Final answer:
Tensional stress, occurring at divergent plate boundaries, leads to lengthening of the crust.
Step-by-step explanation:
Tectonic stress that will result in a lengthening of the crust is called tensional stress. This type of stress occurs at divergent plate boundaries, where two plates move away from each other. As the plates separate, the crust stretches and thins, causing the lengthening of the crust.
For example, the Mid-Atlantic Ridge is an underwater mountain range caused by tensional stress. This ridge is formed as the crustal plates in the Atlantic Ocean move apart, allowing magma from the mantle to rise and create new crust, resulting in the lengthening of the oceanic crust.
Other examples of tensional stress include the East African Rift System and the Basin and Range Province in the western United States.