrons. These elements are considered to be stable
Isotopes have…
Atomic mass
Atomic Number (Number of protons)
C14 – Isotope; … C12 – Element; …
Radioactive Decay
The nucleus of many isotopes is unstable. The nucleus of an unstable isotope will release
energy in its process to become more stable.
The isotope (parent element) will slowly release energy from its
nucleus and change into a stable form (Daughter element) over
time.
A half-life is is the time required for half of the isotope (Parent Material)
…
Radioactive Dating
The Half-Life of an isotope… as radioactive dating.
If a scientist knows the half-life of a radioactive isotope and the ratio of parent material
(Isotope) to daughter material (Decay product = stable form),
he/she can determine the absolute age of a rock sample.
RADIOACTIVE DECAY DATA
PARENT MATERIAL DAUGHTER MATERIAL (ISOTOPE) (STABLE DECAY PRODUCT)
C 14 N14
Half-life: 5,700 years
The skull of a Smilodon was extracted from a tar pit in Southern California. The skull
has been preserved well and C14 and N14 are both present in the skull. The ratio of C14 to
N14 is 1:1 and therefore the skull is estimated to have an absolute age of 5,700
years old.
C14 = isotope N14 = stable decay product
Radioactive DatingOne quarter (1/4) of a Smilodon
skull is composed of C14. The other three quarters (3/4) is composed of N14. How old is the Smilodon skull?
C14 has a half life of 5,700 years.
5,700 years
5,700 years
After one “half-life” period, ½ of the skull would contain…
C14 = isotope
N14 = stable decay product
Radioactive Dating
5,700 years
5,700 years
C14 = isotope
N14 = stable decay product
A fossilized Smilodon Skull weighs 100 kg. If the skull is estimated to be 17,100 years old, How much of the original isotope (C14) remains
in the skull?
5,700 years
If the skull is estimated to have an absolute age of 17,100 years, …
C14 =
C14 =N14 =
C14 =N14 =
C14 = N14 =
Isotope Half-lifeEach Radioactive isotope has its own
unique half-life.
C14 has a half life of 5.7 X 103 yearsU 238 has a half life of 4.5 X 109 years
Certain isotopes are useful in dating rocks and/or geologic events of specific times in
geologic history.
Since U238 has a much longer half-life than c14, it is used to age very old rock and/or
geologic events.
C14 is useful in dating rocks and/or geologic events in earth’s recent past.
The process of radioactive decay from isotope to stable decay product is NOT
ALTERED by process that change the Earth.
WILL NOT alter the half-life of an isotope