Post on 10-Jan-2016
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Lab 525 ONLINE
LESSON
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Do take notes as we peruse through this
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
PPROTON (positively charged)
NNEUTRON (neutral)
e-ELECTRON (negatively charged)
e-
NUCLEUS
The electron orbits the nucleus which contains the protons and neutrons
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
12C
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
14C
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…
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
ALPHA
BETA
ELECTRON CAPTURE
e-
P P
P
P
N N
N
N
e-
DECAY TYPES
ALPHA P P
N 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
BETAe-
PN
A neutron turns into a proton
This will change the element
DECAY TYPES
ELECTRON CAPTURE
P N
e-
A proton turns into a neutron
This will change the element
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
238U/206Pb
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…
238U92
For instance… Uranium 238 is a parent element and it decays over time to to Lead 206…
238U92
The decay process starts out with 92 protons and 146 neutrons for a total of 238 particles in the nucleus…
238U92
And includes a series of steps before uranium turns into lead…
238U92
2 protons and 2 neutrons are ejected from the nucleus and changes the element… uranium to thorium
234Th90
A neutron is converted to a proton and changes thorium to paladium…
234Pa91
And so on and so on…
234U92
230Th90
226Ra88
222Rn86
218Po84
214Pb82
214Bi84
214Po84
210Pb82
210Po84
206Pb82
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…
ln(n0/nt)(100000000)
T =0.0154
ln(n0/nt)(100000000)
T =
“nt” Original amount of parent material
“n0” Leftover amount of parent material (daughter amount subtracted from parent material)
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
ln(n0/nt)(100000000)
T =
Values are then placed into this
equation
0.0154
ln(n0/nt)(100000000)
T =
And the age of a rock can be calculated…
0.0154
ln(n0/nt)(100000000)
T =
This is referred to as absolute time…
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…