Transcript Lab 25

Lab 25
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LESSON
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lesson…
Radiometric Dating
A question that is
often asked…
How old is the earth?
And the answer is…
Pretty Old
Radiometric dating is
the process of
measuring the age of
rocks
By measuring the age
of rocks, geologists
can determine the
age of a number of
geologic events...
To include the age of
the earth!!!
Radiometric dating is
based on the concept
of radioactivity…the
disintegration (falling
apart) of atoms
What is an
atom?
An atom is the smallest
particle in the universe
with a distinct set of
chemical
characteristics…..
An atom is a structure…it
is a package of particles
Everything in the
universe is comprised
of atoms…bicycles
Cellphones
Dumbasses
Everything
An atom is comprised
of three basic
components
ATOM
P
PROTON (positively
charged)
N
NEUTRON
(neutral)
e
ELECTRON
(negatively charged)
The electron
orbits the
nucleus
which
contains the eprotons and
neutrons
NUCLEUS
RULE: The number
of protons determines
the chemical identity
of an element
For instance…
-all Hydrogen contains 1
proton…
-all Helium contains 2
protons…
-all Uranium contains 92
protons…
Element = a substance
that cannot be broken
down into other
substances by chemical
means…
For instance…
Hydrogen is an
element
Helium is an element
Uranium is an
element
Carbon-12 is a species of
carbon…
CARBON
12C
It contains 6 protons and 6
neutrons…for a total of 12 particles
in the nucleus
CARBON
12C
Carbon-14 is another species of
carbon…
CARBON
14C
Carbon-14 contains 6 protons but 8
neutrons…for a total of 14 particles…
CARBON
14C
The neutron count may vary but the
proton count remains the same…6
protons…
CARBON
14C
SO: The number of
protons determines
the chemical identity
of an element
Decay types are part
of Radioactivity
…there are three
types to be
concerned with…
DECAY TYPES
P
ALPHA
N
BETA
ELECTRON
CAPTURE
P
N
e
N
P
P
N
-
DECAY TYPES
ALPHA
P
N
P
N
2 protons and 2 neutrons are
thrown out of the nucleus…this
reduces the number of protons
and thus changes the element…
DECAY TYPES
A neutron turns into a proton
BETA
e
N
P
This will change the element
DECAY TYPES
A proton turns into a neutron
This will change the element
ELECTRON
CAPTURE
P
N
-
With all 3 decay
types…elements will
change their chemical
identity because of
the change in the
number of protons…
A common decay
series is
238
206
U/
Pb
URANIUM = PARENT
LEAD = DAUGHTER
A decay series is the
progression of the
disintegration of
elements…a start to
finish process…
This beginning to end
process is the essence
of radiometric dating…
238U
92
For instance…
Uranium 238
is a parent
element and it
decays over
time to to Lead
206…
238U
92
The decay
process starts
out with 92
protons and
146 neutrons
for a total of
238 particles
in the
nucleus…
238U
92
And includes a
series of steps
before
uranium turns
into lead…
238U
92
2 protons
and 2
neutrons are
ejected from
the nucleus
and
changes the
element…
uranium to
thorium
234Th
A neutron is
converted to a proton
90 and changes thorium
to paladium…
234Pa
And so on and so
on…
91
234U
92
230Th
90
226Ra
88
222Rn
86
218Po
84
214Pb
82
214Bi
84
214Po
84
210Pb
82
210Po
84
206Pb
82
Through a series
of steps…uranium
is changed to
lead…
So how does
radiometric dating
allow geologists to
date rocks?
A rock is
found in the
field…an
igneous
rock…
It is formed
as a result
of melted
material
from the
interior of
the planet…
That
eventually
cooled…
The minerals inside the rock solidified with
some radioactive material inside the
crystals…
Radioactive
elements
started at 100%
parent material
(dark blue
dots)…
Then over
time…the
parent material
decayed to
daughter
material (light
blue dots) one
atom at a
time…
Over time a reduction of parent material will
occur as well as an increase of daughter
material
Principally, this
means that we can
make a ratio between
parent material and
daughter material…
This ratio can be
placed into an
equation…
T=
ln(n0/nt)(100000000)
0.0154
“n0” Leftover
“nt” Original
amount of parent
amount of
material (daughter parent
amount subtracted material
from parent
material)
ln(n0/nt)(100000000)
T=
0.0154
“T” Age of a rock
The bean counting
exercise involves
counting brown beans
which represents parent
material nt
The amount of parent
material nt is determined
by counting the total
number of
beans…brown and
white…
The white beans
represent daughter
material…
n0 is calculated by
number of brown beans
Values are then
placed into this
equation
T=
ln(n0/nt)(100000000)
0.0154
And the age of a rock
can be calculated…
T=
ln(n0/nt)(100000000)
0.0154
This is referred to as
absolute time…
T=
ln(n0/nt)(100000000)
0.0154
Absolute time is used
in concert with a
Relative Time
Scale…
Periods such
as
Cretaceous
and Jurassic
are relative
time
periods…
Meaning one
period is older
or younger
than another
period…
Cretaceous
rocks are
younger than
Jurassic
rocks…
Ordovician
rocks are
older than
Triassic
rocks…
A Relative
Time scale is
older at the
bottom
And younger
at the top…
The Relative
Time scale
was
developed to
show the
relationship of
sedimentary
rocks…
Superimposed
relative to each
other…
The Relative
Time scale was
developed with
no absolute
time values…
However, with
radiometric
dating…real
absolute time
values can be
assigned to
each relative
time period…
Geologic Time
Scale…
Lab 25
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LESSON
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