8-2: the relative ages of rock

8-2: The Relative Ages of Rock

7th Grade Life Science


• Relative Age of Rocks

• This is when the age of rocks are compared to the age of rocks around it.

• Top is youngest.

• Bottom is oldest.


• Absolute Age of Rocks

• This refers to the actual number of years that have passed since the rock formed.

How old? 1 million years? 1000 years?


• Law of Superposition

• This law says the bottom layers are older than the top layers.

• Only true for horizontal layers.


Grand Canyon Sequence

8-2: The Relative Ages of Rock • Other Relative Dating

Techniques • Scientists also use:

1. Clues from igneous rocks

2. Faults 3. Gaps in the

geologic record 4. Inclusions


• Clues from Igneous Rocks

• An extrusion is when lava cools on the surface of the earth.

• Extrusions are always younger than the rocks found below it.


• An intrusion is when magma pushes itself into rock below the surface, and then cools and hardens.

• Intrusions are always younger than the rocks around and below it.


• Clues from Faults

• A fault is a break in the Earth’s crust.

• Faults are always younger than the rock it cuts through.


• Cross Cutting Relationships

• Many of these examples are of things cutting across rock layers.

• This principle states that if it cuts across a layer, it is younger.

8-2: The Relative Ages of Rock • Gaps in the Geologic Record

• Sometimes layers are deposited, then erosion takes parts of the layers away, and then more layers are put down.

• This forms what is called an unconformity.


• An unconformity indicates a gap of time, of unknown duration, has taken place.

• During that time erosion removed some of the rock.

• Then more rock was deposited on top.


• Inclusions

• Inclusions are pieces of rock INSIDE another rock.

• Inclusions are always older than the rock they are in.

8-2: The Relative Ages of Rock • Using Fossils to Date Rocks

• Index fossils are fossils that are wide spread around the world AND only lived for a short period of time.

• Trilobites - 100 million years (Cambrian)

• Ammonites - 65 million years (Jurassic)


• Careful – the Coelacanths was considered an index fossil until it was caught in 1938 near Madagascar.

• So now rock with a fossil of Coelacanths could be any age since they still exist – not an index fossil anymore.

• Coelacanths - 70 million years? (Jurassic)

8-2: The Relative Ages of Rock Summary

• Scientists use several techniques to find relative ages of rocks.

• These include extrusive and intrusive rocks.

• Also, faults, inclusions, and unconformities.

• Index fossils are also used.

8-2: the relative ages of rock

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