Determining Relative Age

Section 17.1 by Andrea Robson and Christian Eib

Principle of uniformitarianism

• a man in the 18th century named James Hutton invented it

• “current geologic process such as volcanism and erosion are the same processes that were at work in the past”

How it works

• Layers of rock are called strata. Once the order of the formation of rocks is known, the relative age of the rocks can be determined

• Relative age indicates that any one layer is older or younger than another layer, NOT THE EXACT AGE OF THE ROCK!

Law of Superposition

• An undeformed sedimentary rock layer is older than the layers above it and younger than the layers below it.

• If the layers aren’t horizontal then the law is not easily applied, then scientists need to find clues:

1.) particle size2.) shape of bedding plane

3.) ripple marks

Uh oh, unconformities

• Are a break in the geologic record due to erosion or faulting.

• 3 types of unconformities are : 1.) nonconformity – stratified rests on

unstratified rock 2.) angular – the boundary between tilted

and horizontal layers 3.) disconformity – boundary between layers

of rock that have not been deposited continuously

Nonconformity

Angular

Disconformity

Cross-cutting relationships

• Is used to determine the relative ages of rock layers, faults, and intrusions

• You can tell of a rock or intrusion is younger than the rock it cuts through

Cross-Cutting Relationships