Final answer:
The type of stress that occurs when plates move apart is tension, which is typical at divergent plate boundaries where the Earth's crust is being pulled apart, leading to thinning and potential volcanic activity.
Step-by-step explanation:
Tensional stress is observed at divergent plate boundaries, where the lithospheric plates are moving away from each other. This movement is due to the mantle's convection currents, which cause the crust to pull apart. These areas are characterized by features such as mid-ocean ridges and rift valleys. Tensional stress leads to the thinning of the crust, which can result in volcanic activity as magma rises to fill the space created by the separating plates.
Compression stress, on the other hand, happens when plates are pushed together, not when they are pulling apart. While shear stress refers to the movement of plates past each other in opposite directions, such as along transform boundaries. Faulting can be a result of any of these stresses when the strength of the rock is exceeded and it fractures. When considering Earth's tectonic activities, tension is a critical stress type in the dynamics of plate movement.