What occurs during the folding of crustal rocks?

During the folding of crustal rocks, the primary result is that they bend and form folds. This process typically occurs in response to tectonic forces that exert pressure on the Earth's crust, causing the rocks to deform rather than break. The bending can create features such as anticlines (upward arches) and synclines (downward troughs), which are characteristic of folded rock formations.

Folding is a crucial aspect of geological processes as it reflects the dynamics of Earth's lithosphere in areas where tectonic plates interact. Unlike fracture-related phenomena where rocks break into faults or move into the mantle through subduction, folding results in altered shapes of existing rock structures without causing them to lose integrity. Therefore, the correct choice highlights the nature of this deformation, distinguishing it clearly from other geological processes involving breaking or shifting of rock.