Final answer:
Shear stress occurs when a rock mass is pushed in opposite parallel directions, causing a horizontal shift of rock layers due to shear forces.
Step-by-step explanation:
The type of stress that occurs when a rock mass is pushed in parallel and opposite directions is known as shear stress. This type of stress is caused by shear forces, which are forces that move horizontally past each other in opposite directions.
An example of shear stress can be visualized by holding a book between your hands and moving the covers in opposite parallel directions without causing any deformation vertically. As rocks experience shear stress, they may undergo shear deformation, characterized by a shift of layers tangent to the direction of the applied forces. This shear deformation results in changes to the rock formation that can eventually lead to features like strike-slip faults.