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How do igneous dikes help establish the relative ages of nearby rocks?
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geochronology
The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.
In geochronology, igneous dikes are essential for establishing relative ages through the principle of crosscutting relationships. Because a dike forms when magma intrudes into and solidifies within existing fractures, the dike must be younger than the surrounding rock units it cuts across.12
Geologists use this relationship to determine the sequence of events in complex geological settings:
- Crosscutting: Any rock unit or structural feature (such as a fault) that is intersected by a dike must predate the intrusion of that dike.1
- Complex Sequences: In areas where older crust has been fractured and invaded by multiple dikes, the dikes serve as chronological markers. Even if the region undergoes subsequent heating or deformation, the presence of these "relic" dikes allows geologists to distinguish and order the older, predike units.1
- Sills: While dikes are typically discordant (cutting across bedding), related bodies called sills form parallel to existing layers. To confirm a sill is younger than its host rock, geologists look for fragments of the host rock trapped within the intrusive body.1