Final answer:
When tensional force is applied to rock layers, it develops a fault. Tensional forces lead to stretching and subsequent fracturing of the rock, differentiating from folds which form under compressional stress.
Step-by-step explanation:
When a tensional force is applied to rock layers, it creates a feature known as a fault. Tensional forces occur as the Earth's crust is being stretched, leading to the formation of faults in the rock structure. This process is a result of the rocks being subjected to stress, such as those found at divergent tectonic plate boundaries where the ground is pulling apart. The strain put on the rocks leads to fractures once they surpass their elastic limits, and these fractures are called faults.
From the context of geological structures, folds are typically associated with compressional stress, where rocks are pushed together. A clear difference between these two types of stress is important: fold-related structures such as anticlines and synclines usually form under compressional stress, whereas faults often form under tensional or shear stress.