Extensional stress occurs when the Earth's crust is stretched and pulled apart. This can happen when two tectonic plates move away from each other, or when a single plate is stretched by the forces of mantle convection.

When extensional stress occurs, the crust thins and weakens, and eventually breaks to form faults. As the faults move apart, the blocks of crust between them are tilted and uplifted, forming fault-block mountains.

Fault-block mountains are characterized by their steep, blocky cliffs and the long, flat valleys between them. The Basin and Range Province in the western United States is a classic example of fault-block mountains.