Why are cross-cutting relationships used in relative dating?

The Principle of Cross-Cutting Relationships states that rock formations that cut across other rocks must be younger than the rocks that they cut across. Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.

Are faults older than dikes?

Cross-cutting relationships may be compound in nature. For example, if a fault were truncated by an unconformity, and that unconformity cut by a dike. Based upon such compound cross-cutting relationships it can be seen that the fault is older than the unconformity which in turn is older than the dike.

Which is older fault A or fault B?

4. (a) Is fault A older or younger than the sandstone layer? Fault A cuts the sandstone layer so the fault is younger. The conglomerate is cut and displaced by fault B; thus fault B is younger.

Why does cross cutting happen?

Intrusional relationships occur when an igneous pluton or dike is intruded into pre-existing rocks. Stratigraphic relationships may be an erosional surface (or unconformity) cuts across older rock layers, geological structures, or other geological features.