How do dikes help geologists determine which rocks formed first?

In the field of geochronology, geologists determine the relative age of rock units by applying the principle of crosscutting relationships.

Because a dike is formed when magma intrudes into and solidifies within existing fractures in the Earth's crust, it must necessarily be younger than the rock units it cuts across. By observing these physical intersections, geologists can establish a clear sequence: the host rock, which was already present to be fractured and invaded, is older than the dike that intrudes it.1

This method is a fundamental tool for visual observation in the field, allowing researchers to deduce relative ages without the need for complex analytical equipment. Even in complex geological settings where multiple intrusions or deformation events have occurred, the presence of these crosscutting bodies provides a reliable record to distinguish between older and younger units.1