How do crosscutting relationships reveal the relative ages of faults and igneous intrusions?

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dating Layered strata in an outcropping of the Morrison Formation on the west side of Dinosaur Ridge, near Denver, Colorado.

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 Intrusions: When magma rises through the crust, it cuts across existing rock units. Consequently, the resulting igneous rock—such as a dike or sill—must be younger than the host rock it intrudes. In the case of sills, the presence of host rock fragments within the intrusive body further confirms its younger age.1
Faults: Similarly, faults represent fractures that offset existing rock layers. If a fault displaces a rock unit or an igneous intrusion, the faulting event must have occurred after those units were formed. Conversely, if an igneous dike cuts across a fault line without being offset, the intrusion is younger than the fault.1

By observing these visual relationships, geologists can establish a chronological sequence of events without the need for complex analytical equipment.1