8-2: The Relative Ages of Rock
7th Grade Life Science
8-2: The Relative Ages of Rock
• 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.
8-2: The Relative Ages of Rock
• 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?
8-2: The Relative Ages of Rock
• Law of Superposition
• This law says the bottom layers are older than the top layers.
• Only true for horizontal layers.
8-2: The Relative Ages of Rock
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
8-2: The Relative Ages of Rock
• 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.
8-2: The Relative Ages of Rock
• 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.
8-2: The Relative Ages of Rock
• Clues from Faults
• A fault is a break in the Earth’s crust.
• Faults are always younger than the rock it cuts through.
8-2: The Relative Ages of Rock
• 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.
8-2: The Relative Ages of Rock
• 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.
8-2: The Relative Ages of Rock
• 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
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)
8-2: The Relative Ages of Rock
• 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.
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