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.