Absolute Time
Historical Methods
Erosion and Sedimentation Scientists estimate the
amount of time it would take for the needed erosion or sedimentation to occur
Only effective for young geologic features
Tree Rings
One Ring forms each year Spring Wood (growing
season) Summer Wood (dry
season) Ring thickness depends on
temperature and rainfall Ring Patterns can be
correlated from tree to tree
Can be used to determine the dates of construction for different artifacts
Varves
A sediment that is deposited on a yearly cycle In any large body of water
Clearest in glacial lakes Summer and Winter lake
levels will differ Glaciers
As ice retreats (melts) in the summer and accumulates in the winter it will leave a varve as well
Radiometric Dating
Atoms with the same number of protons but different numbers of neutrons are called isotopes. Heavy isotopes (those with more neutrons) are
unstable and emit radiation. The original atom is called the parent The new atom is called the daughter
e.g.
Half-Life: The amount of time it takes for half the radioactive atoms in a sample to decay to a stable product
The proportions of parent isotopes vs. daughter isotopes in a sample can be used to date that sample
100% 50% (1/2)
25%
(1/4)
12.5%
(1/8)
Formation 1 half life2 half lives
3 half lives
Example: Carbon-14 has a half-life of 5730 years. If I extract all of the Carbon-14 and Nitrogen-14 out of a leaf and I find that what I have is only ½ Carbon-14, how old is my leaf?
¼ Carbon-14, ¾ Nitrogen-14?
Parent isotope
Daughter isotope
Half-Life (yrs.) Effective Range (yrs.)
Material to Date
Carbon-14 Nitrogen-14 5730 100-70,000 Once living things
Uranium-238
Lead-206 4.5 billion >10 million Igneous
Oldest rocks
Rubidium-87
Strontium-87 47 billion >10 million Igneous/ Metamorphic
Oldest rocks
Potassium-40
Argon-40 1.3 billion >50,000 Igneous Rocks
Types of Decay
Alpha Decay:When the parent isotope emits a Helium atom
along with the daughter isotope.
238U 4He + 206Pb
Circle the alpha particlePut a box around the parent isotopeUnderline the daughter isotope
Beta DecayWhen the parent isotope emits an electron
along with the daughter isotope.
14C e - + 14N
Circle the beta particlePut a box around the parent isotopeUnderline the daughter isotope
Why is radiometric dating so important? Because it can be verified by different
methods it has proven to be very accurate
This helps scientists to put absolute dates on events that have happened in the past
What was a historical method of absolute dating?
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1. Erosion
2. Sedimentation
3. Tree Rings
4. Varve
5. All Of The Above
Why is radiometric dating important?
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1. It allows us to accurately measure how much glacier melt there is
2. It allows us to measure the ages of things accurately even if they are really old
3. It allows is to see how many electrons there are in atoms
Radioactive element X has a half-life of 1000 years. How old is something if only 25% of it is left?
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1. 1000 years
2. 2000 years
3. 3000 years
4. 4000 years
5. 8000 years
What is alpha decay?
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1. When the parent isotope emits an electron along with the daughter isotope.
2. When an atom fuses together
3. When an atom gains an electron
4. When the parent isotope emits a Helium atom along with the daughter isotope.
What is beta decay?
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1. When the parent isotope emits an electron along with the daughter isotope.
2. When an atom fuses together
3. When an atom gains an electron
4. When the parent isotope emits a Helium atom along with the daughter isotope.
What is electron capture?
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1. When the parent isotope emits an electron along with the daughter isotope.
2. When an atom fuses together
3. When an atom gains an electron
4. When the parent isotope emits a Helium atom along with the daughter isotope.
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