7-2 Notes Absolute Ages of Rocks Chapter 7, Lesson 2

7-2 Notes 7-2 Notes Absolute Ages of RocksAbsolute Ages of Rocks

Chapter 7, Lesson 2Chapter 7, Lesson 2

What is Earth’s Age?What is Earth’s Age?

• Scientists discovered and used a natural “clock” to date the age of Earth, meteorites, and the moon.

What is Earth’s Age?What is Earth’s Age?• Scientists used this

natural clock to determine the age of bog bodies.

The Lindow Man has The Lindow Man has been Carbon-14 dated been Carbon-14 dated to sometime between 2 to sometime between 2 BCE and 119 CEBCE and 119 CE

The Tollund Man is the The Tollund Man is the naturally mummified corpse naturally mummified corpse of a man who lived during of a man who lived during the 4the 4thth century BCE. century BCE.

Atoms and IsotopesAtoms and Isotopes

• Atoms are the microscopic building blocks of all matter on Earth.

Atoms and IsotopesAtoms and Isotopes• Atoms have 3 small parts:

– protons (positive charge) and neutrons (no charge) are located in the nucleus (center)

– electrons (negative charge) orbit in clouds around the nucleus


• An element is defined by the number of protons it has (listed as the atomic number on the periodic table).

Carbon’s atomic number is 6.It has 6 protons.


• An isotope is when atoms of an element have the same number of protons, but differing number of neutrons.

Atoms and IsotopesAtoms and Isotopes• Normal carbon is called carbon-12 and it

has 6 neutrons.

• Carbon isotopes:– carbon-13 has 7 neutrons– carbon-14 has 8 neutrons




• Isotopes may be stable or unstable.

• When they are unstable, they are called radioactive, and they can’t keep themselves together – they decay.

Radioactive Decay


• Radioactive decay occurs when an unstable nucleus changes into another nucleus by emitting particles and energy.


• The isotope that undergoes radioactive decay is the parent isotope.

• The stable form of the element that forms is the daughter isotope.

Parent Daughter


• This decay is the natural clock that scientists use to find the ages of Earth’s rocks.


• Parent isotopes decay into daughter isotopes at a constant rate called the decay rate.



• The half-life of an element is the calculated length of time it takes for half a specific amount of a parent isotope to decay.


• Half-life of the carbon-14 isotope:

1/1 at start

1/2 leftafter5730years

1/4 leftafter

another5730years

1/8 leftafter

another5730years


Radiometric DatingRadiometric Dating

• Scientists use radiometric dating to calculate absolute ages of rocks and minerals.


– Comparing the amount of parent to daughter material determines the number of half-lives the material has been through.


– Igneous rock is most commonly used for radiometric dating because it came from melted rock, which “resets” its natural clock.



• Rock grains from continental shields, where the oldest rocks on Earth occur, are estimated to be 4.0 to 4.4 billion years old.

Radiometric DatingRadiometric DatingAbout 4.5 billion years ago, there is

evidence that a rogue planet, Orpheus/Theia collided with Earth and formed our moon. This reset the age of

many of Earth’s rocks.


• Scientists used radiometric dating to determine the ages of meteorites and the Moon.


• The ages of meteorites recently collected in Antarctica are 4.5 - 4.6 billion years old.


• The ages of rocks collected from the moon are about 4.6 billion years old.


• Earth is approximately 4.6 billion years old.


50% of Americans believe the UNIVERSE is less than 10,000 years old…


That would mean the universe began AFTER the domestication of the dog…?


• The closeness of calculated ages of Earth, the Moon, and meteorites helps confirm that the entire solar system formed at the same time.

The isotopes of an element have a different number of what?

A protons

B neutrons

C electrons

D atoms

7.2 Absolute Ages of Rocks

1. A

2. B

3. C

4. D

What important feature of radioactive decay has allowed geologists to date rocks?

A the isotopes of an element may be stable or unstable

B the nucleus gains or loses protons

C parent isotopes decay into daughter isotopes

D the decay occurs at a constant rate


1. A

2. B

3. C

4. D

What do scientists use to measure the absolute age of a rock?

A radiometric dating

B amount of carbon in the rock

C absolute dating

D relative dating


1. A

2. B

3. C

4. D

What type of rock is most commonly used in radiometric dating?

A metamorphic

B igneous

C sedimentary

D minerals

1. A

2. B

3. C

4. D

Which type of rock is the most useful for relative dating?

A igneous

B sedimentary

C magma

D metamorphic

1. A

2. B

3. C

4. D

What term describes time it takes for a sample of a radioactive isotope to decay to half its original mass?

A absolute age

B half-life

C radiometric dating

D relative age

1. A

2. B

3. C

4. D

Which describes a daughter isotope?

A decays into a parent isotope

B is an unstable form of the parent isotope

C is the result of parent isotope decay

D is heavier than its parent isotope

1. A

2. B

3. C

4. D

What percentage of parent isotope remains after 2 half-lives?

A 75%

B 30%

C 37.5%

D 25%

1. A

2. B

3. C

4. D

Which best describes the relationship between the ages of the Earth and the Moon?

A They are about the same age.

B Earth is much older than the moon.

C The moon is much older than Earth.

D Earth is much younger than the moon.

1. A

2. B

3. C

4. D

Scientists believe that the Earth is about ___ billion years old.

A 7.3

B 2.2

C 3.5

D 4.6

1. A

2. B

3. C

4. D

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7-2 Notes Absolute Ages of Rocks Chapter 7, Lesson 2