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Lab 525 ONLINE LESSON
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Do take notes as we peruse through this lesson…
A question that is often asked… How old is the earth?
And the answer is…
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
An atom is comprised of three basic components
ATOM P PROTON (positively charged) N NEUTRON (neutral) e-e- ELECTRON (negatively charged)
e-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 12 C Carbon-12 is a species of carbon…
CARBON 12 C It contains 6 protons and 6 neutrons…for a total of 12 particles in the nucleus
CARBON 14 C Carbon-14 is another species of carbon…
CARBON 14 C Carbon-14 contains 6 protons but 8 neutrons…for a total of 14 particles…
CARBON 14 C 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-e- PP P P NN N N e-e-
DECAY TYPES ALPHA PP NN 2 protons and 2 neutrons are thrown out of the nucleus…this reduces the number of protons and thus changes the element…
DECAY TYPES BETA e-e- P N A neutron turns into a proton This will change the element
DECAY TYPES ELECTRON CAPTURE P N e-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 238 U/ 206 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…
238 U 92 For instance… Uranium 238 is a parent element and it decays over time to to Lead 206…
238 U 92 The decay process starts out with 92 protons and 146 neutrons for a total of 238 particles in the nucleus…
238 U 92 And includes a series of steps before uranium turns into lead…
238 U 92 2 protons and 2 neutrons are ejected from the nucleus and changes the element… uranium to thorium
234 Th 90 A neutron is converted to a proton and changes thorium to paladium…
234 Pa 91 And so on and so on…
234 U 92
230 Th 90
226 Ra 88
222 Rn 86
218 Po 84
214 Pb 82
214 Bi 84
214 Po 84
210 Pb 82
210 Po 84
206 Pb 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… ln(n 0 /n t )( ) T =
ln(n 0 /n t )( ) T = n t Original amount of parent material n 0 Leftover amount of parent material (daughter amount subtracted from parent material) T Age of a rock
The bean counting exercise involves counting brown beans which represents parent material n t
The amount of parent material n t is determined by counting the total number of beans…brown and white…
The white beans represent daughter material… n 0 is calculated by number of brown beans
ln(n 0 /n t )( ) T = Values are then placed into this equation
ln(n 0 /n t )( ) T = And the age of a rock can be calculated…
ln(n 0 /n t )( ) T = This is referred to as absolute time…
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…
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