Transcript Age of the Earth II - PowerPoint Lecture Notes

Methods for Estimating the Age of the Earth
Fossils and evolution
Rates of deposition of sediments
Cooling of the Earth from a molten state
Radiometric Dating
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.
Age of the Earth
Geologic Time
Scale
The Atom
NaCl
1
8
2
7
11 protons
Na = silvery metal
Cl = poisonous gas
NaCl = Salt
8
2
17 protons
Examples of Atomic Numbers
Element
Number of Protons
Na
Cl
C
N
O
Fe
U
11
17
6
7
8
26
92
Phenomenon of Radioactivity
Some elements, such as uranium
(U) and thorium (Th), are unstable:
They decay spontaneously.
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
Title
beta particle =
an electron
discharged from
the nucleus when
a neutron splits
into a proton and
an electron
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
As radioactive elements decay in the Earth, they heat up
the surrounding rocks.
So, there is a source of heat inside the Earth that Kelvin
didn't know about.
Thus: While the Earth was cooling off, it was also being
heated up (due to radioactive decay in its interior), and
the Earth took much longer to cool than Kelvin thought.
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
D
P
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).
Title
Potassium - Argon Dating
Electron Capture
Title
Electron
Capture
Potassium - Argon Dating
 K/Ar clock begins when K bearing minerals crystallize
in a rock.
The new minerals will contain K40, but will not contain
Ar40 (because Ar40 does not combine with other
elements).
As time passes:
Uniformitarianism
K40  Ar40 (half-life = 1.3 billion years)
K40/Ar40 ratio can be measured to determine age
Potassium - Argon Dating
K40/Ar40 ratio can be measured to determine age.
K40  Ar40 (half-life = 1.3 billion years)
K40/Ar40 > 1  less than 1.3 billion years old
K40/Ar40 = 1  1.3 billion years old
K40/Ar40 < 1  more than 1.3 billion years old
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 > 3.8 billion years.
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.
But, rocks on the Earth have been weathered,
metamorphosed and recycled through a very
dynamic planet.
Thus, the Earth's earliest rocks have been
recycled, and we need to look beyond Earth to
find evidence of when the Earth solidified.
Plate Tectonics
Rock
Cycle
Mountain
Building
and
Erosion
Oldest terrestrial rocks (4.0 billion years?) are deep in
the interiors of continents.
Similar Rock Structures
The Earth's earliest rocks have been weathered,
metamorphosed and recycled through a very
dynamic planet.
We need to look beyond Earth to find evidence of
when the Earth solidified.