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