Absolute Dating of Rocks

Absolute Age is the specific age of a rock, fossil, or geologic event from the past

Radioactive Dating is the method by which to determine the absolute age of a rock, fossil, or geologic event from

the past.

Absolute Age and Radioactive Dating

The Smilodon skull is 5,700 years old.

IsotopesAn 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 unequal numbers of protons and

neutrons and usually a higher mass. Isotopes are unstable

as a result.

Atomic mass

Atomic Number (Number of protons)

C14 – Isotope: Unstable form

C12 – Element: Stable form

Proton

Neutron

Radioactive DecayThe 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 the time required for half of the isotope (Parent material) of a given material to change to its

stable form (Daughter Material).Half-life

The half-life of “radioactive material X” is 5,000 years.

Radioactive Dating the half-life of an isotope and the ratio of the amount of isotope vs. stable decay product can be used to determine the age of rock. This

process is known 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 and ½ of the skull would contain N14. After two “half-life” periods, ¼ of the skull would contain C14 and ¾ of the

skull would contain N14.

The skull would be 11,400 years old.

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, the skull went through

three “half-life” periods of C14.

12.5 kg of the original isotope (C14) would remain after 17,100 years.

C14 = 100 kg

C14 = 50 kgN14 = 50 kg

C14 = 25 kgN14 = 75 kg

C14 = 12.5 kg N14 = 87.5 kg

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 processes that change the

Earth.

INCREASING TEMPERATURESINCREASING PRESSURE

CHEMICAL REACTIONS WITH MOLTEN ROCK

WEATHERING/EROSION

WILL NOT alter the half-life of an isotope