Earth Science

Earth History

Determining Relative Ages

Study Tip
A helpful way to think of the Law of Superposition is to think of a stack of newspapers. If you read the newspaper everyday and then put it in a pile, the older newspapers will be on the bottom and newest ones on top.

The Law of Superposition

The Law of Superposition is a rule that scientist can use to determine which layers of rock are older than others.  The law states that rock layers are older than the layers that they are beneath.

Study Tip
A helpful way to think of the Law of Superposition is to think of a stack of newspapers. If you read the newspaper everyday and then put it in a pile, the older newspapers will be on the bottom and newest ones on top.
faults and cross-cutting of rocks

The Principle of Cross-Cutting

The Principle of Cross-Cutting is another rule used to identify the relative age of geological features.  

The rule states that faults and intrusions are younger than the geological features they cut through.

Concept Check
  • What are the two rules scientists use to determine the relative age of rocks?