| Themes > Science > Earth Sciences > Geology > About Geology, Generalities > Principles of Geology > Other Basic Geologic Principles > The Principle of Intrusive Relationships |
Where an igneous intrusion cuts across a sequence of sedimentary rock, the relative ages of these two units can be determined. The sedimentary rocks are older than the igneous rock which intrudes them. (In other words, the sedimentary rocks had to be there first, so that the igneous rocks would have something to intrude.) Or, you could say, the intrusion is younger than the rocks it cuts. Examples of types of igneous intrusions (or plutons) are dikes, sills, stocks, laccoliths, and batholiths. |
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Examples of plutons showing intrusive relationships. Diagram (1): Dike B is younger than Sedimentary Rock A. Erosion surface C is younger than Dike B. Sedimentary Rock D is younger than Erosion Surface C. Diagram (2) Sill B is younger than Sedimentary Rock A. Dike C is younger than sill B. Diagram (3) Stock B is younger than Sedimentary Rock A. Dike C is the youngest. |
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