Transcript Absolute Dating with Radioactivity

Absolute Dating with
Radioactivity
What is Radioactivity?
• When the forces binding protons and
neutrons together are not strong enough,
and the nuclei spontaneously break apart
(decay)
• Isotopes—elements with the same number
of protons, but different numbers of
neutrons
Common Types of Radioactive
Decay
• Alpha Particles—α
particles
– Composed of 2
protons and 2
neutrons
– Emitted from the
nucleus
– Mass number of the
isotope is reduced by
four
– The atomic number is
decreased by two
Common Types of Radioactive
Decay
• Beta Particles—β
particles
– Composed of one
electron
– Comes from a neutron
– Increases protons by
one
– Atomic number
increases by one
Common Types of Radioactive
Decay
• Electron Capture
– An electron is
captured by the
nucleus
– It combines with a
proton and forms a
neutron
– Mass number remains
unchanged
– One less proton;
atomic number
decreases by one
Half-Life
• The time required for
one-half of the nuclei
in a sample to decay
• A common way of
expressing the rate of
radioactive
disintegration
Radiometric Dating
• Radioactivity (unstable nuclei) is a reliable
means of calculating the ages of rocks and
minerals
• The rates of decay for many isotopes have
been precisely measured
• A correct date is not possible unless the
mineral remained a closed system during
the entire period since its formation
Carbon-14
• Half-life is 5730 years old
• Used to date recent events in history
• While an organism is alive, the proportion
of carbon-14 to carbon-12 is constant
• When the organism dies, carbon-14
decays to nitrogen-14 by beta emission
• Scientists compare the proportions of
carbon-14 and carbon-12
Importance of Radiometric Dating
• Have produced thousands of dates for
events in Earth’s history
• Some rocks have been dated at more than
three billion years
• Proven that geologic time is immense
Difficulties in Dating the Geologic
Time Scale
• Not all rocks can be dated radiometrically
• Most sedimentary rock cannot be
accurately dated because the particles
that formed it are much older than the rock
in which they occur
Difficulties in Dating the Geologic
Time Scale
• Metamorphic rock is difficult to date
because the age of a particular mineral in
the rock is not necessarily the same as
when the rock initially formed
• Use cross-cutting relationships