Here's how it works:

* Tensional stress: This occurs when forces pull on a rock from opposite directions, causing the rock to stretch and thin. Imagine pulling on a rubber band; it stretches and thins in the direction of the pull.

* Crustal extension: Tensional stress is often associated with areas where the Earth's crust is being stretched or pulled apart. This is typically found at divergent plate boundaries, where tectonic plates move away from each other.

* Formation of rift valleys and mid-ocean ridges: The stretching caused by tensional stress can lead to the formation of rift valleys, which are valleys formed when the Earth's crust is pulled apart. At mid-ocean ridges, where new crust is formed, tensional stress plays a crucial role in separating the plates.

Here are some examples of how tensional stress affects rock:

* Faulting: Tensional stress can cause rocks to fracture along planes called faults. These faults are often characterized by normal faults, where the hanging wall (the block above the fault plane) moves downwards relative to the footwall (the block below the fault plane).

* Stretching and thinning: Rocks can be stretched and thinned by tensional stress, leading to the formation of folds and other structural features.

* Volcanism: Tensional stress can also lead to volcanism, as magma can rise to the surface through cracks and fissures created by the stretching.

In summary, tensional stress is a powerful force that can shape the Earth's crust, leading to the formation of various geological features.