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Determining Relative Age

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1 Determining Relative Age
Section 17.1 by Andrea Robson and Christian Eib

2 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”


4 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!

5 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 size 2.) shape of bedding plane 3.) ripple marks


7 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

8 Nonconformity

9 Angular

10 Disconformity

11 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

12 Cross-Cutting Relationships

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