Transcript Absolute Dating Powerpoint

Radiometric Dating of Rocks
1896-1898
Radioactivity discovered by:
Henri Becquerel and
Marie and Pierre Curie.
Phenomenon of Radioactivity
Some elements, such as uranium
(U) and thorium (Th), are unstable:
They decay spontaneously.
The Atom
Uranium Nucleus
spontaneously emits a particle from its
nucleus called an alpha particle (2 protons +
2 neutrons).
Alpha Particle
emits a particle from its nucleus called
an alpha particle (2 protons + 2 neutrons).
Uranium - Thorium Decay
238
92
U
spontaneous
decay
“parent”
alpha particle =
2 protons + 2 neutrons =
positively charged ion of
Helium
4
2
He +
234
90
Th
“daughter product”
Thorium:
90 protons +
144 neutrons
Beta Particle Emission
But, 234
Th is also unstable, and it emits a
90
beta particle …
Thorium - Protactinium Decay
234
90
Th
beta particle = an
electron discharged from
the nucleus when a
neutron splits into a
proton and an electron
beta
particle
+
234
91
Pa
Protactinium:
91 protons +
143 neutrons
U
Pb
Series
This process is
called radioactive
decay, and
eventually
uranium (parent)
decays to lead
(daughter
product).
U
Pb
Series
The rate at
which this
process occurs
is measured in
terms of the
“half life”.
Half Life
Half Life = Number of years for 1/2 of the
original number of atoms to decay from U to Pb
The phenomenon of radioactivity
relates to our story about the age
of the Earth in two ways:
(1) As radioactive elements decay in
the Earth, they heat up the
surrounding rocks.
(2) Radiometric Dating
Radiometric Dating
Assume: Rate of decay (half life) of long-lived
elements has not varied since the Earth was formed.
This is an example of uniformitarianism.
The Principle of Uniformitarianism
The physical, chemical and biological laws that operate
today have also operated in the geologic past.
To estimate the age of a rock:
D = amount of daughter product.
P = amount of parent.
For a particular radioactive element in a rock,
determine the present ratio = D/P.
D
P
 Rate of decay (from theory and measurement)
 Make assumptions about original ratios (from theory of
geochemistry).
D
P
D
P
As time passes, the amount of parent decreases, and the
amount of daughter product increases. This provides a way
of estimating the amount of time since the "clock" got
started (i.e., since the rock solidified).
Half-Lives of Radioactive Isotpes
Examples of Very Old Rocks Found on Earth:
3.4 billion year old granites - South Africa.
3.7 billion year old granites - Greenland.
Oldest terrestrial rocks > 4.6 billion years.
Central Canada and Central Australia
Oldest terrestrial rocks (4.0 billion years?) are deep in
the interiors of continents.
Oldest terrestrial rocks (4.0 billion years?) are deep in
the interiors of continents.