Superposition — Science definition
The law stating that in an undisturbed sequence of rock layers, the oldest layer is on the bottom and the youngest layer is on top.
Where it is taught
- Geologic Time & the Rock Record — Earth’s History, Earth & Space Science, Grade 9
In use
Understanding how long ago things happened is one thing; working out the order they happened in is another, and that's where the law of superposition comes in.
Geologic Time & the Rock Record
Questions using superposition
Problem 1
According to the law of superposition, in an undisturbed sequence of rock layers, which layer is the oldest?
- The layer on the very bottom
- The layer on the very top
- The middle layer
- It's impossible to tell
Answer and explanation
The layer on the very bottom
Superposition says undisturbed layers get younger as you go up, so the bottom layer formed first and is the oldest.
Problem 2
The principle of superposition states that in undisturbed layers
- the oldest is on top
- the oldest is at the bottom
- all layers are the same age
- age cannot be determined
Answer and explanation
the oldest is at the bottom
Later sediment is deposited on top of earlier sediment.