Chapter 6, Lesson 2

The Relative Age of Rocks

Chapter 6, Lesson 2

​Pages 228 - 233

How old are Rock Layers?

Evidence of the dynamic changes of Earth's surface are found in the geologic record

Relative Age - the rock's age compared to the age of other rocks (relate it to the rocks around it)

Absolute Age - the number of years that have passed since the rock formed

It is almost impossible to know the exact age of a rock so scientists use both relative and absolute ages together

How old are Rock Layers?

Fossils are often found in layers of sedimentary rock

According to the Law of Superposition, in undisturbed horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Multiple Choice

The number of years that have passed since the rock formed is the rock’s...

• relative age

• superposition

• index fossil

• absolute age

Multiple Choice

The age of a rock compared to the ages of other rocks is the rock’s...

• absolute age

• geologic age

• sedimentary age

• relative age

Multiple Choice

According to the law of ______________ , in horizontal sedimentary rock layers the oldest layer is at the bottom and the youngest layer is at the top.

• quarantine

• folding

• faulting

• superposition

Multiple Choice

Geologists use the law of ____________________ to determine the relative ages of undisturbed sedimentary rock layers.

• crosscutting

• intrusions

• extrusions

• superposition

Clues from Igneous Rocks

Extrusion - lava that hardens on the surface and forms an igneous rock

**Extrusions are ALWAYS younger than the rocks below

Intrusion - magma that pushes into bodies of rock below the surface and hardens into a mass of igneous rock

**Intrusions are ALWAYS younger than the rock layers around and beneath

Clues from Faults

Fault - break in Earth's crust

**A fault is ALWAYS younger than the rock that it cuts through

Multiple Choice

A formation of igneous rock on Earth’s surface is known as...

• a gap

• an intrusion

• a fault

• an extrusion

Multiple Choice

A break in Earth’s crust is called...

• an intrusion

• a fault

• a layer

• an index fossil

Multiple Choice

A mass of igneous rock below the Earth’s surface is called a(n)...

• intrusion

• extrusion

• xenolith

• fault

How do Fossils Show Age?

Index Fossils - fossil that helps geologists match rock layers - widely distributed and represents an organism that existed for a geologically short period of time

Index fossils are useful because they tell the relative ages of the rock layers in which they occur

You can use fossils to match rock layers

How can Rock Layers Change?

Gaps in the geologic record and folding can change the position in which rock layers appear

Motion along faults can also change how rock layers line up making it harder for scientists to reconstruct Earth's history

Gaps in the Geologic Record

When rock layers erode away, an older rock surface may be exposed. Then deposition begins again, building new rock layers

Unconformity - gap in the geologic record

An unconformity shows where rock layers have been lost due to erosion

Sometimes, forces inside Earth fold rock layers so much that the layers are turned over completely - the youngest layer may be on the bottom

Multiple Choice

What type of fossil helps scientists find the relative age of rock?

• trace fossil

• body fossils

• index fossils

• marked fossils

Multiple Choice

Which feature of this rock strata is the youngest?

• B

• D

• A

• E

Multiple Choice

Which feature is the oldest in this rock strata?

• B

• C

• E

• A

Multiple Choice

What is the youngest feature in this sample?

• A

• D

• C

• B

Multiple Choice

Which feature is the youngest in this image?

• E - the fault

• A- the top sedimentary layer

• D - the igneous intrusion

• C - the lowest sedimentary layer

Multiple Choice

What is the oldest feature in this image?

• E- the fault

• A - The top sedimentary layer

• D - the igneous intrusion

• C - the lowest sedimentary layer

Multiple Choice

Which fossil is the oldest?

Multiple Choice

Which fossil is the youngest?

Multiple Choice

Which of the following listed features is the youngest?

• Feature C

• Feature H

• Feature E

• Feature A

Multiple Choice

Which of the following listed features is the oldest?

• Feature G

• Feature F

• Feature E

• Feature H

Multiple Choice

What do features "D" and "A" represent in this image?

• Igneous extrusions

• Igneous intrusions

• A yummy cream center.

• An unconformity

Multiple Choice

What does feature "E" represent in this image?

• an unconformity

• an igneous intrusion

• a tectonic plate

• a fault

Multiple Choice

What is the youngest feature in this diagram?

• Feature "E" - fault

• Feature "I" - rock layer

• Feature "A" - rock layer

• Feature "C" - rock layer

Multiple Select

Why could there be missing layers of rock in the geologic record?

• Erosion could have removed some layers.

• Deposition didn't occur for some reason.

• Animals moved all of the fossils and rocks

• This never happens.

Multiple Choice

A gap in the geologic record is known as a(n)...

• fault

• intrusion

• unconformity

• erosion

Multiple Choice

The position in which rock layers appear can be changed by gaps in the geologic record and by...

• unconformity

• folding

• shuffling

• uplifting