What does the law of superposition help scientists figure out?

The law of superposition helps scientists figure out which rock layers are older or younger than others.

Imagine you have a stack of books. The bottom book has a copyright date of 1990, and the top book has a copyright date of 2020. Which book is older?

The book with the 1990 copyright date is older.

Relative dating is figuring out if something is older or younger compared to something else, without knowing the exact age.

The fossil in the lower rock layer

An igneous intrusion is when molten rock pushes into existing rock layers and then cools and hardens.

Grade 5th Grade · Science · 45 min

I can explain how the law of superposition helps determine the relative ages of rocks and fossils.

Khan Academy · 4:35

Fossils and rock layers | The geosphere | Middle school Earth and space science | Khan Academy

3 key concepts

1
The geologic time scale divides Earth's history into units called eons and eras, which are mapped out using clues found in rocks.
2
The law of superposition states that in undisturbed rock layers, the layers at the bottom are older than the layers at the top.
3
The law of cross-cutting relationships helps determine relative ages when an igneous intrusion or fault cuts through existing rock layers.

10 questions · Multiple choice & Short answer

Quick comprehension check

“Imagine you find two layers of rock. Layer A is on the bottom, and Layer B is on top. According to the law of superposition, which layer is older?”

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