Dating Rock Layers and Fossils

Unit 7.2

Dating Methods

Fossil Dating

●There are two types of dating for rock layers and fossils.
○If the age of a rock is known, then the age of the fossil will be the
same
■Absolute dating

○Sometimes all we can say is whether something is older or younger
■Relative dating

Relative Dating

●Relative dating is the process of determining if one rock or geologic event
is older or younger than another, without knowing their specific ages

●Relative dating is how the majority of fossils are dated, but scientists can
also use absolute dating.

●Law of Superposition

○
Oldest rocks are at the bottom and youngest are at the top

Multiple Choice

Which layer is the oldest?

• E

• D

• C

• A

Multiple Choice

Which of the following layers is the youngest?

• Q

• G

• X

• A

Absolute Dating

●Absolute dating, also called radiometric dating or radioactive dating,
measures the amount of radioactive material in a rock with a fossil.

●Radioactive elements such as uranium or potassium

●Each radioactive atom will release energy and change into a different type
of atom (decay). PARENT → DAUGHTER

Absolute Dating

● A rock starts with a certain number of radioactive

atoms (parent material), after a period of time it will
only have half that number of atoms.

● This length of time is known as the atom’s half-life.

● The half-life is always the same for each element.

● By comparing the amount of parent material to

daughter material, you can determine how much time
has passed.

● This gives an absolute age to the rock and the fossils

near it.

Multiple Choice

What percent of parent isotopes (parent material) remain after 1 half-life?

• 87.5%

• 75%

• 50%

• 25%

Absolute Age of Fossils

If we know that the rock layer
above a fossil is 505 mya and the
rock layer below is 520 mya, then
we know that the fossil is between
505-520 mya.

Multiple Choice

What is the age of the green rock layer in Location A that is located between the clam and the trilobite fossils?

• Younger than 200 million years

• Between 200-400 million years

• Older than 400 million years

• Cannot be determined

Index Fossils

Characteristics of Index Fossils:

1. Easily Recognizable

2. Widespread- large

geographic area

3. Short Lived- found

in only a few layers

Index Fossils

Which of the fossil would
make a good index fossil?

Multiple Choice

Which of the following would make a good index fossil?

Review Time!

Answer the following questions based on the previous lesson.

Multiple Choice

Describes a group of living things that are no longer living.

• fossil

• extinct

• endangered

• dinosaur

Multiple Choice

Where can the oldest layer be found?

• The top layer

• The middle layer

• They are all the same age

• The bottom layer

Multiple Choice

Which fossil layer is the youngest? 

• Layer A

• Layer B

• Layer C

• Layer D

Multiple Choice

The Law of Superposition helps scientists to determine what?

• The absolute age of rock layers

• The age of fossils

• The relative age of rock layers

• the super powers of rocks

Multiple Choice

Look at the diagram, what layer did animal E first show up in? (Hint: Layer 1 is at the bottom)

• Layer 4

• Layer 5

• Layer 6

• Layer 7

Multiple Choice

Which rock layer contains the newest fossils?

• F

• E

• B

• C

Multiple Choice

Which layer formed first?

• Rock Layer W

• Rock Layer X

• Rock Layer Y

• Rock Layer Z

Multiple Choice

Scientists use radioactive decay to measure...

• relative age

• absolute age

• half-lives

• time of day

Multiple Choice

The length of time required for half of the radioactive atoms in a sample to decay is its...

• era

• age

• half-life

• eon

Multiple Choice

How does absolute age differ from relative age?

• only absolute dating indicates which rock is older than another

• only absolute dating finds the age in years

• only relative dating finds the age in years

Multiple Choice

If the half-life of a radioactive group of atoms is 100 years, then how old is the rock if there have been two half-lives?

• 25 years

• 200 years

• 400 years

• 800 years

Multiple Choice

The original element that undergoes radioactive decay is known as the...

• daughter material

• half-life

• parent material

• carbon-14

Multiple Choice

The element that is the product of radioactive decay is known as the...

• daughter material

• half-life

• parent material

• carbon-14

Multiple Choice

What is the half-life of this radioactive isotope?

• 5 seconds

• 10 seconds

• 15 seconds

• 20 seconds