In which situation is the principle of cross-cutting relationships useful in determining relative age?

Answer 1

There are many geological situations where cross-cutting relationships are found and can help in establishing relative age of events. In the field, it takes thoughtful consideration to work out these episodes. Here are some examples that help illustrate the principle that a cross-cutting event always occurred after the rocks that have been crossed.

1. Where there are layered sedimentary rocks that are cross-cut by a fault, the sediments were already there before the faulting occurred. A fault is just a break in the rocks where one side slides relative to the other... but clearly, such a break cannot happen BEFORE the rocks are already there! Rocks first, faulting (the cross-cutting event) happens after rock formation.
2. Similarly, mineral veins can cut across rocks. Veins occur where a rock fractures (a fracture is just a crack in the rock), then hot groundwater moves through the fractures and minerals (such as quartz or calcite) that were dissolved in the groundwater grow in the crack and fill it with the new mineral. Veins are often quite easily seen in rocks, as the minerals within them often are quite different in color or style from the surrounding rock. But clearly, from the principle of cross-cutting relations, the rocks are older than the veins, so fracturing of the rocks came after the rocks formed.
3. In some situations, sedimentary rocks (which typically form in horizontal or nearly horizontal layers) can be folded into spectacular, accordion-like fold shapes, where they are no longer horizontal at all. If these folded sedimentary rocks contain veins, the veins can be used to tell whether they formed before or after the folding events. If the veins are folded too, along with the sedimentary rocks, then the order of formation would be 1) rocks deposited, 2) veins formed, 3) rocks+veins folded. If instead the veins are not folded but cut across the folds, then the order is necessarily 1) rocks, 2) folds, and 3) veins last, because the veins cut across both the rocks and the folds.

I don't exactly know how to answer the question as it is written, because there are MANY situations where this principle can be applied. I have only written about three of them, but I hope this illustrates how useful the principle can be.