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How do crosscutting relationships reveal the relative age of a fault or igneous dike?
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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.
The principle of crosscutting relationships is a fundamental concept in geochronology used to determine the relative ages of geologic features. It states that any geologic feature that cuts across another must be younger than the unit it disrupts.1
- Igneous Dikes: When magma rises through the crust and solidifies within a fracture, it forms a dike. Because the dike must intrude into existing rock to occupy that space, the dike is necessarily younger than all the rock units it cuts across.1
- Faults: Similarly, if a fault offsets a series of rock layers or an igneous dike, the fault must be younger than the features it displaces. If a dike cuts across a fault, however, the dike must be younger than the faulting event.1
By observing these physical intersections, geologists can establish a chronological sequence of events—such as deposition, intrusion, and deformation—without needing absolute isotopic dating.12